two-step laser mass spectrometry

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PART ONE

TWO-STEP LASER MASS SPECTROMETRY


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TWO-STEP LASER MASS SPECTROMETRY Renato Zenobi and Richard N. Zare


Department of Chemistry, Stanford University, Stanford, CA 94305

Abstract Two-Step Laser Mass Spectrometry (L2MS) is a new mass spectrometric method where the two essential steps of any mass spectrometric analysis, vaporization and ionization, are performed by lasers. In the First step, the output of a pulsed C0 2 laser is focused onto a thin film of sample deposited on a chemically inert substrate, such as glass, quartz or ceramics. This causes rapid heating of the substrate and results in desorption of intact, neutral analyte molecules from its surface. In a second step, a pulsed ultraviolet laser causes 1 + 1 Resonance-Enhanced Multiphoton Ionization (REMPI) of the desorbed neutrals. The ions are then mass-analyzed in a reflectron-type time-of-flight

mass

spectrometer.

Under

suitable

experimental

conditions,

fragmentation in both the desorption and ionization steps can often be avoided. The method is well suited for the analysis of nonvolatile, polar, and thermally labile species up to high molecular weight. Because of the optical selectivity of the REMPI ionization process, L2MS is also an ideal methodology for the analysis of complex mixtures. In this article, the process of laser-induced thermal desorption is considered both from an analytical and surface science perspective. First, experimental findings from the literature and models to explain the process are contrasted. Second, experimental and theoretical results on laser surface heating of dielectric materials are presented. Third,

4

preliminary results on kinetic energy distributions of desorbed molecules are discussed. A number of analytical applications of L2MS are presented. They include a study of terpenoid compounds in amber samples, an investigation of polycyclic aromatic hydrocarbons (PAHs) in meteorite samples, and a spatially resolved organic analysis of Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

the Allende meteorite, which again focuses on PAHs. The most important impact of these studies is that they document the possibility to analyze trace amounts of organics in small quantities of complex mixtures without prior sample preparation, extraction, purification and separation steps. This not only decreases analysis time, but also greatly reduces the danger of contamination inherent in trace analysis. Modern, laser-based methods such as L2MS have great potential for studying very small quantities of precious samples and for analyzing the distribution of organic material in a wide variety of samples with unprecedented spatial resolution.

5


Table of Contents Abstract

3

Table of Contents

5

PREFACE

8

INTRODUCTION AND BACKGROUND

9

A. B. C. D.

Concept of Two-Step Laser Mass Spectrometry Resonance-Enhanced Multiphoton lonization Time-of-Flight Mass Spectrometry Analytical Laser Mass Spectrometry

CHAPTER 1: EXPERIMENTAL A. Experimental Configurations B. Trigger Scheme C. Sample Preparation CHAPTER 2: DESORPTION MECHANISM 2.1 Scope of Laser Desorption 2.1.1 Review of Previous Experimental Work A. Small Molecules on Metal Surfaces B. Desorption from Thick Cryogenic Films C. Matrix-Assisted Desorption D. Laser Ablation E. Large Molecules on Dielectric Substrates F. Resonant Desorption 2.1.2 Models of Laser-Induced Thermal Desorption A. Resonant Desorption Model B. Shock-Wave Model C. Thermal Equilibrium Model D. Non-Equilibrium Model E. The Role of Collisions

9 14 16 18 21 21 26 29 33 33 33 34 37 39 41 42 44 46 46 48 49 52 55


6

2.1.3 Possible Experimental Tests

57

2.2 Surface Temperature Measurement of Dielectric Materials heated by Pulsed Laser Radiation Abstract A. Introduction B. Experimental C. Results and Discussion D. Conclusions

59 59 59 61 64 73

2.3 Pulsed Heating of Surfaces: Comparison between Numerical Simulation, Analytical Models, and Experiments Abstract A. Introduction B. One-Dimensional Heat Flow Model C. Finite Difference Method D. Results and Discussion E. Conclusions

74

2.4 Preliminary Measurements on Kinetic Energy Distributions of Desorbing Molecules A. Introduction B. Experimental C. Results and Discussion D. Conclusions

90

CHAPTER 3: SELECTED ANALYTICAL APPLICATIONS 3.1 Analysis of Aromatic Terpenoids in Amber Samples A. Introduction B. Experimental C. Results and Discussion D. Conclusions

74 74 75 76 78 89

90 92 98 106 108 108 108 110 112 117


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3.2 Application of Two-Step Laser Mass Spectrometry to Cosmogeochemistry: Direct Analysis of Meteorites Abstract A. Introduction B. Experimental C. Results and Discussion

118 118 118 120 122

3.3 Spatially Resolved Organic Analysis of the Allende Meteorite Abstract A. Introduction B. Experimental C. Results and Discussion D. Conclusions

130 130 130 132 137 142

POSSIBLE FUTURE DIRECTIONS A. Isotopic Ratios B. Spatial Resolution C. Alternative Ionization Methods D. Identification of Analyte Molecules

143 143 144 145 145

REFERENCES

147

8

PREFACE This article consists of two major parts: experimental and theoretical work aimed at obtaining a deeper understanding of the laser desorption mechanism (Chapter 2), and


selected analytical applications of two-step laser mass spectrometry (Chapter 3). In addition, the concepts of this experimental methodology are outlined in "Introduction and Background", and many of the experimental details are given in Chapter 1. Four sections in this article are based on the following published material: R. Zenobi, J. H. Hahn, and R. N. Zare, Chem. Phys. Lett. 150, 361 (1988) (section 2.2); J.-M. Philippoz, R. Zenobi, and R. N. Zare, Chem. Phys. Lett. 158, 12 (1989) (section 2.3); J. H. Hahn, R. Zenobi, J. L. Bada, and R. N. Zare, Science 239, 1523 (1988) (section 3.2); and R. Zenobi, J.-M. Philippoz, P. R. Buseck, and R. N. Zare, Science 246, 1026 (1989) (section 3.3). We would like to thank a number of people who have contributed significantly to the work presented here: Jean-Michel Philippoz and Jong Hoon Hahn for their major contributions during many stages of the research, Peter R. Buseck (Arizona State University), Jeffrey L. Bada, and Kurt Marti (both UC San Diego) for their collaboration in the course of the work with meteorites. We thank them for sharing their knowledge with us. We are also grateful to Rob Bucenell, Laurie Kovalenko, Rick Maechling, and Evan Williams for assistance in the lab; they and Christine Leach helped to proofread the manuscript. This work would not have been possible without the financial support from the Center for Materials Research (CMR) at Stanford, grants from IBM, Applied Biosystems, Inc., and NASA, and a fellowship from Zentenarfonds der ETH Zurich.

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INTRODUCTION AND BACKGROUND Mass spectral analysis is one of the most powerful analytical methods, characterized by both high sensitivity and high information content. In most types of Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

mass spectrometers, electron impact or chemical ionization is used to obtain ions from the sample molecule. To apply these techniques, the sample must be present as a vapor in the ion source region. This is usually done with a capillary inlet for highly volatile compounds, or with a direct inlet probe for substances with lower vapor pressure. In the latter case, the sample is heated to several hundred degrees, and the resulting density in the source region is quite sufficient for analysis. However, the direct inlet method is not applicable for studying the vast number of organic and bio-organic compounds that are nonvolatile and thermally labile. This unsatisfying situation led to the development of a number of different methods that allow production of ions from such compounds. They include field desorption (FDMS, [1]), plasma desorption (PDMS, [2 - 4]), sputtering of substances as secondary ions by bombardment with energetic primary ions (SIMS, [5 - 9]) or atoms (FAB, [10; 11]), direct chemical ionization (DCI, [12; 13]), thermospray [14; 15}, electrospray [16; 17], and laser desorption (LD, [18 - 21], see also section D of this chapter). All these techniques have in common that ions are created directly from the condensed phase or by chemical ionization in the selvedge region, i.e., there is no separation between desorption and ionization.

