Using JCP format - Department of Chemistry - University of Utah

JOURNAL OF CHEMICAL PHYSICS

VOLUME 114, NUMBER 15

15 APRIL 2001

Multiphoton ionization and photoelectron spectroscopy of 1,3-trans-butadiene via its 3 d ␲ Rydberg state Jianbo Liu and Scott L. Andersona) Department of Chemistry, University of Utah, 315 S. 1400E. RM Dock, Salt Lake City, Utah 84112-0850

共Received 19 December 2000; accepted 29 January 2001兲 Resonance-enhanced multiphoton ionization 共REMPI兲 and photoelectron spectroscopy 共PES兲, have been used to study the 1 A g (3d ␲ ) Rydberg state of 1,3-trans-butadiene in the two photon energy range from 61 000 to 66 400 cm⫺1. The 1 A g (3d ␲ ) spectrum is dominated by the ␯ 4⬘ , ␯ 6⬘ , and ␯ ⬘9 vibrational modes, with some excitation of the ␯ 8⬘ mode, as well. Photoelectron spectroscopy shows that the dominant ionization pathways are diagonal, i.e., they produce cations in the same vibrational level that was populated in the Rydberg state. Weaker off-diagonal ionization is also observed, with ⫹ ⫹ excitation of the ␯ ⫹ 4 , ␯ 6 , and ␯ 9 modes. The relative intensities of diagonal and off-diagonal PES bands are observed to be strongly dependent on the angle between the laser polarization and the detection axis. It is possible to use REMPI to generate state-selected cations, however, the nascent ions are quite efficiently photodissociated by the REMPI laser. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1356736兴

I. INTRODUCTION

4s Rydberg state, because either s or d final states are twophoton allowed.29 Subsequently, Taylor et al. reinvestigated this state using 2⫹1 REMPI with both linear and circularly polarized light,30 concluding that this state is actually a 3d ␲ Rydberg state. The present paper reports a REMPI and photoelectron spectroscopy study of this Rydberg state. We have obtained the 2⫹1 REMPI spectrum of 1,3-trans-C4H6 in the two photon energy range from 61 000 to 66 400 cm⫺1. The spectrum covers the ␯ 4⬘ ⫽0 – 3, ␯ ⬘6 ⫽0 – 3, ␯ 8⬘ ⫽0 – 1, and ␯ 9⬘ ⫽0 – 2, energy range, and reveals a number of new vibronic bands. The photoelectron spectrum has been measured following ionization through each band, providing a probe of the nature of these intermediate levels. In addition, mass spectroscopy has been employed to study photofragmentation of the nascent ions.

Resonance-enhanced multiphoton ionization 共REMPI兲 and photoelectron spectroscopy 共PES兲 have proven to be useful techniques for the study of molecular photophysics and photochemistry. In the REMPI process, the neutral precursor is pumped with one or more photons to a particular vibrational level of an intermediate electronic state, and subsequently ionized by a single additional photon. By using different numbers of photons to reach the intermediate state, many states that are forbidden in single photon spectroscopy are accessible, and PES can be a useful probe of the nature of these states.1,2 In the case where the intermediate state is a ‘‘good’’ Rydberg state where the Franck–Condon factors for ionization of the Rydberg state are diagonal 共⌬␯⫽0兲, the ion is formed predominantly in the same vibrational level that is selected in the intermediate state. In this manner, REMPI can be a powerful method for vibrational state-selected ion formation.3 The 1,3-trans-butadiene molecule is a good example. As the simplest of the chemically important polyenes, its electronic spectra have been the subject of numerous experimental studies, including ultraviolet 共UV兲 absorption spectroscopy,4–9 infrared 共IR兲, and Raman,10–14 electron impact spectroscopy,15–21 photoelectron spectroscopy,22,23 single photon ionization,24,25 and multiphoton 26–30 ionization. Extensive semiempirical and ab initio calculations have also been carried out to probe the electronic structure of butadiene, and to serve as a model for large, more complex systems.31–44 From the absorption spectrum, McDiarmid first identified a new Rydberg state, tentatively assigning its origin at 7.482 eV 共60 343 cm⫺1兲.6 Subsequently, Mallard et al. observed this state in the 2⫹1 REMPI spectrum with origin at 61 436 cm⫺1, and assigned it as the

