The Journal of
Physical Chemistry
0 Copyright, 1989, by the American Chemical Society
VOLUME 93, NUMBER 14 JULY 13,1989
LETTERS Observation of Translationally Hot, Rotationally Cold NO Molecules Produced by 193-nm Laser Vaporization of Multilayer NO Films Lisa M. Cousins, Robert J. Levis, and Stephen R. Leone*.+ Joint Institute for Laboratory Astrophysics, National Institute of Standards and Technology and University of Colorado, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: March 15, 1989) This Letter presents results for the rotational and spin-orbit state excitation of NO molecules which are ejected at hyperthermal velocities by 193-nm laser vaporization of cryogenic multilayer NO films condensed on MgF,. For molecules with translational energies of 0.14 and 0.56 eV, the average rotational energies are 0.014 and 0.017 eV, corresponding to temperatures of 160 and 180 K, respectively. The spin-orbit population ratio F2/F1for the 0.14-eV molecules is 0.35 (T= 170 K); however, the population ratio for the 0.56-eV molecules is higher, 0.7 ( T = 500 K). The disequilibrium between translation and rotation may be due in part to a collisional cooling mechanism (adiabatic expansion) which occurs immediately following vaporization. Other dynamical ejection mechanisms are also discussed.
Introduction
further evidence for the mechanisms responsible for the high velocities of the ejected particles. In this Letter, we present the This report is part of a growing effort to characterize the first results of a study of the rotational and spin-orbit state mechanisms of ejection of translationally fast molecules formed distributions of translationally fast NO molecules ejected from by UV laser vaporization of cryogenic multilayer films.'-9 Several cryogenic NO films deposited on a MgFz window. features are common to this interesting phenomenon. Particles The onset of ejection of fast molecules from cryogenic films are ejected with supersonic velocities, sometimes with energies occurs for film coverages greater than one monolayer, where above several electronvolts. The ejection threshold and velocity energy transfer begins to play an important role.'A6 depend simultaneously on film thickness and laser f l ~ e n c e . ' ~ * ~ ~intermolecular ~~ The velocity distributions are narrower than Maxwell-Boltz(1) Domen, K.; Chuang, T. J. Phys. Rev.Lett. 1987,59, 1484. mann$-' and their angular distributions are peaked toward ( 2 ) Domen, K.; Chuang, T. J. J . Chem. Phys. 1989, 90, 3318, 3332. The process appears to be initiated by absorption to (3) Natzle, W. C.; Padowitz, D.; Sibener, S. J. J . Chem. Phys. 1988.88, electronic states in the film, but the velocities of the ejected 7975. (4) Cousins, L. M.; Leone, S. R. J . Mater. Res. 1988, 3, 1158. molecules are wavelength independent over a wide range of photon (5) Cousins, L. M.; Leone, S. R. Chem. Phys. Lett. 1989,155, 162. energiesS4v5Typically, the ejection yields are high. ( 6 ) Harrison, I.; Polanyi, J. C.; Young, P. A. J . Chem. Phys. 1988, 89, The degree to which the ejection phenomenon should be 1498. characterized by either thermal or photochemical mechanisms (7) Cho, C. C.; Polanyi, J. C.; Stanners, C. D. J . Phys. Chem. 1988, 92, 6859. is not clear. Studies of the internal state distributions may provide (8) Kutzner, J.; Lindeke, B.; Welge, K.H.;Feldman. D. J . Chem. Phys. ?Staff Member, Quantum Physics Division, National Institute of Standards and Technology.
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1949, 90, 548. (9) Simpson, C. J. S. M.; Minchington, C.; Kingslcy, C. R.; Parker, A. W. To be published.