A. CONCEPT OF TWO-STEP LASER MASS SPECTROMETRY In Two-Step Laser Mass Spectrometry (L2MS) we take a different approach: the desorption and ionization events take place spatially and temporally separated. This

10 general strategy has first been applied to the trace detection of atoms from surfaces [22-26], then molecules from surfaces [27 - 31]. Using two different laser pulses for the two tasks gives the experimentalist the freedom of choosing ideal light sources, and the ability to optimize each process individually. Many variations are possible, but the Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

approach we have chosen uses a pulsed C 0 2 laser for desoiption, followed, after a suitable time delay, by the frequency-quadrupled output of a pulsed Nd.YAG laser which causes resonance-enhanced multiphoton ionization (REMPI) of the desorbed neutrals. These ions are then mass-analyzed in a time-of-flight mass spectrometer. Figure 1.1 shows the L2MS time-of-flight mass spectra of two high-molecular weight compounds of complicated organometallic complexes. Such compounds typically present problems to classical mass spectrometric analysis because they are nonvolatile, may be thermally labile, and are often subject to extensive fragmentation when ionized by electron impact The spectra nicely document several of the following advantages of L2MS over other mass spectrometric techniques: (i) Soft ionization: REMPI yields simple mass spectra that are often dominated by parent ion peaks, (ii) High selectivity in ionization by using REMPI. This allows us to discriminate against background, such as water, carbon dioxide, or pump oil contamination inside the instrument. (Hi) The infrared (IR) laser pulse causes desoiption of intact neutral molecules from the substrate surface. The IR photons are not energetic enough to induce photochemical reactions, e.g. photodissociation, of the adsorbed species, (iv) High sensitivity. Since the number of neutrals is several orders of magnitude greater than the number of ions desorbed in one-step desorption-ionization methods, combining C 0 2 laser desoiption with the high efficiency of REMPI ionization and the high throughput of time-of-flight mass spectrometers allows us to obtain sensitivities in the range of

11


(a)

1132

MW = 1132

1232

(b)

MW = 1232

JL 100

50 Time of Flight Figiue 1.1

(^s)

Examples of mass spectra obtained with L2MS: (a) H4(DPA) and (b) Zn2(DPB). DPA = l,8-bis[5-(3,9,13,19-tetraethyl-4,8,14,18-tetramethyl) porphyrinyl]-anthracene; DPB = l,8-bis[....]-biphenylene.

12 femtomoles per laser shot or less [32]. (v) No matrix ionization effects, (vi) Large dynamic range of quantitative analysis [32]. (vii) Short analysis times. A complete mass spectrum can in principle be obtained for a single sequence of desorption and ionization laser pulses, (viii) Theoretically, there is no upper mass limit due to the use of


time-of-flight mass separation. We have further expanded the range of applications of this methodology to the direct analysis of mixtures of organic compounds (see also chapter 3). Minimal sample amounts and essentially no sample preparation are needed. The approach mainly makes use of the selectivity in REMPI ionization. Specific classes of compounds within a complex mixture are ionized (see section B of this chapter). Figure 1.2 shows a schematic diagram of the ion source region of the instrument. The sample is mounted on the tip of a long teflon of delrin rod. This probe is introduced through a vacuum interlock into the system without breaking high vacuum. Sample introduction takes about 1 minute and the spectrum can be recorded immediately thereafter. The sample is positioned in the hole of the first electrode (repeller) in the ion extraction region, with the sample surface flush with the repeller plate. The desorption laser beam is focused with a 25 cm focal length lens and hits the sample surface at a 45° angle, off-axis with respect to the center of the sample rod. The sample probe can be rotated about its axis in order to expose fresh sample surface to the desorption laser beam. The output of the first laser causes desorption of intact neutral molecules from a spot about 1 mm in diameter. For ionization, the output of a second laser is focused by a 25 cm focal length cylindrical lens to a ribbon, 2 mm from the sample surface. The ions formed are then extracted into a time-of-flight tube and mass analyzed.


13

^

2 p.

3

4>

o

a 9

o w

.5* it,

14

B. RESONANCE-ENHANCED MULTIPHOTON IONIZATION In the second step of our methodology, desorbed molecules are ionized by 1+1 REMPI with the fourth harmonic (266 nm) of a Nd:YAG laser (= 1 mJ/pulse; 10 ns


pulse width). The appropriate delay between the desorbing C0 2 laser and the ionizing Nd:YAG laser pulse is chosen so that the ionizing laser pulse intercepts as many molecules as possible. In 1+1 REMPI, one photon causes a molecule to make a transition to an electronically excited state and a second photon ionizes the excited molecule (see fig. 1.3 a). Wavelength selectivity results from the photon energy being resonant with an intermediate state while such selectivity is lost in the direct one-photon ionization process [33]. The ionization efficiency of a resonant mechanism is greater by many orders of magnitude than a nonresonant multiphoton process for molecules which lack a chromophore in the proper wavelength range (fig. 1.3 c). REMPI has other important attributes for mass spectrometry; it can provide very efficient "soft ionization" [21; 32; 34 - 41] in which the parent molecular ion dominates the mass spectrum because ions with very little internal excitation are formed (fig. 1.3 a). This is in sharp contrast to electron impact ionization where soft ionization can be obtained only with a significant decrease in ionization efficiency. In contrast to direct one-step ionization of molecular adsorbates, the two-step laser methodology where the desorption and ionization processes are spatially and temporally separated allows a much larger fraction of the molecules to be converted to parent ions by optimizing separately the two laser sources. In addition, this technique avoids variations of the ionization efficiency with the substrate surface (matrix effect).

15 Energy A

l\


fragments

IP

S1

S1

virtual S1

(a)

(b)

(c)

Figure 1.3: Different scenarios of multiphoton ionizadon. (a) 1 + 1 REMPI: The first photon is resonandy absorbed by the molecule, and the second photon causes ionization. (b) Formation of fragments when more than two photons are absorbed, (c) Nonresonant two-photon ionization through a virtual state.

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REMPI also has the capabilities to give structural information when fragmentation is purposely induced. This can be accomplished by using higher laser power densities which result in further photons being absorbed by the ion; the extra internal energy of the ions leads to fragmentation (fig. 1.3 b). Because the mass fragmentation patterns are Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

not expected to be identical to those found in El mass spectra, the process has been studied in detail for a variety of molecular systems [42]. When several additional photons are absorbed, fast unimolecular reactions of highly excited ions lead to a competition between dissociation and further photon absorptions. This is commonly referred to as a "ladder switching" mechanism for the production of smaller photofiragments. By simply changing laser power, REMPI has been used for the hard and soft ionization of a variety of biomolecules [36; 37]. In particular, the structure-specific (A, B, X and Y) fragments for a variety of small peptides could be obtained by REMPI [43].

C. TIME-OF-FLIGHT MASS SPECTROMETRY The introduction of lasers as pulsed ionization sources in mass spectrometry has led to a renaissance for time-of-flight (TOF) mass spectrometry. TOF mass spectrometers are ideally suited for detecting pulses of ions, allow a complete mass spectrum to be recorded in a single shot, have a theoretically unlimited mass range, and are popular for high-sensitivity applications because of their very high ion throughput. The principle of this type of mass analyzer is that ions which are produced during a laser pulse are accelerated by electric fields into a field-free drift tube. Light ions reach high velocities and will arrive at the detector at the end of the drift tube first, while heavy ions will leave the acceleration field with lower velocity and take longer to reach

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the detector. The flight time is therefore a measure of the mass of the ions: it is proportional to the square root of the mass. A source region design consisting of two different electric fields for the extraction of the ions was introduced by Wiley and McLaren [44] and is used in virtually all instruments today. Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

A drawback of TOF instruments is their poor resolution, typically around m/Ara ■= 200. Many methods for improving the resolution have been suggested. The most widely used is the so-called "reflectron" which was introduced by Mamyrin in the 70s [45; 46]. It can be shown that in linear TOF instruments the best space-time focusing of an ion pulse is accomplished close to the source. In reality, this is impractical because it would result in an unreasonably short flight time before the ions hit the detector, and the resolution would be dictated by the time resolution of the detector and associated electronics. In a reflectron, the ions are focused close to the source, but are then allowed to spread out again. The trick is to introduce an ion mirror in the beam path, so that the ions reach another, secondary space-time focus after a long flight time. The ion mirror consists of two adjacent homogeneous electric fields. Let us consider a pulse of ions with the same mass, but slightly different velocities moving from the first focus towards the ion mirror. Faster ions will reach the reflector first, but will penetrate farther into it, while slower ions reach it later, but do not penetrate so far. This essentially allows the slow ions to leave the reflectron first, i.e., to pass the fast ones. The fast ions will catch up with the slow ones at the second focal point. This is precisely the location where the detector is placed. Experimentally, a resolution of over 10,000 has been reported by several laboratories [47; 48; 49; 50], mostly in instruments with molecular beam sample introduction.

