Experimental and Theoretical Investigation of

AIAA-2001-2806

Experimental and Theoretical Investigation of Nonequilibrium in Laser Induced Plasmas a,b

a,b

a,b

a,b,

G. Colonna , A. Casavola , L.D. Pietanza , A. De Giacomo , V. A. c a,b Shakhatov , M. Capitelli a

b

Dipartimento di Chimica, University of Bari, Via Orabona 4, 70126 Bari, Italy Centro di Studio per la Chimica dei Plasmi del CNR, Via Orabona 4, 70126 Bari, Italy c Centro Laser, S.P. per Casamassima, km.3, 70010 Valenzano (Bari), Italy

Expansion of the material ablated from a TiO (titanium oxide) surface with a nanosecond pulsed laser is studied by emission spectroscopy. The quantities measured are the time of flight and a space resolved spectrum. The Boltzmann plot approach is used to obtain temperatures and concentrations of Ti (titanium) atoms and ions. A semi-empirical collisional-radiative model is shown to be very useful to eliminate spurious effects from the spectrum. A self-consistent model coupling collisional/radiative kinetics and fluid dynamic equations of the plume expansion can improve the understanding of the physics during the expansion ad can be used as a powerful tool to extract quantitative information from the emission spectrum even in the absence of LTE (local thermodynamic equilibrium).

Nomenclature L P0 Pb tR T0 Tb x z

30

dispersion is found to be of the order of cos θ. Under LTE approximation it is possible to estimate theoretically the plasma composition [8] and the mass fluxes toward the substrate. The strong linking between theory and experiment is evident when the analysis of experimental data contradicts LTE conditions. One of the most commonly method used for controlling the parameters of the laser induced plasma is optical emission spectroscopy [11-17]. It is based on the study of the spectral distribution of lines intensities and on the broadening detection in emission spectrum. Electron temperature Te and number density Ne are determined from relative atomic and ionic lines intensities measurements using Boltzmann’s and Saha-Boltzmann’s plot techniques [17,18]. The validity of this technique is based on the assumptions of the existence of LTE conditions and of an optically thin plasma behavior. Furthermore, Stark broadening detection and spectral lines displacement allow to estimate electron temperature and number density without considering LTE [15, 16, 18, 19]. Discrepancy between experimental data derived from the different methods do not allow to make an unequivocal conclusion about the presence of LTE in the laser induced plasma [14]. Even if in many experiments the distribution function of atoms and ions, in laser induced plasma, can be represented in a good approximation as a Boltzmann distribution, for a wide range of the expansion time, this does not justify the initial assumption about the existence of LTE conditions. Temporally and spatially resolved optical emission spectroscopy measurements of atomic and ionic state distribution functions show that electron, ion and atom temperatures can

Distance between substrate and target Initial pressure Buffer pressure time interval for matter evaporation Initial temperature Buffer temperature distance from the target matter evaporated per unit surface and unit time

Introduction The flexibility of the experimental set-up and the large quantity of condensed material that can be treated by laser, make pulsed laser deposition (PLD) a very promising technique for thin film production [1]. In fact, among conventional techniques [1, 2] which are commonly employed for synthetizing titanium dioxide (TiO2) thin films, PLD has been found to be the most satisfactory [3-6]. In particular, plasma assisted pulsed laser deposition has been successfully used for growing TiO2 thin films [7]. This same system has been investigated theoretically using a one-dimensional time-dependent fluid dynamic model, both in free flow and in local thermodynamic equilibrium (LTE) approximations [8]. The validity of the one-dimensional approach has been proved in refs. 9 and 10 where angular Copyright © 2001 by G.Colonna. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission 1 American Institute of Aeronautics and Astronautics

essentially differ. The presence of LTE conditions in the plume is commonly accepted, specially in the case of laser induced breakdown spectroscopy (LIBS) technique, where the plume expands in atmospheric pressure air. This condition is necessary when the LIBS is used to perform a quantitative analysis of solid surfaces. On the contrary, the plume expansion (low density, high speed) presents characteristics similar to the expansion of high enthalpy nozzle flow, where strong nonequilibrium vibrational (vdf) and electron energy distributions (eedf) have been found [20] in N2 plasma.