II. EXPERIMENT

The experiments were performed in a homemade photoelectron/photoion time-of-flight spectrometer. The laser beam was generated by Continuum Nd:YAG-pumped dye laser 共NY82S/ND6000兲 operating with the exciton DCM dye, with fundamental wavelength from 600 to 656 nm. The dye laser output was frequency doubled by an Inrad Autotracker, producing average UV pulse energies of 5–7 mJ. The laser beam was focused with a 15 cm focal length lens into the center of the ionization region. 1,3-trans-butadiene 共Scott, 99%兲, seeded 1:10 in He, was expanded into the source chamber through a pulsed nozzle operating at 10 Hz, collimated by a skimmer, and then introduced to the ionization region in which the molecular beam is crossed by the laser beam. The experimental apparatus and procedures for multiphoton ionization spectroscopy measurement 共i.e., signal versus wavelength兲 have been described in detail previously.45,46 Briefly, the photoelectrons were produced in

a兲

Electronic mail: [email protected]

0021-9606/2001/114(15)/6618/7/$18.00

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© 2001 American Institute of Physics

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Spectroscopy of 1,3-trans-butadiene

J. Chem. Phys., Vol. 114, No. 15, 15 April 2001

an ionization region formed by a pair of planar grids separated by ⬃1 cm. The upper grid was grounded and the lower grid was set at a negative dc potential 共⫺5 V兲 to maximize collection efficiency of the electrons. After acceleration the electrons enter a 0.75-m-long magnetically shielded, fieldfree flight tube ending in an electron multiplier. By adding a third grid to the ionization region, this spectrometer can be changed to a Wiley–McLaren mass spectrometer by which the multiphonon ionization 共MPI兲 time-of-flight 共TOF兲 mass spectrum can be measured. At the exit of the drift tube, both electron and ion signals were detected by a dual microchannel plate, amplified, collected by a Tektronix 500 M digital oscilloscope, and transferred to a PC for analysis. The wavelength step size used in the REMPI scanning was 0.02 nm and the counting time for each step was 5 s. No attempt was made to normalize the spectra for variations in laser intensity 共⫾20%兲. Photoelectron kinetic energy spectra were also measured with the same instrument. The only difference was that no acceleration fields were used, so as not to degrade the electron energy resolution. The collection efficiency (⬃10⫺4 ) is determined by the solid angle subtended by the detector. For each PES, the laser intensity was kept as low as possible to avoid peak broadening due to space charge effects. Spectra were corrected for background by subtracting TOF spectra obtained with the molecular beam on and off, each signal averaged for 10 000 laser shots. In order to examine the angular dependence of the PES bands, the direction of the laser polarization vector was rotated by passage through a double Fresnel rhomb. The energy calibration of the TOF PES was carried out by using 1⫹1 MPI PES of iron atoms, as described previously.47 Fe atoms were produced by photolysis of Fe共CO兲5共Aldrich), and the PES peaks of Fe associated with two different REMPI transitions (z 7 D 03 ⫺e 7 F 4 at 318.023 nm, and ␣ 3 H 4 ⫺ ␮ 3 G 03 at 312.044 nm兲 were measured. The photoelectron energies of these peaks were calculated based on the energy levels of Fe and Fe⫹ given by Sugar and Corliss.48 The result is a set of seven accurately known PES calibration peaks, spanning the kinetic energy range from 2.0 to 2.4 eV—near the energy associated with the main PES peaks in butadiene. The Fe peaks were least-squares fit by an empirical expression 关Eq. 共1兲兴 to determine the parameters t 0 and E 0 , KE⫽

冉冊

me L 2 t⫺t 0

2

⫹E 0 ,

共1兲

where KE is the kinetic energy of photoelectron, L is the length of flight path, t is the measured time of arrival, t 0 is an adjustable parameter representing the actual time of the ionization event, m e is the mass of the electron, and E 0 is a parameter accounting for stray fields along the flight path. The absolute accuracy of the calibration is ⫾5 meV, in the KE range covered by the Fe⫹ calibration peaks 共KE⬃2.0– 2.5 eV兲, with relative uncertainty of ⫾1.5 meV. Note, however, that for the higher KE electrons generated when pumping the high vibrational levels of the C4H6 关 1 A g (3d ␲ ) 兴 state, the calibration is extrapolated, with unknown uncertainty.