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D. ANALYTICAL LASER MASS SPECTROMETRY For the most part, the term "laser desorption" has been used in analytical chemistry to designate techniques where ions for mass spectrometric analysis are created in a


one-step desorption/ionization process by irradiating a solid sample with a laser. Initial experiments were carried out by Vastola [51; 52], Kistemaker [53], and Hillenkamp [54; 55] using pulsed ruby and C0 2 lasers, and by ROllgen [56] using a cw-COz laser. These early experiments stimulated much interest because they demonstrated the production of parent ions direcdy from a wide variety of nonvolatile and thermally labile compounds, such as oligosaccharides, glucosides, peptides, etc. However, in almost all of the cases, quasi-molecular ion peaks, [M + H]+, [M + Na]+, and [M + K] + dominated the mass spectra. The same was true for experiments where lasers with shorter wavelengths and shorter pulse widths were used for ion production [57]. These direct one-step methods have been widely used since the introduction of a few commercial laser microprobe mass spectrometers, the most successful of which was the "LAMMA" instrument developed by Leybold-Heraeus [58; 59; 60]. Much of this work has been reviewed extensively by Conzemius and Capellen in 1980 [61], and since then by Cotter [62; 63]. The recent development of matrix-assisted laser desorption mass spectrometry by Hillenkamp [64; 65; 66], Chait [67; 68; 69], and Tanaka [70] is another form of a one-step desorption/ionization method. It has been very successful in the production of parent ion peaks of very high mass compounds, such as proteins. Although the mass resolution in theses studies did not exceed 500, it has been claimed that quasimolecular ions, mosdy [M + H] + are also produced here [67].

19

One-step laser desorption/ionization techniques have also been used in a number of studies employing Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometers. Reports from various groups have focused on surface analysis [71; 72], on the study of surface reaction kinetics [73; 74], on polymer analysis [75; 76; 77], and on Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

the mass spectrometry of biomolecules [78; 79; 80; 81; 82]. The field has recendy been reviewed by Wilkins [83; 84]. Soon it became clear to researchers that the number of desorbed neutral molecules exceeds the number of ions produced in the laser desorption process by about two orders of magnitude; this has also been shown experimentally [85]. Therefore, the next logical step was to post-ionize laser-desorbed neutral molecules. This was first attempted by electron impact ionization in a sector instrument [86]; however, pulsed ionization with lasers has proven to be much more efficient for the ionization of a short pulse of neutral molecules leaving a surface. Two-step laser mass spectrometry was first demonstrated by Letokhov and coworkers in 1982/83 [28; 87], and has later been adapted by other researchers. TOF or reflectron TOF instruments are used in almost all experiments. Recently, a commercial system, developed by Bruker (Germany) has even become available. It is tailored after the initial design by the Schlag group [30; 47], incorporating a reflectron and a pulsed molecular beam for cooling the desorbed species and transporting them into the ionization region. Recent reviews about Two-Step Laser Mass Spectrometry have been presented by Grotemeyer [88], Lubman [89; 90], Nogar [91], Shibanov [21], and Zare [92]. Mass spectra of a wide variety of compounds, including free and derivatized amino acids, peptides and proteins, sugars, purines and pyrimidines, steroids, chlorophylls, and organometallic complexes have appeared in the literature using this method.

20 In our experimental setup, the sample is deposited as a thin film on an infrared-absorbing dielectric substrate. Irradiation with a C 0 2 laser pulse causes rapid heating of the substrate. This leads to desorption of intact, neutral analyte molecules from the substrate surface. We have chosen an approach where no molecular beam is Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

used following desorption, and therefore dilution of the analyte molecules is avoided. By maximizing the spatial overlap of the ionization laser with the desorption plume, ultrasensitive detection of organic molecules on surfaces can therefore be achieved [32]. This approach also yields "lukewarm" rather than cold analyte molecules which makes the REMPI process somewhat less selective. A range of compounds whose chromophores absorb in a similar wavelength range can therefore be ionized and detected. This feature allows us to perform organic analyses of complicated mixtures.

21

CHAPTER 1: EXPERIMENTAL Whereas the introductory chapter presented the concepts of Two-Step Laser Mass Spectrometry, the present chapter is intended to give a comprehensive account of the


experimental setup and methods used for sample preparation. The presentation is based on lab reports that have been compiled during the course of this work and is quite detailed. It is written with the intention of transferring knowledge that hardly ever appears in scientific publications, but is equally important to the success of the experimental work. These technical details are not important for understanding the work described in later chapters; all of the later chapters contain separate brief experimental sections that refer to the present description.

A. EXPERIMENTAL CONFIGURATIONS During the course of this work, the experimental configuration has changed several times. This development is broken up into five major stages, as summarized in table 1.1. In the following, the performance of the various parts of the experiment will be briefly described for each stage, and important improvements will be highlighted. Stage I. This stage used a small, multiline C0 2 laser system (Pulse Systems, LP-30) with a pulse energy of » 10 mJ, and a pulse width of 10 us for desorption. Its output was focused by a ZnSe lens (f = 250 mm) onto the sample which resulted in a power density of « 105 W/cm2 in a 1 mm-diameter spot on the surface. Ionization was accomplished by the frequency-quadrupled output of a Nd:YAG laser (Quanta-Ray, DCR-2) with 10 ns pulse width, suitable as a precise zero time for time-of-flight mass analysis. Its output was slightly focused by a quartz lens to a diameter of 6 mm which resulted in a power density of about 106 W/cm2 in the ionization volume.

22

Table 1.1: Development of Experimental Configurations


Stage used for

C0 2 laser system

YAG laser system ion optics and optics

I

fig. 1.1

LP-30

DCR-2

linear TOF mass spectrometer (Resolution 100-200)

II

chapter 3.2

LP-30

DCR-2

Reflectron TOF (Resolution => 550)

m

chapter 3.1

LP-30

DCR-11

Reflectron TOF

IV

chapter 3.3

LP-30

DCR-11 cylindrical lens, optical translation stage

new source region einzel lens design slit reflectron with adjustable voltages (Resolution «= 600)

Allmark-853

DCR-11

inverted TOF geometry cooled and rotatable surface holder

V chapter (UHV) 2.4

23

The spatial beam profile was the familiar "donut" shape, which is a linear combination of the

TEMQJ

and TEM10 modes. The laser beam entered the room from another lab

through a hole in the wall and was directed into the vacuum chamber by a series of turning prisms. This required careful alignment every time the system was operated. Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

The ions were formed in the source region of a Wiley-McLaren [44] type time-of-flight mass spectrometer (extraction field «* 130 V/cm, acceleration field » 400 V/cm). The most serious limitation of this setup was the mass analysis of the ions by a time-of-flight drift tube which was only 30 cm long. This resulted in a mass resolution of only 100 200. This configuration was used forrecordingthe spectrum shown in fig. 1.1. Stage II. In this stage, a reflectron system was added to the existing time-of-flight source region. Its design and construction has been described in detail [93] and will not be repeated here. In addition, a two-cylinder ion lens just after the extraction region was added for spatially focusing the ions, two pairs of deflection plates were installed for steering the ion beam, and the existing detector was replaced by a microchannel plate array detector (Galileo Electro-Optics, FID 2001 in a chevron configuration). Although intended for a mass resolution of over 1000, this instrument only performed up to these specifications for the ionization of gas-phase species. When desorbed particles were ionized, the typical resolution was « 550. This stage of the experimental system was used for the work described in chapter 3.2.