focus the laser beam, a PLD vacuum chamber, a system for gas feeding and pumping out and a pressure controlling system and a spectroscopic system, as depicted in Figure 1. A pulsed KrF excimer laser, with an emission wavelength of 248 nm, is used as the source of monochromatic radiation for evaporating the target material. The laser is operated at a repetition rate of 10 Hz, with a pulse duration of 30 ns. The energy of the laser is varied up to 1.0 J and is controlled by means of an energy meter. Using mirrors and spherical lenses the laser beam is directed through a quartz window into the PLD chamber and focused onto the target at 45˚ from the surface normal to reduce the plasma shield effect on the incoming laser pulse. The fluence of the laser beam is varied up to 6 -2 J cm , by adjusting its degree of focusing on the target and the laser energy output.

Experimental section Set-up The experimental set-up consists of an excimer laser with an optical system in order to steer and

Fig.1. Experimental set-up. The PLD chamber is a typical stainless steel vacuum chamber of cylindrical geometry. It is equipped with holders for the target and the substrate, ports for the laser beam, gas pumping out and feeding and pressure gauging. The target holder is continuously rotated to avoid the formation of laser induced craters locally on the target. The emission spectrum from the laser induced plasma (LIP) is detected by two different methods. Time-integrated optical emission spectroscopy is utilized to identify chemical species generating in the laser-induced plume and to investigate LIP until 6 mm from the target. Spatially - and - temporally

resolved optical emission spectroscopy is used to investigate the time evolution of the spectral line intensity and broadening, atomic state distribution functions (ASDF) and ionic state distribution functions (ISDF), and to estimate the electron number density depending on time, close to the target (0,6 mm). To record a spectral distribution of the line intensity in emission spectra, in a range of wavelengths from 250 nm up to 700 nm, is applied a high resolution monochromator (HR 640, Instruments S.A., division Jobin Yvon) equipped with a holographic grating 2400 gr/mm and an intensified charge coupled device 2

American Institute of Aeronautics and Astronautics

(ICCD, ORIEL Instruments), for time resolved measurements, or an optical multichannels analyzer (OMA IRY 700 GmbH), for time integrated measurements. The detection of the ICCD output signal is accomplished by a Stanford Research DG 535 programmable pulse generator, connected, through a general purpose interface bus (GPIB), to a personal computer (PC) for data acquisition and processing. To exclude a timing jitter of the excimer laser and spectroscopic systems, a portion of the laser beam by glass wedge is split off and detected by a fast photodiode in order to trigger the pulsed generator. A digitizing fast oscilloscope (Tektronix TDS 620 A) and a photodiode are used to calibrate and control the gate width and time delay after the laser irradiation. A gate width of 15 ns is used to maximize the spectral line intensity while maintaining good temporal resolution. Accuracy in establishment of a time delay td is 4 ns for a range from 100 ns to 600 ns. To optimize the signal-to-noise ratio the detection of signals is carried out in accumulation mode. The average value of the signal from the laser induced plasma is obtained over 20 consecutive laser pulses in time integrated OES (optical emission spectroscopy) and 150 consecutive laser pulses in time resolved OES, at the repetition rate of 10 Hz. A quartz lens optically projects the plume image of the LIP, with 1:1 magnification, on the entrance of an optical fiber placed outside of the deposition chamber. So the UV-grade optical fiber (core diameter 0.6 mm) is placed on the image plane of the plume and it is mounted on an optical table, permitting vertical and horizontal micro-movements to collect light emitted from different regions of the LIP. The aperture of the optical fiber is aligned with centerline of the plume to ensure that the fluorescence signal is collected perpendiculary respect to its symmetry axes. The output of the optical fiber is coupled to the slit of the monochromator. The slit width is set at 100 µm. The maximum spectral resolution of the optical system is determined as 0.2 Å monitoring the lines of a mercury lamp and an He-Ne laser for the slit width used in the experiment. The spectral response of the optical system including lens, optical fiber, monochromator and ICCD is controlled with a tungsten halogen lump. To investigate the spectral contour and width of the investigated lines the instrumental function of the optical system was measured by the mercury lamp and a He-Ne laser. The line profile is well fitted by a Voight profile, characterized by a spectral width 0.9 ± 0.08 Å (FWHM). ASDF and ISDF are recovered from relative atomic and ionic line intensity measurements. For each selected spectral line, a computer code calculates the area by the fitting of spectral line contours with an automatic baseline correction.