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˜ 1 A 兴 and TABLE I. Experimental and calculated frequencies for C4H6 关 X g ⫹ ˜ 2 C4H6 关 X B g 兴 . ˜ 2 C4H⫹ 6 关 X Bg兴

˜ 1A 兴 C4H6 关 X g

Calculationb PES Calculationb Absorptiona 共This work兲 Absorptionc 共This work兲共This work兲 Ag ␯ ⬙1 ( ␯ ⫹ 1 ) ␯ 2⬙ ( ␯ ⫹ 2 ) ␯ ⬙3 ( ␯ ⫹ 3 ) ␯ 4⬙ ( ␯ ⫹ 4 ) ␯ ⬙5 ( ␯ ⫹ 5 ) ␯ 6⬙ ( ␯ ⫹ 6 ) ␯ ⬙7 ( ␯ ⫹ 7 ) ␯ 8⬙ ( ␯ ⫹ 8 ) ␯ ⬙9 ( ␯ ⫹ 9 ) Au ␯ ⬙10( ␯ ⫹ 10) ⬙ (␯⫹ ␯ 11 11) ␯ ⬙12( ␯ ⫹ 12) ␯ ⬙13( ␯ ⫹ 13) Bg ␯ ⬙14( ␯ ⫹ 14) ⬙ (␯⫹ ␯ 15 15) ␯ ⬙16( ␯ ⫹ 16) Bu ␯ ⬙17( ␯ ⫹ 17) ⬙ (␯⫹ ␯ 18 18) ␯ ⬙19( ␯ ⫹ 19) ⬙ (␯⫹ ␯ 20 20) ␯ ⬙21( ␯ ⫹ 21) ⬙ (␯⫹ ␯ 22 22) ␯ ⬙23( ␯ ⫹ 23) ⬙ (␯⫹ ␯ 24 24)

3087 3003 2992 1630 1438 1280 1196 894 512

3104 3022 3011 1638 1420 1265 1182 866 499

¯ ¯ ¯ 1610 1469 1279 1258 927 515

¯ ¯ ¯ 1606 ¯ 1269 ¯ ¯ 517

3154 3072 3050 1593 1454 1252 1249 908 503

1013 908 522 162

1013 903 515 166

¯ 1001 ¯ ¯

¯ ¯ ¯ ¯

1018 987 421 182

976 912 770

960 902 749

¯ ¯ ¯

¯ ¯ ¯

1025 890 503

3101 3055 2984 1596 1381 1294 990 301

3104 3024 3019 1580 1362 1272 967 288

3125 3074 3026 1477 1330 1251 1006 ¯

¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯

3154 3081 3048 1477 1326 1234 979 287

a

See Ref. 49. Calculated at B3LYP/6-311⫹⫹G** and scaled by 0.9638. c See Refs. 12 and 14. b

III. RESULTS AND DISCUSSION

The ground state of neutral C4H6 关 ˜X 1 A g 兴 is planar with dominant electronic configuration ...(6b u ) 2 (7a g ) 2 (1a u ) 2 (1b g ) 2 . The C4H6 molecule has a total of 24 vibrational modes, all of which are well known for the ground electronic state 共see Table I兲.49 The excitation of a single electron from the 1b g (2p ␲ ) orbital into a 3d atomic-like orbital leads to the 1 A g (3d ␲ ) Rydberg state. The removal of an electron from the 1b g (2p ␲ ) orbital results in ˜ 2 B ). Also included in Table I are the cation ground state (X g ˜2 vibrational frequencies ␯ ⫹ for C4H⫹ 6 关 X B g 兴 determined by 12 matrix-isolation infrared 共IR兲 and Raman spectroscopy.14 Note that ␯ 4 , ␯ 6 , ␯ 8 , and ␯ 9 , which are important in the present study, represent the CvC symmetric stretching (A g ), CH bending (A g ), CH2 rocking (A g ), and C–C–C deformation (A g ) modes, respectively.49 A. 2¿1 REMPI spectrum

Figure 1 shows the 2⫹1 REMPI spectrum of the A g (3d ␲ ) state. The main spectroscopic features observed in the present REMPI spectrum are consistent with those reported in the REMPI study of Mallard et al.,29 however, supersonic cooling in the present study has improved the resolution and signal-to-noise considerably, relative to the 1