Stage III marks a major upgrade in laser equipment: a new Nd:YAG laser, complete with harmonic generator was purchased (Spectra-Physics, DCR-11). Although with less power than the DCR-2, its output energy of up to 40 mJ (at 266 run, 10 Hz

24

repetition rate, 10 ns pulse width) was still more than enough for ionizing aromatic molecules by REMPI without fragmentation. However, stable operation of the laser can only be achieved near maximum flashlamp power and optimum Q-switch delay. An alternative way of varying its power was therefore found: instead of aperturing or Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

defocusing the beam, a rotatable polarizer was mounted in the laser beam path. This allowed smooth adjustment of the UV power without affecting the stability of the laser, its beam profile, or the size of the focus. Soon after the purchase of the new YAG laser, a delay generator (Stanford Research Systems, DG-535) was added to the experiment (see chapter 1 B). Results presented in chapter 3.1 were obtained during this stage of the development Stage IV. Many small improvement in the ion optics of the instrument led to a very stable and reproducible operation of the reflection with a mass resolution of » 600. The changes include making the deceleration voltage (middle grid) of the reflectron adjustable by adding an external voltage divider, improving the flatness of the first reflection grid by clamping it with two metal plates that only expose the grid in places where the ions pass through it, and redesigning the entire source region. Ion trajectory simulations and a computer program which optimized the resolution for different geometries aided the new design of the ion source. The results can be summarized as follows: (i) The smallest dispersion in flight time is obtained for a geometry where the ionization takes place exactly in the middle between the repeller and the extractor plates. In order to ionize close to the surface (- 2 mm), the distance between repeller and extractor plate was chosen to be only 4 mm. (ii) In order to obtain a real focusing of the ion beam, a true Einzel lens with small gaps between the lens elements must be

25

used, with negative bias voltages on the middle element. The "two cylinder lens" described in stage II was therefore replaced with a design where the first cylinder was separated from the grounded last plate of the Wiley-McLaren source configuration. This is exactly the geometry shown in fig 1.2. Even if the resolution did not improve as much Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

as we hoped, this modified source andreflectronconfiguration allowed for a much more stable and reproducible operation of the apparatus. The overall sensitivity of the instrument also improved. Another new feature in the ionization laser optics is evident in fig. 1.2: the focusing element has been changed to a cylindrical lens so that a ribbon of UV radiation, rather than a line, is formed in front of the sample. The focal lenght of the lens was 25 cm, and the beam waist in the focus was measured to be 240 urn. Additionally, a micrometer-actuated holder was built for the final turning prisms in the YAG laser beam path. All these changes helped to render the operation of the system more reproducible, more reliable, and more sensitive. At the same time, a computerized data storage and manipulation program, "MASSACRE", written by Brian Hintzman, was installed. The results described in chapter 3.3 have been obtained in the configuration of stage IV. Stage V. This stage in the development consists of a whole new ultra-high vacuum (UHV) chamber that was constructed for the purpose of studying the desorption mechanism. Purchasing a new C0 2 laser (Alltech, model Allmark-853) was required, because the stability andreliabilityof the desorption laser was fundamentally important to these experiments. This laser has pulse energies of up to 2.2 J, a pulse duration of » 120 ns (with 2-3 us tail), and is capable of repetition rates up to 25 Hz. It is specially

26 designed for laser marking, and features a very good shot-to-shot stability (± 5%), a low beam divergence (3.5 mrad), and a spatial beam profile approaching a perfect "top hat." The remaining features of the setup of stage V, the time-of-flight mass spectrometer, the UHV system, and the liquid-nitrogen-cooled rotatable surface-holder Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

will be described in the experimental section of chapter 2.4.

B. TRIGGER SCHEME An accurate time delay between the desorption and ionization laser pulses is crucial for sensitive detection of surface adsorbates, and for precise measurement of their velocities. A variety of trigger schemes were used during the early stages of this work, all involving some electronic delay generators. The most accurate and convenient trigger scheme was implemented as early as stage in (see above), and is described here. It employs a digital multichannel delay generator (Stanford Research Systems, DG-535) as the master clock. This precise and versatile device allows triggering with both positive and negative pulses of adjustable amplitude and duration. The problem with delaying the optical output of the ionization laser with respect to the desorption laser pulse is the fact that most YAG lasers take several hundred microseconds between the trigger of the flashlamp and the optimum time for Q-switching the cavity. In contrast, C0 2 lasers have only an internal delay of a few microseconds with respect to the electronic trigger. If simultaneously triggered, the resulting delay between the two lasers would be far too long to ionize particles with velocities of 200 - 300 m/s a few millimeters above the surface. Therefore, the firing of the first (C02) laser must happen after the second (YAG) laser has been triggered.

27

Figure 1.4 schematically shows the triggering. Both laser pulses are detected with nanosecond time resolution using appropriate fast devices. At = 0 (no delay) is established by adjusting the value in one of the channels (channel A, see fig. 1.4) of the delay generator. This delay is then stored and the pyroelectric detector (PY) is removed Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

from the C0 2 laser beam. The C0 2 laser is triggered by the output of another channel, whose value was zero up to this point (Channel C in fig. 1.4). Only the delay of this channel is adjusted from now on; its display therefore directly indicates the actual delay between the two laser pulses. The signal from a fast photodiode (PD), which intercepts a reflection from the YAG laser beam, then serves to trigger the data acquisition in the digital storage oscilloscope. This causes less jitter than directly initiating the data acquisition from a third channel of the delay generator. The rather long (= 200 us) time interval between the flashlamp trigger of the YAG laser and its optical output affects the day-to-day stability of the laser delay (± 2 us). The jitter is small (± 0.1 us) once the lasers are warmed up. The present trigger scheme allows for easily reducing both of these instabilities by directly triggering the Q-switch of the YAG laser with another channel of the DG-535 delay generator. However, this was never attempted because an accuracy of 100 ns is better than needed for all experiments.

28


Trigger

Scheme BS1

=>

C 0 2 LASER Ch. C: t0+At

PY DG53S Ch.A: t o +202us

BS2 YAG LASER

■mzz.

;j:!'«m..m\» ) v*.m.'wfr

PD

data

check At-0 trigger LeCROY9400 OSCILLOSCOPE

Figure 1.4: Trigger scheme for laser systems and data acquisition. The beamsplitters (BS) are a sodium chloride plate (BSl) and a quartz plate (BS2); PY is a pyroelectric detector, and PD a fast photodiode.

29 C. SAMPLE PREPARATION Often, the method of sample preparation is crucial for obtaining good mass spectra with the Two-Step Laser method. Initial experiments were all performed on soluble


pure materials. The task here was to create a thin film of the sample on a suitable, IR absorbing substrate. For the purpose of quantitative analysis, homogeneity of this film is also very important. The simplest sample preparation method was to dissolve the sample, dispense a few microliters on the desired substrate, and let the solvent evaporate. A variety of materials have been used as substrates. Best results were obtained with glass and quartz plates, vials, and tubing, as well as MACOR (machinable ceramic) platters. Nitrocellulose membranes and TLC plates also yielded good spectra in certain cases. Plastic materials are also promising candidates as substrates for infrared laser desorption, but unfortunately, plasticizers were liberated by the desorption laser pulse. In addition, many plastic materials only absorb weakly at 10 |im wavelength. One attempt was made with gennanium as a substrate and a desorption laser wavelength of 1.06 jun (YAG fundamental) during the change from stage II to stage HI; this was not successful. Metals are not suited for these experiments because they tend to bind adsorbates more strongly, and exhibit a lower threshold for plasma formation than insulators. For the purpose of preparing homogeneous sample films, a spin evaporation technique using glass tubing or vials [92] was employed. Alternatively, it was possible to prepare quite homogeneous films on roughened substrate surfaces. Rough surfaces were obtained by either grinding the material with diamond tools, or by etching it with hydrofluoric acid. The most convenient shape of the substrates were 7 mm-diameter