different distances from the target and delay time from the laser pulse, in order to characterize the thermodynamics and the kinetics of the processes involved as result of the interaction of laser radiation with TiO target. A series of characteristic titanium emission lines from the excited neutral atoms and from single charged ions (Ti I and Ti II) was observed. Spectral lines, corresponding to the transitions between the excited states of oxygen atoms O I and ions O II, were not observed because their intensities are too low with respect to the corresponding ones of Ti I and Ti II lines or they are overlapped by too strong nearby lines to be clearly identified in the spectral range investigated (250-700 nm). In addition no molecular species such as TiO, TiO2, O2 or molecular oxygen ions were observed in the LIP at the distances from the target investigated in the present work (until 6 mm). For the determination of titanium ASDF and ISDF evolution during LIP expansion, several lines not overlapping with nearby lines and having emission time shorter or at least comparable with the time of residence of the LIP in the probe volume, were chosen. The velocity of LIP expansion was estimated by Time of Flight (TOF) [21-23] measurements, corrected for spontaneous emission, of selected 6 atomic lines. The velocity estimated is 1.3 10 cm/sec at 0.6 mm from the target and is reported in fig.2, where the temporal interval investigated by OES is also signed.

Intensity (a.u.)

tobs

0

Time (s) Fig.2.

TOF measurement of Ti I line, at 0,6 mm in vacuum at the laser fluence of 6 J cm

-2

The electron number density was determined experimentally by the quadratic stark broadening [24] considering a Voigt shape of the spectral lines and it 18 17 -3 varies between 10 and 10 cm during the temporal evolution of LIP from 100 to 300 ns, at the laser -2 fluence of 6 J cm . An example of the temporal

Results The emission spectrum of LIP has been observed at 3 American Institute of Aeronautics and Astronautics

2 10-7 4 10-7 6 10-7 8 10-7 1 10-6

dependence of spectral line shape is presented in fig.3, where the line width decreases rapidly as function of time, from its maximum value of 1.5 Å at 100 ns to 0.9 Å at 280 ns, which coincides with the width of the instrument function of our spectroscopic equipment. To estimate the thermal energy and to characterize the thermodynamic conditions of the LIP, the distribution functions of the electronic excited states of titanium atoms and ions were recovered from the experimental spectra at different values of laser fluence and oxygen pressure. Assuming a Boltzmann distribution, the temperatures of Ti I and Ti II, corresponding to each measured distribution function, can be determined using the Boltzmann plot technique by the measurements of intensities of the spectral lines. The time resolved measurement of the excitation temperature, showed in Fig.4, put in evidence that, at 0.6 mm from the target, after 100 ns from the laser pulse, a significant difference between the temperatures of Ti I and Ti II arises as a consequence of the expansion of the LIP at low pressure. As will be explained below, also if the electron number 15 -3 density is high (>10 cm ) and a Boltzmann distribution is assumable for titanium species, the LTE assumption is not valid, because the expansion rate is so fast that different kinetic process roles take place for atoms and ions. The same phenomena is observed by spatial resolved measurement performed by integrated time OES and reported until 6 mm.

collisional ionization and three bodies’ recombination, radiative recombination and photoionization, spontaneous emission, reabsorption and stimulated emission. The model is based on some experimental input parameters as electron temperature and temporal spectral line profiles. Good agreement is obtained between calculated and measured ASDF and ISDF as is shown in Fig 5. The fitting of experimental measurements by the results of the theoretical model demonstrates that the different roles of the radiative processes for ions and atoms are the cause of the discrepancy in the excitation temperature of ions and atoms.

35000

Temperature (K)

30000 25000 20000 15000 10000 5000

1

100

150

200

250

300

Delay time (ns)

Intensity (n. u.)

Fig.4.

0.5

laser fluence of 6 J cm TiO target in vacuum

t = 0 ns t = 40 ns t = 200 ns 0 402.67

402.8

402.93

Wavelength (nm) Fig.3.