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J. Liu and S. L. Anderson

FIG. 1. 2⫹1 REMPI spectrum for C4H6 关 1 A g (3d ␲ ) 兴 in the two-photon energy range of 61 000–66 400 cm⫺1.

diffusion flame used in Mallard’s experiment. As a result, many new vibronic bands are resolved. The peak position of each band is listed in Table II, and we have also included in Table II the peak positions of the bands observed in the REMPI study by Mallard et al. The assignment of the majority of REMPI bands is identical to an assignment in the

TABLE II. REMPI band positions, transition energies and vibrational assignments for the C4H6 关 1 A g (3d ␲ ) 兴 Rydberg state. Laser ␭ 共nm兲

Energy 2 ␯ vac 共cm⫺1 )

⌬E a 共cm⫺1兲

⌬E b 共cm⫺1兲

Assignment

325.540 322.865 321.662 320.741 320.403 319.051 317.243 316.428 315.200 314.704 314.062 312.755 312.292 311.063 310.298 309.330 306.739 305.044 304.135 303.475 301.870

61436 61945 62177 62356 62421 62686 63043 63205 63451 63552 63682 63948 64043 64296 64454 64656 65202 65564 65760 65903 66254

0 509 741 920 985 1250 1607 1769 2015 2116 2246 2512 2607 2860 3018 3220 3766 4128 4324 4467 4818

0 512 ¯ ¯ 975 1258 1612 ¯ ¯ 2110 ¯ 2525 ¯ 2867 ¯ 3212 ¯ 4137 ¯ 4472 4840

0 00 9 10 6 10 9 01 8 10 9 20 6 10 4 10 6 10 9 10 6 20 9 01 4 10 9 10 6 10 9 20 6 20 4 10 9 20 4 10 6 10 6 20 9 10 4 20 6 30 4 10 6 20 4 30 9 01 4 20 6 10 4 30

earlier study. The assignment of the newly observed bands is generally straightforward, as described below. ˜ 1 A transition is The origin band of the 1 A g (3d ␲ )⫺X g ⫺1 resolved at 61 436 cm . Our value is identical to that obtained in the previous REMPI experiment of Mallard et al., but very different from the origin reported in the UV absorption spectrum of McDiarmid 共60 343 cm⫺1兲.6 We note that the molecular designations of the Rydberg orbitals in butadiene are ns (a g ), n p (a u ,b u ), nd (a g ,b g ), and n f (a u ,b u ), and as discussed by Johnson, the highest occupied molecular orbital 共HOMO兲 1b g (2p ␲ ) is equivalent to a d orbital in the molecule-centered atomic symmetry.26 As a consequence, transitions from the HOMO 1b g (2p ␲ ) to n p and n f Rydberg orbitals are one-photon allowed, and transitions from the HOMO to ns and nd orbitals are two-photon allowed. It seems clear that McDiarmids was observing an np or n f Rydberg state. The 3d ␲ state has also been extensively investigated at various levels of semiempirical and ab initio theory. Nascimento and Goddard reported a transition energy of 7.68 eV 共61 943 cm⫺1兲 based on a generalized valence band–configuration interaction 共CI兲 calculation,34 Hollauer and Nascimento reported a transition energy of 7.67 eV 共61 863 cm⫺1兲 from a multiconfiguration self-consistent field–CI calculation,35 Serrano-Andre´s et al. reported a transition energy of 7.55 eV 共60895 cm⫺1兲 from a complete active space self-consistent field calculation,40 and Watts et al. reported a transition energy of 7.69 eV 共62 024 cm⫺1兲 based on an equation-of-motion 共EOM兲 coupled-cluster singletsand-doublets method with a noniterative treatment of triple excitations, CCSD 共T兲.42 All these calculations are in good agreement with our assignment of the 1 A g (3d ␲ ) origin band. The prominent vibronic bands in the 3d ␲ state are attributable to progressions associated with excitation of the ␯ 4⬘ , ␯ ⬘6 , and ␯ ⬘9 . A band attributed to excitation of the ␯ 8⬘ mode, which has not been previously observed, is also resolved at 62 356 cm⫺1 as a shoulder to the red of the 9 20 band. Vibronic bands resulting from combination excitations of ␯ ⬘6 and ␯ ⬘9 , ␯ ⬘4 and ␯ ⬘9 , and ␯ 4⬘ and ␯ 6⬘ are also identified. We note that the energies of both overtones and combination bands are within a few wave numbers of the sum of the individual vibrational fundamentals, indicating that these modes are quite harmonic for up to three quanta of excitation. Several weak hot bands are discernible in Fig. 1, which are assigned as transitions with one quantum of ␯ ⬙9 in the ˜X 1 A state. g The frequencies of ␯ 4⬘ , ␯ 6⬘ , ␯ 8⬘ , and ␯ 9⬘ for the 3d ␲ state measured in the present study are 1607, 1250, 920, and 509 cm⫺1, respectively. All of these vibrations are totally symmetric (A g ). We note that the values of ␯ 4⬘ , ␯ 6⬘ , and ␯ 9⬘ determined here are in excellent agreement with the frequencies extracted from Mallard et al.’s MPI work 共1612, 1258, and 512 cm⫺1兲.29