30

platters with a cylindrical 5 mm-diameter recess. MACOR exhibits some porosity and resorbs sample solution into its pores. This can be very useful in cases where a sample will be used for a long time, for example while tuning the experiment. It is not useful for quantitative work where complete desorption in one laser shot is desired. Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

For experiments that involve low surface coverages (submonolayers) it isabsolutely imperative to clean the substrates carefully and regularly. In many cases, quite elaborate cleaning procedures had to be used. The steps involved wiping the substrates with wetted Q-tips, ultrasonicating them in various solvents, rinsing them with methanol and distilled water, and treating them with chromic acid. Before experiments, the chromic acid was rinsed away with distilled water and the substrates were dried at 360°C (normal glass) to 450°C (pyrex glass, quartz, and MACOR) in an oven. All substrates and samples were only manipulated with clean stainless steel instruments, never by hand. Fingerprints have been found to show intense signals if analyzed by L2MS. More difficult is the preparation of samples from insoluble material. These include insoluble chemicals, pulverized solid material, and macroscopic solid chunks of mixtures (for example rocks of geochemical interest) that are directly analyzed. From pure materials, only low-quality "films" could be produced by rubbing the sample onto a roughened substrate surface. A more successful sampling method was to prepare a paste or pill with a binder material. In this case, the desorption must liberate the analyte molecules from this paste or pill, and not from the substrate (if any) which will be effectively buried under the sample. As a binder, glycerol was found to be the most useful material. It does not interfere with the mass spectra, and has a high viscosity.


31

(a)

(b)

/T\ (d) Figure 1.5: Tools for sampling unexposed surfaces of geological material: (a) small rock splitter, built from a modified vise (1) for small samples (2); (b) crankhandle rock splitter for larger samples (2); (c) miniature lathe built from a handdrill (3) and an optical translation stage (4); (d) sample holder for machining very small samples with miniature lathe, machined from aluminum rod (5). A diamond tool (6) grinds small samples (7) which are glued to the aluminum holder (5). Different planes of cuts (8,9) to avoid contamination from the exterior surface of the sample are obtained by adjusting the position of the sample holder (5) in the jaws of the handdrill.

32

Pulverized material was well mixed with a drop of glycerol and pressed to a 7 mmdiameter pellet in a small stainless steel press that was squeezed in a vise. Solid material was investigated by cleaving samples and desorbing molecules from unexposed interior surfaces. For this purpose, two home-built "rock splitters"( shown Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

in fig. 1.5 a and b) were used. Splits of the samples were collected in aluminum foil and mounted on 7 mm diameter sample holders in a way such that the interior surface faced the desorption laser beam. A variety of glues was tested for sample mounting. Best results were obtained with "Crystalbond 904" (Armeco Products, Inc.), a thermoplastic polymer that showed no mass spectral interferences at all. A few chips of Crystalbond were softened on a heated substrate; the samples were squeezed into the thermoplast while it was cooling after removing it from the heating plate. This mounting technique proved to be very convenient, especially for small samples. The polymer was strong enough so that small samples could subsequently be machined by diamond tools (see fig. 1.5 c and d). In a few cases, pieces of filter paper soaked with Crystalbond were used for mounting very coarse powders.

33

CHAPTER 2: DESORPTION MECHANISM 2.1 Scope of Laser Desorption


Some of the central questions in experiments like two-step laser mass spectrometry concern the surprising fact that even large, nonvolatile, and thermally labile molecules can be desorbed from surfaces with little or no decomposition by using pulsed laser irradiation. Why and under which conditions does this happen? Which parameters determine the efficiency of the desorption process, the amount of decomposition, the translational energy distribution of the desorbed species, their angular distribution, and their vibrational/rotational excitation? What is the influence of the surface-adsorbate system, the laser pulse duration, shape, and energy, and the way the film of adsorbed molecules was prepared? Many of these questions are still unanswered. Before going into details about how to determine some of the characteristics of the desorption process, a literature review of laser desorption studies relevant to our experimental regime is presented.

2.1.1 Review of Previous Experimental Work The earliest laser desorption experiments had quite different motivations. On one hand, the mass spectrometry community was exploring novel ionization sources. Experiments by Vastola [51; 52], Kistemaker [53], and Hillenkamp [54; 55] using pulsed ruby or O0 2 lasers were aimed at producing parent ions or cationized species of polar, nonvolatile molecules in the source region of a mass analyzer in a one-step desorptionionization process. Rollgen [56] used a cw-C02 laser for the same purpose (see also section D of the introduction). On the other hand, the "snapshot" capabilities of short

34

laser pulses were becoming popular in the surface science community to study surface adsorbates and their reactions, as described in the landmark paper by Cowin et al [94]. In the late 60s and early 70s, desorption of neutral adsorbates had already been studied in relation to production of clean surfaces [95], thermal desorption [96; 97; 98; 99], and Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

photodesorption [100; 101]. In addition, some interesting studies of vaporizing solids have been conducted [102; 103]. The term "laser desorption" is used in the literature for a variety of experimental situations. They can be distinguished based on the • laser wavelength (IR-VIS-UV), the energy associated with the photons, and the possibility of inducing photochemical reactions in addition to the desorption process; • laser pulse duration, ranging from sub-picosecond to continuous-wave; • laser power density, with effects ranging from submonolayer desorption of adsorbates to the formation of dense plasmas on the surface; • nature of the substrate (metal-semiconductor-insulator), and its thermal properties and optical characteristics at the wavelength of the laser light; • adsorbatefilm thickness; and • surface-adsorbate interaction (binding energy; chemisorbed vs. physisorbed surface species).

A. SMALL MOLECULES ON METAL SURFACES Much work has been devoted to studying laser desorption of small molecules from metallic surfaces [104; 105]. In the majority of these experiments, laser wavelengths have been used which are not in resonance with any particular eigenstate of the adsorbed molecular systems. The laser irradiation therefore causes rapid heating of the metal

35

substrate which leads to thermal desorption of the adsorbate. The calculation of the surface temperature rise is quite straightforward in this case and involves the solution of the one-dimensional heat transport equation (see chapter 2.2). In principle, the heat capacity c and the thermal conductivity k are temperature dependent For metals Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

however, this dependence is only weak, and of opposite sign for c and k, so that it is a good approximation to treat these parameters as temperature independent. Several authors have given solutions to this problem for a variety of laser pulse shapes [94; 97; [106; 107; 108; 109; 110; 111]. The systems studied include: D2 and Hj on polycrystalhne W [94]; D2 on Pd (100), Hj on Ni (111, 100 and 110) [99]; Hj, CO and methanol on Ni (100) [95]; CO on Pd (111) [98]; CO onFe(llO) [ill]; CO on Cu (100) [112; 113]; Xe, Ar, 0 2 , and CO on polycrystalline Cu [114]; and Hjand D2on Ru (001) [115; 116; 117]. Note that in the majority of these cases, there is a chemisorption interaction between the surface and the adsorbate. The lasers used in theses studies, Nd:glass, NdrYAG, ruby, and excimer la sers, have pulse widths in the nanosecond range. In only a few casestime-of-flightdis tributions of the desorbing molecules were measured. Discrepancies between the maxi mum surface temperature and the kinetic energy of the desorbing species were generally observed. Burgess et al. [112] excluded the possibility of collisions at low desorption fluxes and attributed their observations to incomplete equilibration of the CO molecule with the Cu surface. Wedler and Ruhmann [111] used a Maxwell-Boltzmann distribution to fit their experimental time-of-flight data, but found that the fit was not satisfactory at low velocities. Cowin et al. [94] observed non-Boltzmann behavior for desorption of substantial fractions of a monolayer. They attributed the narrow velocity distributions and peaked angular distributions to near-surface collisions of the desorbed species.