Temporal evolution of atomic and ionic temperature: experimental values for atoms (squares) and ions (circles) are compared to results of numerical modeling, plotted respectively as dashed and solid curves at the

Line shape of Ti I 402.7 at different delay time detected with the gate width of 15 ns.

In order to explain the discrepancy in atomic and ionic excitation temperature a collisional radiative model, described in detail in a previous work [24], was applied to the experimental results in vacuum. In this model were considered the collisional excitation and de-excitation of atoms and ions with electrons, 4 American Institute of Aeronautics and Astronautics

-2

at 0.6 mm from the

approach has been performed considering an hydrogen plasma (H, H+ and e-). With this model it is possible to calculate the levels distributions, solving a system of master equations (one equation for each level and one for ion and electron densities) where the kinetic terms are due to electron-atom collisions (e-A) and radiative processes. The electron energy distributions have been calculated solving the Boltzmann equation. In this way the rate coefficients of e-A collisions are calculated integrating over the electron energy and, as a consequence, the effects due to non-maxwellian electron distributions can be taken into account. With this model we can investigate the simultaneous temporal evolution of both electrons and levels distributions (self-consistent model) during the thermal relaxation of the plasma. The collisionalradiative model has been developed for hydrogen atoms, because of the availability of input data [25]. Two radiative processes have been considered in the master equations of the cr model:

6

ln( Inm/ gnmAnmvm)

4

2

0

-2 0

2 104

4 104

6 104

Energy (cm-1) Fig.5.

1) spontaneous emission H(i)! H(j)+ h" con i>j

Boltzmann plot of ASDF: experimental (circles) and calculated ASDF (triangles).

Theoretical Section

2) radiative recombination H + + e ! " H(i ) + h#

Model The model built to describe the laser ablation is divided in two parts: the fluid dynamics of the plume expansion and the kinetic modelling of the plasma. To study the fluid dynamics of the plume, we have used the one-dimensional, time dependent Euler equations for compressible fluids. Experimental data show that the angular dispersion is very small and therefore the one-dimensional model is sufficient to describe the flow properties. This model has been applied to study the laser induced plume from a TiO target in local equilibrium. To model the gas expansion in the equilibrium approximation we have calculated the thermodynamic quantities, the equilibrium constants and the composition of the considered mixture. The limit of this method relies on the fact that the plasma, during its expansion, can be far from local thermodynamic equilibrium (LTE). As a first attempt to investigate the plasma kinetics in conditions far from LTE, we take into account the ionization and recombination processes of atomic titanium.

and two collisional processes: 3) the electron impact excitation (forward) and deexcitation (backward)

H(i)+ e ! (" ) # H(j)+ e ! (" ! " ij ) 4) electron impact ionization and three body recombination

H(i)+ e ! (" ) # H + + e ! (" ! " i ) + e ! where εij and εi represent, respectively, the threshold energy for the transition i ! j and the ionization energy from the i-th level. Atom-atom and ion-atom collisions have been neglected assuming that the electron-atom collision frequency dominates the kinetics. In the electron kinetics we have also introduced electron-ion and electron-electron Coulomb collisions. The inelastic cross sections and Einstein coefficients for radiative decay have been taken from an available database [25].

Ti + e! " Ti + + e! + e! The ionization rate coefficients have been calculated considering a Maxwellian electron energy distribution and a Boltzmann distribution of the atom excited levels at the gas temperature (thermal rate), and the cross sections have been derived from the Grizinskii model. The recombination rate has been estimated by detailed balance principle. A further improvement in the kinetic model consists in coupling the fluid equations with a collisional-radiative (cr) model [23, 24]. This kind of

Results The laser induced plasma has been investigated, in three different approximations: a) free flow, where the composition is kept frozen; b) local thermodynamic equilibrium, where the equilibrium composition is calculated locally, c) chemical kinetics, where the time evolution of concentrations are calculated selfconsistently with the flow. 5

American Institute of Aeronautics and Astronautics

1 018 -3

1 017

Ti

2.0 10-4

(a)

1 016

!/! 0

Concentration (cm )

(b)

(a) +

1.0 10-4

Ti (b)

1 015

2.35 10-7

2.5 10-7

0.0 100

2.65 10-7

time (s) Fig 6:

0 100

Ti and Ti+ concentration versus time. Comparison between (a) experimental data (Pb= 10-6 Torr, Elaser= 6 J/cm2) and (b)