B. MPI time-of-flight photoelectron spectra

a

This work. (⌬E is energy difference measured with respect to the origin band 0 00 ,3␲ ). b Data from Refs. 29 and 30.

Photoelectron spectra were taken pumping through the major vibronic bands associated with the 3d ␲ state, as de-




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FIG. 2. Photoelectron spectra taken following REMPI via various vibronic resonant states of C4H6 关 1 A g (3d ␲ ) 兴 .

picted in Figs. 2共a兲 and 2共b兲. The ion internal energy E int associated with each photoelectron band is calculated from the positions of the band center by Eq. 共2兲 E int⫽ 共 2⫹1 兲 h ␯ ⫺KE⫺IP,

共2兲

where h ␯ is the photon energy, KE is the measured electron kinetic energy, and IP 共ionization potential兲 is 9.072 eV according to the NIST chemistry WebBook.50 The PES in Figs. 2共a兲 and 2共b兲 have been transformed to an ion internal energy scale for ease of comparison, and a summary of the ion internal energies corresponding to the dominant peak in each PES is given in Table III. Matrix isolation IR and Raman measurements for C4H⫹ 6 have provided fundamental frequencies for 14 vibrational modes, as listed in Table I.12,14 To aid assignment of the ion vibrations, we carried out ab initio calculations at the B3LYP/6-311⫹⫹G** level of theory, using GAUSSIAN98.51 As a check on the calculated cation frequencies, frequencies were also calculated for neutral C4H6 , where a complete set of experimental data is available. As often is the case, the ab initio frequencies are systematically higher than the neutral ground state frequencies. To eliminate this systematic error,

the calculated values have been scaled by a factor of 0.9638. The same factor was then used to scale the cation ab initio frequencies. The B3LYP vibrational frequencies are generally in excellent agreement with experiment. With the aid of the existing calculated and experimental cation frequencies, the assignment of the photoelectron bands is straightforward. The first PES result from pumping through the vibrationless level of the 3d ␲ state. The most intense peak in this PES corresponds to the formation of vibrationless cations, with ⬎80% state purity. Some weak peaks at higher internal energy correspond to formation of cations with excitation in 9 20 and 4 10 9 10 . The major photoelectron band observed for each REMPI transition is clearly assignable as the diagonal ionization transition, i.e., production of cations with the same vibrational excitation that was pumped in the intermediate state. This is reasonable because the intermediate is a Rydberg state and should, therefore, have geometry similar to that of the cation. The diagonal ionization transition typically accounts for 60%–80% of the total ionization. The remaining structure in the PES is due to off-diagonal ionization ⫹ transitions, principally involving combinations of ␯ ⫹ 4 , ␯6 , ⫹ and ␯ 9 . Generally speaking, the intensity of the off-diagonal


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J. Liu and S. L. Anderson

˜ 2 TABLE III. Multiphoton ionization PES of C4H⫹ 6 关 X B g 兴 via the C4H6 关 1 A g (3d ␲ ) 兴 Rydberg state. Intermediate state C4H6 关 1 A g (3d ␲ ) 兴 0 00 9 10 9 20 6 10 4 10 4 10 9 10 6 20 4 10 6 10 4 20 4 10 6 20 4 20 6 10 4 30 a