36 Another favorite molecule in desorption experiments has been nitric oxide (NO). In detailed, state resolved spectroscopic studies of NO desorbing from Ru (001), Pt (111), and platinum foil by King and coworkers the existence of at least two components in the NO velocity distribution has been discovered [118; 119]: A "fast" component with Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

moderate internal excitation, and a "slow" component with a "colder" internal energy distribution. The kinetic energy of the slow component shows no dependence on the rotational quantum states, and corresponds to the calculated maximum surface temperature. The rotational state populations of this slow component are well described by a single Boltzmann distribution. In contrast, the translational energy of the fast component increases with J, but shows no dependence on the maximum surface temperature. These fast NO molecules are rotationally excited, have non-Boltzmann rotational population distributions, a significant vibrational excitation, and a population inversion of the spin-orbit states. However, all these observations were only made if visible (X = 532) light was used for desorption, not with infrared light (YAG fundamental, 1.06um). Similar experiments have been carried out by other groups [120; [121; 122; 123; 124]. In general these experimental findings were attributed to a "hot electron" mechanism where optically excited electrons of the metal substrate scatter into an unoccupied valence state of the adsorbate [122; 125; 126]. Therefore, nonthermal processes must be considered when elucidating laser desorption mechanisms from metal surfaces. Hall and coworkers have studied laser desorption of ethylene and methanol on Ni surfaces [108; 127; 128; 129] with a different goal in mind: in an experimental procedure called "temperature-programmed pulsed-laser-induced desorption", laser-desorbed species are monitored while the surface temperature is held constant or ramped slowly.

37

These studies yield real-time information about the dynamics of a number of adsorption and desorption processes, surface reactions, and the kinetics of surface diffusion. Surface diffusion has also been studied in detail by other groups [115; 116; 117; 130; 131]. Laser desorption of small molecules from metal surfaces is an important starting Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

point for our later discussion, because many parameters, such as surface coverages, binding geometries, and binding energies are well defined. Moreover, the kinetic and internal energy distributions of small desorbed molecules can be measured with great detail. Compared to the case of large molecules interacting with dielectric surfaces, the binding energies to metals are stronger, and the photodesorption process is further complicated by electron-hole pair formation in the metal substrate.

B. DESORPTION FROM THICK CRYOGENIC FILMS A different regime of laser desorption has been exploited in experiments where molecules are desorbed from thick cryogenic films. In most of these studies, pulsed ultraviolet (YAG or excimer) or C0 2 lasers were used. The systems studied include CH3Br [132; 133], CH3I [134], NO [135; 136; 137], Cl2 [138], methanol [139], trans-1,2dichlorocyclohexane [140; 141; 142], and benzene [143]. Photochemical effects can play an important role in the desorption from thick condensed films. Cowin and coworkers [132] observed photolysis fragments from the adsorbed CH3Br. Their measured fragment velocity distributions were much broader than the analogous ones for gas-phase photodissociation, with significant amounts of low-velocity fragments. They interpreted their results as a one-photon photodissociation process with collisions just above the surface being responsible for slowing down the

38

desorbing fragments. Thermal desorption was thought to play a minor role. Similar observations were made by Feldmann and coworkers [134]. They could distinguish between a "direct" photodissociation mechanism and an "explosive" mechanism above a critical laser power density. Two different groups have reported UV photodesorption Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

of NO from condensed films. Leone and coworkers desorbed about 2-1015 molecules/pulse so that their velocity and rotational population distributions are strongly affected by collisions [136]. Two components, a slow and a fast one were identified in the velocity distributions. The relative contribution and average energy changed with film thickness and laser fluence. Average rotational temperatures were low, for most cases below 200K [136; 137]; this is in agreement with the idea of the desorption resembling a molecular beam expansion in this case. In contrast, Natzle et al. [135] used a much lower laser power density so that collisions above the surface were avoided. They also observed a slow, Maxwellian, and a fast, non-Maxwellian contribution in their velocity distributions, but the rotational temperatures for the fast peak was much higher, TR *» 2500 K (for v = 2). The two velocity components also had different angular distributions, near cos9 for the slow component, and about cos4© for the fast one. The slow peak was interpreted as originating from laser-induced thermal desorption. Several mechanisms for the production of the fast peak were suggested [135]: photodissociation of NO dimers, desorption of vibrationally excited NO monomers, lattice relaxation, and, at lower coverages, excitbn migration. In these studies with NO, no laser-induced dissociation of the molecule was observed. Laser desorption from cryogenic films has also been suggested for the production of fast pulsed molecular beams [136; 138].

39 Photochemical reactions can be avoided by using infrared radiation for desorption. Hess and coworkers have studied IR photodesorption from cryogenic films of methanol [139], trans-1,2-dichlorocyclohexane [140; 141; 142], and benzene [143]. The mechanism for energy deposition is resonant excitation of internal vibrational modes of the adsorbate Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

molecule (see also section F). Time-of-flight distributions of the desorbed molecules were measured as a function of laser wavelength and fluence. Molecular beam-like distributions were found for desorption from thick layers, but a clear transition to Maxwellian behavior for submonolayer desorption yields was seen. Absolute desorption yields were not determined, and the possible influence of substrate heating was neglected, even at very low surface coverages. Desorption from cryogenic layers mimics the desorption of thick films of large organic molecules. In this case, there is mainly a weak van-der-Waals interaction between the molecules in the film, and it can be expected that at least the "slow", thermal component mentioned above is relevant to this experimental situation.

C. MATRIX-ASSISTED DESORPTION Matrix-assisted desorption is a one-step methodology where desorption and ion formation happen simultaneously during laser irradiation of the sample. It is generally carried out by using a small concentration of analyte molecules ("guest") in a large excess of matrix ("host"). This method has first been described by Hillenkamp and coworkers [64 - 66], and has recently been successfully reproduced and extended by Chait and coworkers [67 - 69] (see also Introduction chapter). Almost all of these studies used frequency quadrupled YAG lasers (266 nm wavelength) with short (=10 ns) pulse

40

widths. The most widely used matrix is nicotinic acid, although other substances have been shown to be at least as effective [67]. A slight variation of this method used finely dispersed metal powder mixed into the matrix for absorbing the laser light, at a wavelength of 337 nm in this case [70]. A recent study reporting laser desorption of Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

large DNA fragments from frozen aqueous solutions is another example of matrix-assisted desorption [144]. As proposed by Vertes and coworkers [145; 146], the mechanism of matrix-assisted desorption is a competition between the rate of sublimation of the guest molecules and the rate of energy transfer to the guest molecules. According to the theory, the optimal conditions for a successful experiment are a low heat of sublimation of the host molecules, an inefficient vibrational energy transfer between the host and the guest molecules, subcritical concentration of the guest molecules, a high laser irradiance in a short time, and a cold initial sample temperature. The study also predicts that thermally degraded analyte molecules will exist in the matrix after a desorption experiment. Thermal degradation products have indeed been identified by both laser ionization mass spectrometry and IR spectroscopy in an experiment where small peptides were laser-desorbed from a thick film [147]. A similar conclusion can be drawn from a study where pyrolysis of the analyte was suppressed by adding various sugars to the sample slurry

as

pyrolysis

"quenchers"

[148; 149].

The

theory

of

matrix-assisted

desorption/ionization does not make any statements about the mechanism of ion formation. Matrix-assisted desorption is similar to our experimental situation in that thermally labile, high-molecular weight compounds are studied. Remaining solvent molecules or surface contaminants which escape the REMPI ionization may play the role of the

41

matrix that serves to lift off the heavy species (this is sometimes referred to as a "hovercraft" mechanism). However, it is difficult to estimate the amount of these small matrix molecules, and so we do not know if there are enough of them for a true


matrix-assisted desorption.