1.0 10-1 8.0 10-2

TiO Ti Ti+

Calculating the mass flux over the substrate it is possible to predict qualitatively the stoichiometric composition of the deposited film (see fig 7).

flow speed (M)

1.0

0 100

+

Ti Ti

4 10-3

8 10-3

1.2 10-2

x(m)

Fig 9:

Mach number profile at different times. Pb= 10-1 Pa, Tb=300 K, P0=106 Pa, T0=10000 K, tR=5 10-5 s; (a): z=1000 Kg/m2/s; (b): z=100 Kg/m2/s

TiO

We can investigate the effect of different laser fluences by considering different rates of material evaporation (z) from the target. On figures 8 and 9 we report the temporal evolution of density and Mach number profiles for different values of the evaporation rate (z): as greater is z, as higher are the gas density and the flow speed, in accordance with experimental results. In fact, for slow release of matter, the front expansion of the plume is strongly

-2.0 10-2 0 100 1 10-52 10-53 10-54 10-55 10-56 10-5 time (s) Mass flux versus time. Pb= 10-1 Pa, Tb=300 K, P0=106 Pa, T0=10000 K, t =5 .10-5 s, z=1000 Kg/m2/s. R

6 American Institute of Aeronautics and Astronautics

2.0

0.0

2.0 10-2

Fig 7:

t=2.5E-07 (a) t=7.5E-07 (a) t=2.0E-06 (a) t=2.5E-07 (b) t=7.5E-07 (b) t=2.0E-06 (b)

3.0

4.0 10-2

0.0 10

8 10-3

z=100 Kg/m2/s

6.0 10-2

0

4 10-3 x(m)

Density profile at different times. Pb= 10-1 Pa, Tb=300 K, P0=106 Pa, T0=10000 K, tR=5 10-5 s; (a): z=1000 Kg/m2/s; (b):

Fig 8:

theoretical results (Pb= 10-1 Pa, Tb=300 K, P0=106 Pa, T0=10000 K, tR=5.10-5 s, z=1000 Kg/m2/s). The first application of the theoretical model is the study of an apparatus which works in vacuum and has been built to investigate pulsed laser deposition (PLD) mechanisms. In this case the fluid dynamic equations have been coupled with local equilibrium conditions. A comparison between experimental and theoretical concentrations of Ti and Ti+ is reported on figure 6, at a distance of 0.6 mm from the target, as a function of time. A qualitative agreement can be observed. Comparing measured data and theoretical results is quite hard because of many difficulties in experiments and in the model. In fact the experimental concentrations are affected by instrumental errors, and they are determined using the Boltzmann plot technique, which supposes LTE. Moreover the fluid dynamic model needs initial conditions that are not available from experiments.

mass flux (mol/m2)

t=7.5E-07 (a) t=2.0E-06 (a) t=6.0E-06 (a) t=7.5E-07 (b) t=2.0E-06 (b) t=6.0E-06 (b)

slowed down. Fig. 11:

Ti

Atomic and ion concentrations as a function of time at x=2mm from target. Comparison between equilibrium flow (triangles) and kinetics (circles).

18

3.5 10

18

Ti + (cm-3)

-3

Ti (cm )

3 10

Further from the target the situation is different. Fig 11, reports the atoms and ions concentrations as a function of time at x=2 mm for equilibrium flow (triangles) and kinetic model (circles). In this case, the composition is far from equilibrium at short time (t< 300 ns) but it gets very close to the equilibrium composition for longer time (t about 300 ns). In this condition we can expect non-equilibrium distributions. For this reason we introduce the collisional-radiative model in the fluid dynamic code. For simplicity we consider an hydrogen plasma. Even if this system is not realistic, the results are qualitatively comparable independently on the kind of atoms considered. On figures 12, 13 we show electron energy (eedf) and level population distributions at 0.6 mm from the target. At the initial time (t=0s) the distributions are imposed to be in local equilibrium with the gas. During the expansion deviations from equilibrium can be observed. In the eedf they are quite small, due to the strong contribution of electron-electron and electron-ion Coulomb collisions imputable to the high ionization degree. On the contrary, atomic levels are very far from Boltzmann distributions, being the tail underpopulated.