E viba 共cm⫺1兲

Ionic state ˜ 2 C4H⫹ 6 关 X Bg兴

0 518 1039 1269 1606 2157 2557 2825 3221 4080 4452 4750

0 00 9 10 9 20 6 10 4 10 4 10 9 10 6 20 4 10 6 10 4 20 4 10 6 20 4 20 6 10 4 30

The vibrational energy E vib listed corresponds to the diagonal ionization transition peak in each PES.

transitions increases with the level of vibrational excitation selected in the intermediate state. The PES obtained after REMPI through the 9 20 transition is unique, in having a peak appear corresponding to cations with very high vibrational excitation 共0.920 eV⫽7420 cm⫺1兲, in addition to the diagonal ionization peak at 0.129 eV共1039 cm⫺1兲. There are two possible explanations for such an outlying peak. One possibility is that this peak corresponds to a 2⫹2 photon process, rather than the 2⫹1 process. At the intensities used here, such an above threshold ionization process would require a long-lived autoionizing state at the 3 photon level, increasing the chance of absorbing an additional photon before ionization. The total 4 photon energy would then be 15.48 eV, indicating an ionization process requiring 13.86 eV to produce the 1.62 eV electron. 13.86 eV matches the appearance energy of the fragment 15 C4H⫹ 4 共13.84⫾0.07 eV兲 from butadiene. Despite the close energetic match, this above-threshold dissociative ionization mechanism seems unlikely. It would require a long-lived autoionizing state resonant with the third photon, and only for the 9 20 transition. More importantly, it is quite unlikely that dissociative ionization would produce a sharp peak in the photoelectron spectrum. Rather, we expect that the large geometry change in dissociative ionization should result in a broad range of electron kinetic energies. Similarly, it is possible to reject the possibility that the sharp peak represents an excited electronic state of the cation, because there are no electronic states accessible at the 3 photon level; neither are there states that would give a 1.62 eV photoelectron in 4 photon above-threshold ionization. The conclusion is, therefore, that the peak at high internal energy must be attributed to a high vibrational level of butadiene cation in its ground electronic state. The presence of such a peak following REMPI via the 9 20 transition is indicative that the ␯ 9⬘ ⫽2 level of the 3d ␲ Rydberg state must be mixed with a high vibrational level of some other Rydberg or valence state, thus making the high cation vibrational level Franck–Condon accessible. The most likely candidate for such mixing is the 1 A u (3px) Rydberg state. The origin transition energy of the

FIG. 3. Photoelectron spectra taken following REMPI C4H6 关 9 20 , 1 A g (3d ␲ ) 兴 with different direction of laser polarization.

via

3 px state is at 6.80 eV.16–18,20,21 For the 1 A g (3d ␲ )9 20 transition in question, the mixing would be with a level of the 1 A u (3px) state with about 0.94 eV of vibrational energy. Given that the 1 A u (3px) state is also a Rydberg state, and should have diagonal ionization Franck–Condon factors, we would expect that such mixing would result in some cations with vibrational energy very close to the observed 0.92 eV peak. Less likely states for mixing are the 1 A u (3py) and 1 B u (3pz) Rydberg states, with origin band transitions at 6.66 and 7.07 eV, respectively,5,6,16–18,20,21. If the mixing were with these Rydberg states, and ionization were diagonal, cation vibrational energy peaks would be expected at 1.08 or 0.67 eV, respectively. Finally, mixing might be with some unknown valence state, however, mixing in valence character would tend to result in nondiagonal ionization, and we would not expect a single, sharp outlying peak. The photoelectron spectra were all measured with two laser polarizations: parallel and perpendicular to the electron detection axis. For all PES, with the exception of that obtained by REMPI through the 9 20 level of the 3d ␲ state, there is no significant polarization dependence. The spectra presented in Fig. 2 are for perpendicular 共90°兲 polarization. The result for the 9 20 transition is shown in Fig. 3. There is little change in the relative peak heights between polarizations, with the exception of the outlying peak at E int⫽0.92 eV, which is much weaker in the parallel polarization. The fact that the outlying peak has different polarization dependence 共i.e., produces photoelectrons with a different angular distribution兲 compared to all other PES bands, is further evidence that the outlying peak results from a fundamentally different type of ionization process, presumably resulting from mixing of some other electronic state at the intermediate level. The ion vibrational frequencies determined from the ⫹ ⫺1 ⫺1 PES measurement, i.e., ␯ ⫹ 4 ⫽1606 cm , ␯ 6 ⫽1269 cm ,




FIG. 4. Multiphoton ionization mass spectrum of C4H6 via resonant state C4H6 关 0 00 , 1 A g (3d ␲ ) 兴 at 325.540 nm.