D. LASER ABLATION It is worth mentioning the somewhat related area of laser ablation, sometimes also termed "explosive desorption". Here, pulsed lasers are used to ablate between 0.1 (xm and several microns of material per laser shot from metal, semiconductor, polymer, or even biological [150] material surfaces. Laser ablation of polymers has received much attention recently because of the possibility to pattern them for applications in microelectronics [151]. In virtually all of the work in this field, UV and VUV lasers were used with fluences of up to 10 J/cm2, and short pulse widths, ranging from nanoseconds down to 160 fs [152]. Strong absorption of the light wavelength by the ablated material seems to be a prerequisite. Some of the most striking observations were the absence of thermal damage to the substrate, and that very clean crater rims were formed. Together with other experimental evidence such as narrow angular distributions of the ablated material, this laid the basis for the formulation of a photochemical model to explain the process for polymers [153]. The model suggests that the UV photons excite each monomer unit directly from an attractive to a repulsive potential energy surface. This would cause chemical decomposition in the polymer and a change in the volume occupied by the monomers, and eventually lead to the explosive ablation of the sample. When different wavelengths or polymers are used so that no optical absorption occurs,

42

ablation can still be observed, but in an irreproducible manner, and with accompanying thermal damage [151]; thermal effects are thought to be responsible for the ablation process in this case. A thermal mechanism for ablating polymers has also been observed in systems where a nonabsorbing polymer was either doped with light-absorbing Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

molecules [154] or deposited on an absorbing substrate [155]. In the latter study, the formation and expansion of a gas layer could be detected at the substrate-polymer interface by using time-resolved reflectivity measurements. UV laser ablation of biological tissue has great potential for laser microsurgery [150; 153]. Laser ablation is mentioned here more for the sake of completeness, not so much for direct comparison with the situation in laser-induced thermal desorption. In fact, the laser fluences used in laser ablation experiment would probably cause damage of most substrates used in our experiments.

E. LARGE MOLECULES ON DIELECTRIC SUBSTRATES Experimental and theoretical investigations of laser-induced thermal desorption of large organic molecules from dielectric substrates are directly related to the experimental regime of interest to us, but unfortunately, are scarce. A few large organic molecules have been studied by Letokhov and coworkers [28; 87; 156]. For the most part, however, these studies involved the detection of ions obtained by direct sputtering from molecular crystals by excimer or YAG laser irradiation. In one case, a crude velocity distribution measurement of sputtered adenine has been reported [87]. Five data points with very large error bars were fitted to a Maxwell-Boltzmann distribution with a temperature of 1250°C, in contrast to an estimated maximum surface temperature of

43

£350°C. This was interpreted with a nonthermal sputtering mechanism. Another study focused on the ultrasensitive detection of naphthalene and anthracene on graphite surfaces by C0 2 laser desorption and UV laser post-ionization [28]. An adsorption energy of 0.46 eV was reported for naphthalene on graphite, and the range of desorption Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

energy densities before thermal decomposition occurs was given for both molecules. Velocity distributions for laser desorbed neutral molecules from films of tryptophan, adenine, and tocophenol have been reported for 248 and 193 nm desorption laser wavelengths [157]. A strong dependence on the desorption wavelength has been found, with fitted translational temperatures ranging from 300 K for 248 nm to 1500 K for 193 nm. Experiments in our laboratory have focused on the intact desorption of thermally labile species such as PTH-amino acids [34] from glass substrates by a pulsed C0 2 laser. Velocity distributions were also measured and gave essentially thermal distributions with temperatures only slightly above room temperature. However, in both of these studies the exact nature of the films was unknown and the desorption yields were poorly determined. Nevertheless, of importance to us is that in these cases the interaction between the sample and the substrate (or the sample and itself) was physisorption. Laser-induced thermal desorption of physisorbed molecules has also been studied theoretically by Tully [158] and Zeiri [159]. These studies used stochastic trajectory calculations for desorbing diatomics, and heating rates up to 1015 K/s. The energy partitioning into translation, vibration , and rotation of the desorbing molecules, as well as their angular spread were determined. One of the important conclusions is that desorption of molecules with translational temperatures colder than the surface temperature will be a common feature in cases where adsorption proceeds without a

44

barrier. Other findings include a strong dependence of the amount of vibrational energy in the desorbing molecule on its oscillator frequency [158], and pronounced effects of the adsorption geometry and the surface roughness on the desorption kinetics [159].


F. RESONANT DESORPTION The possibility of desorption by resonant excitation of molecular adsorbates on surfaces has been studied extensively and is covered by a number of excellent reviews [160; 161]. One of the original driving forces in this work was to try to realize the intriguing possibility of molecule-selective desorption. However, recent work by Chuang and coworkers suggests that efficient molecule-selective desorption will be difficult to achieve, at least on a metal substrate. Desorption does occur following resonant excitation of vibrational modes of adsorbates. However, the excitation seems to rapidly decay into heat, leading to thermal desorption rather than causing specific desorption of the originally excited species. For example, no specific isotope selectivity has been found in laser stimulated desorption of mixtures of NH3 and ND3 from metal substrates, where infrared photons selectively excited the N-H stretching vibration [162]. Similarly, the lack of isotopic selectivity has been observed when carbon dioxide isotope mixtures were desorbed from NaCl films with tunable IR laser radiation [163]. It is clear that the energy deposition is by resonant vibrational excitation and not by directly heating the substrate by the laser light, as desorption studies of CH3F [164; 165], pyridine [166], ammonia [167], and a number of other molecular systems from infrared-transparent NaCl and KC1 substrates showed. But this fact does not contain any deeper significance for the mechanism of resonant IR desorption.

45

The reason to mention these studies is that heavy organic molecules have a large number of vibrational modes and, by chance, one of them can very well be in resonance with the desorption laser wavelength. This would probably cause more efficient desorption. Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

Desorption can also be induced by populating electronically excited states of surface adsorbates. In a recent experiment, electronic, thermal, and "explosive" (=ablation) desorption have been observed in the same system [168]. Another study followed the real-time desorption dynamics of a monolayer of rhodamine B on fused silica substrate with second harmonic generation [169], using 6 picosecond light pulses at 560 nm to induce desorption, a wavelength that is resonant with the S0 —» S, transition of this molecule.

46

2.1.2. Models of Laser-Induced Thermal Desorption A number of ideas and theories have been put forward in the last decade to account for the experimental observations described in the previous section. They can be divided


roughly into a resonant desorption model, a shock-wave model, a thermal equilibrium model, a non-equilibrium model, and a collisional regime model. A short account of the main properties of these different ideas will be given, with special emphasis on the case of thermal desorption of large organic molecules from opaque, nonconductive substrates.

A. RESONANT DESORPTION MODEL For resonant desorption with infrared light, a number of different theoretical approaches have been developed [170; 171; 172]. The field has recendybeen reviewed extensively by Piercy et al. [173]. The most widely used at present is a generalized master equation approach. A quantum mechanical model for the adsorption system is used, and the laser light is resonantly coupled into a vibrational mode of the surface adsorbate. A Hamiltonian is set up for a low coverage, homogeneous adsorbate of physisorbed molecules with both translational and internal vibrational degrees of freedom. The adsorbate is allowed to interact with both the laser radiation and the phonon bath of the solid. A quantum statistical approach is then used to calculate the probabilities of transitions induced by these interactions. Based on these transition probabilities, desorption rates as a function of temperature and laser intensity are computed. All of the experimentally studied systems mentioned in the previous paragraph

47

have received much attention from theoreticians. For the most part, these theoretical studies tried to calculate the dependence of the description yields on the incident laser power density, and the photodesorption line shape if tunable laser radiation was used for desorption. The theory is quite sophisticated and acknowledges the importance of many Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

processes, such as lateral vibrational coupling of the adsorbates [174; 175], phononassisted tunneling between bound vibrational states, and coherent multiphoton processes. A salient question is whether the desorption takes place with isotopic selectivity, or nonselectively, by "resonant heating" of the surface [162; 167; 176]. Isotope selectivity has not been observed so far, but is predicted to be possible when low coverages, low initial surface temperatures, and dielectric substrates are used [173]. Most of the systems modeled to date consider substrates that are transparent for the incident laser light, and quite substantial laser power densities (105 - 106 W/cm2). Jedrzejek has even suggested the possibility of resonant desorption followed by multiphoton excitation of a strongly anharmonic surface-adsorbate bond [177]. Exceedingly high laser power densities (more than 1010 W/cm2) are predicted to be required for this mechanism to occur [178]. This led to the surmise that surface roughness could enhance the electric fields at the surface. Even though large organic molecules have enough vibrational modes such that by coincidence one of them could be in resonance with the wavelength of the desorption laser, the role of the temperature jump induced on an opaque substrate surface is by far more important for desorption at such high laser power densities. Moreover, for opaque substrates, the onset of plasma formation upon pulsed laser irradiation also lies in the range of 105 -10 6 W/cm2. Even though it has not been tested experimentally, a resonant mechanism will therefore contribute negligibly to desorption from opaque substrates.