18

2.5 10

+

Ti

18

18

2.2 10

50

18

2.5 10

2 10 150

250

350

time (ns) Fig. 10: Atomic and ion concentrations as a function of time at x=0.2 mm from target. Comparison between free flow (dashed line) and kinetics (circles). On figure 8 we can see that at the time t=6 10-6 s the expansion has reached the substrate only for high release rate case (z=1000 Kg/m2/s). Some similarities with the expansion of high enthalpy nozzle flows (low density and high speed) can be observed, therefore nonequilibrium level and electron energy distributions (eedf) can be present. The first attempt to investigate the possibility of non-equilibrium in the plume expansion, has been carried out coupling the ionization/recombination kinetics of atoms with the fluid dynamic equations. The model has been applied to the expansion of a Ti/Ti+ plume. On fig 10 we have reported the atoms and ions concentrations calculated with the free flow approximation (dashed line) and with the ionization kinetics (circles) as a function of time. At this position, 0.2 mm from the target, the pressure is very high (~1.4 106 Pa) and therefore the concentrations are very close to the local thermodynamic equilibrium even at very short time (100 ns).

t=0

100

-8

t=10 s t=10-7 s t=10-6 s

10-2 10-4 10

-3/2

)

18

eedf (eV

2.8 10

-6

10-8 10-10 10-12 0

1 1017

+

Ti

-3

1 1017

0

-1 1017 150

250

350

time (ns) 7 American Institute of Aeronautics and Astronautics

20

Fig. 12: Electron energy distribution as a function of time et x=0.6 mm from the target.

Ti 0 50

10 electron energy

Ti + (cm-3)

Ti (cm )

2 1017

[5]

C.M. Dai, C.S. Su and D.S. Chuu, “Composition and chemical reactions of titanium oxide films deposited by laser evaporation”, J. Appl. Phys. 69 (1991) 37663768 [6] H. O. Sankur and W. Cunning, “Deposition of optical thin films by pulsed laser assisted evaporation”, Appl. Optics 28 (1989) 28062808 [7] V.A. Shakhatov, A. De Giacomo, V. D'Onghia, “Plasma assisted pulsed laser deposition of titanium dioxide”, in ALT ’99 International Conference on Advanced Laser Technologies, V.I.Pustovoy and V.I. Konov Eds., Proceedings of SPIE 4070 (1999) 394 [8] G. Colonna, A. Casavola, M. Capitelli, “Modeling of TiO Plume Expansion Under Laser Ablation”, in ALT ’99 International Conference on Advanced Laser Technologies, V.I.Pustovoy and V.I. Konov Eds, Proceedings of SPIE 4070 (2000) 293-299 and G. Colonna, A. Casavola, M. Capitelli, “Modeling Plasma Libs Expansion” Spectrochimica Acta part B, in press. [9] R. Kelly, “Gas dynamics of the pulsed emission of a perfect gas with applications to laser sputtering and to nozzle expansion”, Phys. Rev. A 46 (1992) 860 [10] R. Kelly, A. Miotello, “Pulsed laser sputtering of atoms and molecules. Part I: basics solutions for gas dynamic effects”, Appl. Phys B 57 (1993) 145-158. [11] S.S. Chu, M. Ye and C. P. Grigoropoulos, “Spectroscopic characterization of laser-induced

100

H level distribution

10

-2

10-4 10-6 10 10

t=0 t=10-8 s

-8

t=10-7 s

-10

t=10-6 s

10-12 0

4

8

12

Level energy Fig. 13: Levels distributions as a function of time at x=0.6 mm from the target.

Conclusions In this paper we are giving an overview of the state of art of the research on nonequilibrim distributions during the expansion of the plume produced ablating a metallic surface with nanosecond laser pulses. Both theory and experiments shows the evidence of nonequilibrium distributions, but the linking is still poor. Due to the high speed of the plume expansion and to its low density, measurements resolved in space and time are affected by large errors, when available. In the future we plan to project experiments appropriate to validate the models and to resolve the spectra in a larger region.

th

[12]

Acknowledgements The present paper has been partially supported by ASI (Agenzia Spaziale Italiana) under contract I/R/038/01 and by CNR/P.F. “Materiali Speciali per Tecnologie avanzate” (contract N° 97.01005.34)