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above threshold, dissociative ionization of the neutral butadiene, or photofragmentation of neutral butadiene, followed by ionization of the fragments. The photoelectron spectroscopy results clearly indicate that the first mechanism is dominant, because the photoelectrons clearly correspond to vibrational levels of the butadiene cation. Photodissociation of butadiene cations is not surprising, as there are a number of low-lying electronic states. Dunbar ⫹ reported production of C3H⫹ 3 when C4H6 is one-photon ex52 2 2 ˜ B states. Woodward et al. reported cited to ˜A A u and C u that both one photon and multiphoton absorption by ⫹ ⫹ 53 ˜2 C4H⫹ 6 关 X B g 兴 can lead to C2H4 and C2H3 fragment ions. ⫹ Bunn and Baer also observed production of C2H4 and C3H⫹ 3 by single and multiphoton dissociation of vibrationally state˜2 selected C4H⫹ 6 关 X B g 兴 , using the photoelectron–photoion coincidence technique.54 From a spectroscopic point of view, the fragmentation in the REMPI process is interesting. However, given our ultimate purpose of using REMPI to prepare vibrational stateselected cations for ion–chemistry studies, the facile photodissociation makes REMPI of butadiene through the 3d ␲ Rydberg state marginally useful, at best. IV. CONCLUSIONS

⫺1 and ␯ ⫹ 9 ⫽518 cm , are in excellent agreement with the matrix isolation and theoretical frequencies. Of these three frequencies, ␯ ⫹ 4 is associated with CvC stretching and is more sensitive to removal of the 2p ␲ electron. This is reflected in the decreased frequency for ␯ 4 in the Rydberg state 共1607 cm⫺1兲 and in the cation 共1606 cm⫺1兲, compared to the value in the neutral ground state 共1630 cm⫺1兲.

C. MPI time-of-flight mass spectrum

Figure 4 shows the REMPI mass spectrum obtained pumping the origin transition of the 1 A g (3d ␲ ) Rydberg state. Note the extensive fragmentation, with almost no surviving parent ion. Fragmentation proceeds all the way down to C⫹ , indicating absorption of many photons. The appearance energies for these range upward of 19 eV,50 and the actual energy deposited may be higher because the various fragments are not necessarily produced via the lowest energy pathways. To examine this behavior, experiments at various laser powers were performed. Even with less than 500 ␮J/ pulse focused with a 15 cm lens, the entire set of fragments is observed, including the rearrangement species CH⫹ 3 and . The implication is that UV absorption by the cation C2H⫹ 4 and its photofragments is highly efficient, so that at any laser intensity large enough to allow 2⫹1 REMPI, the cation will be photodissociated with near-unit probability. The present REMPI mass spectrum shows substantially more fragmentation than the spectrum reported by Zandee and Bernstein,27 which was taken by REMPI through the origin of the 1 B g (3s) Rydberg state. In Zandee and Bernstein’s study, the most abundant fragment ions are C4H⫹ n , while our ions are ⫹ and C H . fragmented down to CH⫹ 2 n n Conceivably, the fragmentation could result from several processes: photofragmentation of nascent butadiene cations,

We have performed a 2⫹1 REMPI study of 1,3-transC4H6 in the two photon energy range from 61 000 to 66 400 cm⫺1. The REMPI spectrum allows identification of a number of new vibronic bands. Photoelectron spectroscopy reveals that with one exception, all observed vibronic levels of the 1 A g (3d ␲ ) Rydberg state are well behaved and unmixed. The exception is the ␯ 9⬘ ⫽2 level, which appears to be mixed with a high vibrational level of some lower states. REMPI mass spectrometry has also been investigated, revealing the extensive fragmentation of the parent ion. ACKNOWLEDGMENTS

This work was supported by the National Science Foundation under Grant No. CHE-9807625. The authors thank Ho-Tae Kim for help in setting up the PES instrument. 1

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Using JCP format - Department of Chemistry - University of Utah

Using JCP format - Department of Chemistry - University of Utah

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Using JCP format - University of Utah Chemistry