48

Lin and coworkers have developed a theory for laser desorption in the case of electronic excitation of the adsorbate [179]. The theory is also based on a master equation approach and can be applied to a variety of possible scenarios leading to desorption. However, as pointed out by the authors, UV resonant desorption has only Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

received little attention from experimentalists compared to IR resonant desorption.

B. SHOCK-WAVE MODEL In the shock-wave model, rapid thermal expansion is thought to cause desorption of the molecules by momentum transfer. The velocity reached by the top surface atoms can be estimated by using the linear thermal expansion coefficient of the substrate material, y, some characteristic time for the duration of the laser pulse, x, and the energy penetration depth d = 1/a for the thickness of the heated slab, where a is the optical absorption coefficient of the material. This is similar to how the magnitude of the surface buckling is estimated in thermal deflection spectroscopy for the time-resolved measurement of temperature transients on surfaces [180; 181]. The change in thickness, Ad of the heated layer (thickness d) is given by

Ad = y d A T

(2.1)

The velocity of the surface atoms can then be estimated by

M

1

AT

v=— =y

x

,,,x (2.2)

ax

Here, the last factor represents the rate at which the surface is being heated. For typical

49 experimental conditions, this velocity is estimated to be about 0.02 m/s (taking y= 5-10"7 K 1 , a = 2.7-103 cm 1 , hence d = 3.7 nm, and a heating rate of = 1010 Ksec 1 ). This velocity is much too small compared with experimental values. We thus conclude that a shock wave mechanism is not important in the regime of our experimental Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

parameters. However, for much higher heating rates, larger penetration depths, and larger thermal expansion coefficients, this mechanism may play a role. Desorption caused by a gas-phase pressure wave ("blastwave") has already been considered in section 2.1.1 C, "matrix-assisted desorption", as the so-called "hovercraft" mechanism. An optoacoustic shock has been hypothesized to be responsible for the photodesorption of CH3Br [182], HjS [183], HjO, HBr, and Xe [184] from LiF substrates using excimer laser radiation at 193, 222, 248, and 308 nm wavelength. LiF is transparent at these wavelengths, but color-center defects were proposed as the chromophores that absorbed the laser radiation.

C. THERMAL EQUILIBRIUM MODEL The equilibrium model is based on the fact that for the same molecule adsorbed on a surface, the rates of desorption and decomposition show a different temperature dependence. In an equilibrated surface-adsorbate system, decomposition and desorption are therefore competing channels for energy disposal according to kdes = v d e $ e x p { ^ |

(2.3)

kdec = v d « e x p | ^ }

(2.4)

50

where k are the rates, v are preexponentials, and E are activation energies; the indexes "des" stands for desorption, and "dec" stands for decomposition. The simplest case with unimolecular rates has been assumed, although in practice the situation might be more complicated. Depending on the preexponentials and the activation energies, one or the Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

other rate can be dominating at a given temperature. This is shown for a range of realistic values for v and E in Fig. 2.1. For example, for methanol on Ni (100), the following parameters have been determined [108]: v,^ = 1 • 1014sec"\

E ^ = 14kcal / mol = 59kJ / mol

vaec = 2- 10 9 sec"\

Eje,. = 9.0kcal / mol = 38kJ / mol

Curves (a) and (b) in Fig. 2.1 are plotted for these values. It can be seen that around 250 K, the rate for desorption starts to overtake the decomposition rate. If the surface is heated above this temperature in a time that is short compared to the decomposition time at lower temperatures, most of the methanol will desorb, because the desorption rate at higher temperatures is substantially greater than the rate of decomposition. For surface-adsorbate systems with other kinetic parameters than the ones for methanol/Ni (100), the crossover can happen at a different temperature (Fig. 2.1 d), or may not happen at all (Fig. 2.1 c). The competition between different channels for methanol on Ni (100) can also be calculated as a function of rate at which the surface is heated. For conventional heating rates, there is little dependence of the relative yields for the different channels, but for heating rates in excess of 106 K/s, desorption becomes dominant over decomposition.


51

100

I 200

1

r

300 400 Temperature (K)

r

1 600

500

Figure 2.1: Calculated reaction rates as a function of temperature for (a) v = 1 • 1014 sec"1 and Ea = 14 kcal/mol (= 0.6 eV); (b) v = 2-109 sec 1 and Ea = 9 kcal/mol (= 0.4 eV); (c) v = 1 • 1014 sec ' and Ea = 9 kcal/mol (= 0.4 eV); and (d) v = MO12 sec"1 and Ea = 14 kcal/mol (= 0.6 eV). The parameters ofcurves (a) and (b) correspond to the rates for desorption and decomposition, respectively, of methanol on Ni (100).

52 A thermal equilibrium model has also been successfully used by George and coworkers to model the signal increase in LITD signal intensity as the temperature of a hydrogen-covered silicon surface is slowly raised [185]. Using a Gaussian laser pulse profile, they argue that the greater the initial temperature is, the greater an area will be Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

heated to a temperature where substantial hydrogen desorption occurs. For physisorbed molecules, similar calculations have not been performed yet.

D. NON-EQUILIBRIUM MODEL In the non-equilibrium model, it is suggested that inefficient energy transfer between the weak, low-frequency

surface-adsorbate bond, and the stronger,

high-frequency internal modes of the adsorbed molecule creates a bottleneck for energy flow [186], and under certain circumstances, an adsorbed molecule will desorb before its internal modes are equilibrated with the surface temperature. In this case, other weak bonds in the adsorbed molecule will not dissociate, and it will desorb "lukewarm", i.e., with litde internal excitation. An adiabaticity parameter, ^, is used to describe the efficiency of the energy transfer. This concept is borrowed from gas-phase intermolecular energy transfer [187]. For example, the adiabaticity parameter for kinetic-to-vibrational energy transfer during a gas-phase collision is defined as

*

t,.„i t vib

f< 1 "sudden" \ > 1 "adiabatic"

v

' '

where t^j and t ^ are the characteristic times for the colbsion, and for the vibrational

53

period, respectively. In the case of fast collision and a relatively weak bond, t^, < t ^ , the energy transfer will be efficient (high-velocity or "sudden" limit), whereas for l

coi > 'vib' * e energy transfer will be quite inefficient (low-velocity, or "adiabatic

limit"). For a surface-adsorbate system, the energy flow between the low-frequency Advances in Multi-Photon Processes and Spectroscopy Downloaded from www.worldscientific.com by STANFORD UNIVERSITY on 02/04/16. For personal use only.

surface-adsorbate bond and some high-frequency internal mode of the adsorbed molecule is considered. Here the adiabaticity parameter can be written as

$ = 2 n - - = 2«- — t v

(2.6)

where t and t' are the periods of the vibrations in the surface and internal bond, respectively, and v and v' are their corresponding vibrational frequencies. A rate equation for energy flow into the physisorption bond is set up:

£.*M-mt) dt

(2.7)

dt

The first term on the right-hand side is the rate of energy gained by the physisorption bond if there were no flow to the rest of the molecule, and the second term on the right-hand side is the loss term due to energy flow into the molecule. The relevant rate constant is given by k = v exp(-£) [186]. If we set EQ = D in (2.7) (D is the dissociation energy of the surface-adsorbate bond), the equation can be integrated. This approximation actually represents an overestimation of the energy flow into the molecule. We find

54 E(t) = E„(t)-ktD

(2.8)

and if we evaluate this equation at time x, when the molecule desorbs (E 0 (x) = D ), we obtain


E(x) = D-kxD = D(l-kx)

(2.9)

A simple operational criterion for the desorption of cold molecules is kx < 1 [186], and with k = v exp(-£) from above, we find vx