[13]

References [1] [2] [3]

[4]

[14]

D.B. Chrisey and G.K. Huber, “Pulsed Laser Deposition of Thin Films”, A. WileyInterscience Publication, New York, 1994 M. Ohring, “Materials Science of Thin Films”, Academic Press, Boston, 1997 Y.A. Bykovskii, V.M. Boyakov, V.T. Galochkin, A.S. Molchanov, I.N. Nikolaev and A.N. Oraevskii, “Deposition of metal, semiconductor, and oxide films with a periodically pulsed CO2 laser”, Sov. Phys. Tech. 23 (1978) 578-581 A. G. Akimov, A.P. Gagarin, V.G. Dagunov, V.S. Makin and S.D. Pudov, “Composition of an oxide film formed after pulsed heating of a metal”, Sov. Phys. Tech. 25 (1980) 1439-1441

[15]

[16]


titanium plume”, Proccedings of 5 ASME/JSME, San Diego, California, USA, March 15-19 (1999) 1-5 X.T. Wang, B.Y. Man, G.T. Wang, Z. Zhao, B.Z. Xu, Y.Y. Xia, L.M. Mei and X.Y. Hu, “Optical spectroscopy of plasma produced by laser ablation of Ti alloy in air”, J. Appl. Phys. 80 (1996) 1783-1786 B.Y. Man, “Particle velocity, electron temperature, and density profiles of pulsed laser-induced plasmas in air at different ambient pressures”, Appl. Phys. B 67 (1998) 241-245 M.H. Hong, Y.F. Lu, T.M. Ho, L.W. Lu and T.S. Low, “Plasma diagnostics in KrF eximer laser deposition of Ti thin films”, in ”Laser Processing of Materials and Industrial Application II”, D. Shu-Sun and S.L. Wang Eds., Proccedings of SPIE 3550 (1998) 441-449 F. Fuso, L. N. Vyacheslavov, G. Masciarelli and E. Arimondo, “Stark broadening diagnostics of the electron density in the laser ablation plume of Yba2Cu3O7-x and PbZrxTix-1O3”, J. Appl. Phys. 76 (1994) 8088-8096 O.A. Bukin, I.V. Bazarov, N.S. Bodin, A.A. Il’in, V.D. Kiselev, E.A. Sviridenkov, V.I. Tsarev, A.Yu Major, “Influence of the pressure of a gaseous, atmosphere on the characteristics of the emission spectra of a laser plasma

[17]

[18] [19] [20]

[21]

[22]

[23]

[24]

[25]

generated on the surfaces of solid targets”, Quantum Electronics 28 (1998) 685-688. S. Yalcin, D.R. Crosley, G.P. Smith, G. W Faris, “Influence of ambient conditions on the laser air spark, Appl. Phys. B 68 (1999) 121130 H.R. Griem, “Plasma Spectroscopy”, McGrawHill Book Company, New York, 1964 R.H. Huddlestone and S.L. Leonard, “Plasma Diagnostic Techniques”, Academic Press, New York - London, 1965 G. Colonna, M. Capitelli “Self-Consistent Model of Chemical, Vibrational, Electron Kinetics in Nozzle Expansion”, AIAA 20002415. M. Capitelli, G. Colonna, M. Catella, F. Capitelli, A. Eletskii, “On the relaxation of electron energy distribution function in LIBS plasmas”, Chem. Phys. Lett. 316 (2000) 517523 M. Capitelli, F. Capitelli, A. Eletskii, “Nonequilibrium and equilibrium problems in laserinduced plasmas”, Spectrochimica Acta part B 55 (2000) 559-574 G. Colonna, L.D. Pietanza, M. Capitelli, “On the coupling of collisional radiative models and Boltzmann Equation for atomic transient Hydrogen plasßmas”, RGD 2000, 9-14 July 2000, Sydney, Australia. G. Colonna, L.D. Pietanza, M. Capitelli, “Coupled solution of a time-dependent collisional-radiative model and Boltzmann equation for atomic hydrogen plasmas: possible implication with LIBS plasma”, Spectrochimica Acta B, in press. IAEA (International Atomic Energy Agency) database at: http://www-amdis.iaea.org/.