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The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
The ejection velocity increases with deposition rate (film thickness) and laser fluence, as demonstrated by Domen and Chuang1v2for the vaporization of CH212films and by Cousins and Leone for Clp5 and NO! The process can be likened to an explosion caused by a sudden transfer of electronic energy into vibrational and rotational excitation of the molecules in the film. This photoinduced ejection has been observed by Harrison et a1.6 for H2S films, by Cho et al.7 for HBr films, by Simpson et al.9 for cyclohexane films, and by Natzle et al.3 for high coverages of NO films frozen to Ag. Ejection of translationally fast particles also occurs in the laser ablation of polymer films.1° Although the exact mechanisms undoubtedly differ for different systems, the generality of occurrence of this phenomenon suggests that the predominant mechanisms might be similar. In an earlier publication4we demonstrated that translationally fast NO molecules are produced by laser vaporization of cryogenic multilayer NO films which are condensed onto a 30 K MgF2 window. Fast NO molecules with energies varying from 0.1 to 1 eV are ejected when UV laser radiation (193 nm, 0.1-2 MW/cm2) is absorbed by a multilayer film. Translational time-of-flight studies show that the NO translational energies scale monotonically and nonlinearly with laser power and deposition rate. For high deposition rates the translational energies are observed to decrease, which may represent a maximum thickness effect or an experimental artifact due to collisions with the incoming dosing gas. Energy is channeled into translation most likely through a number of condensed-phase energy-transfer processes. In this study, we investigate the internal energy in u = 0 of the NO molecules as a function of translational energy. The extent to which the nascent rotational and translational energies may be affected by collisions following laser desorption has been addressed in several theoreti~al'~-'~ and experimental' 1*12 investigations. The postdesorption collision process may effect a mild supersonic expansion. When the number of desorbing molecules approaches a monolayer (- 1015 molecules/cm2), collisions occur immediately following desorption which shift the nascent translational disttibutions to higher energies. There is simultaneously an increase in the velocity of the desorbed molecules and a narrowing of their angular distribution. Recent calculations by Noorbatcha et al.13-15suggest that the rotational distribution should also be substantially cooled in such an expansion. An experimental study which can be used to compare to the calculations of Noorbatcha et al. can only be accomplished when the desorption yields are high, near one monolayer or greater. The desorption yields, as discussed below, are typically a few monolayers or less in these studies. The time over which the desorption occurs using the 15-ns laser pulse is not known. Therefore, the internal state distributions from this study may reflect a convolution of both initial desorption dynamics and postcollisional rotational cooling. In these studies, we observe rotationally cold (160-180 K), translationally fast molecules produced by the 193-nm laser vaporization of 30 K multilayer NO films. We present the rotational data for u = 0 N O molecules with 0.14-eV translational energy (ET), measured by laser multiphoton ionization (MPI). We also discuss the rotational results for ET= 0.56 eV and the spin-orbit population ratios. The translational energy is determined by time-of-flight (TOF) quadrupole mass spectrometry (QMS), and the rotational spectrum is obtained by using resonant 1 + 1 MPI, via the y(0,O) band of the A2Z+-X2111/2,3/2electronic transition.
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Experimental Section The apparatus used for this experiment is shown in Figure 1. Translationally fast NO molecules are generated repetitively by (10) Srinivasan, R. Science 1986, 234, 559. (11) Cowin, J. P.; Auerbach, D. J.; Becker, L.; Wharton, L. Surf. Sci. 1978, 78, 545 [Erratum: Surf.Sci. 1979,83, 6411. (12) Kelly, R.; Dreyfus, R. W. Surf. Sci. 1988, 198, 263. (13) Noorbatcha, I.; Lucchese, R. R.; Zeiri, Y. J. Chem. Phys. 1987,86, 58 16. (14) Noorbatcha, I.; Lucchese, R. R.; Zeiri, Y.J. Chem. Phys. 1988.89, 5251. (15) Noorbatcha, I.; Lucchese, R. R.; Zeiri, Y . Surf. Sci. 1988,200, 113.
Letters BOXCAR - DIGITIZER
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Figure 1. A schematic of the laser vaporization-TOF apparatus used to probe the rotational distributions of the vaporized NO molecules.
pulsed laser vaporization of NO films in a source chamber. A small fraction (approximately one to several layers) of a multilayer 30 K NO film is desorbed with a 193-nm excimer laser. The film is continually regenerated by depositing N O gas (99.5% pure) on the transparent MgF2 window with a slow flow (=(1-20) X 1017molecules/s). The source chamber is configured so that the laser irradiates the film from behind, through the transparent window first; this geometry is chosen to minimize the production of ions, in case plasma formation plays a role. The average NO translational energy is selected by changing the N O deposition rate, which may affect the film structure or the film thickness." The laser is pulsed at 3-5 Hz and the beam is mildly focused to a 1.2 cm X 0.8 cm rectangle to give an incident laser fluence of a 2 5 mJ/cm2. The spatial and temporal homogeneity of the beam are not ascertained. The rotational energy of the fast NO molecules is probed at a distance 19 cm downstream from the vaporization window, in a differentially pumped central chamber. A deflector composed of two stainless steel screens, biased at 0 and +400 V, is placed parallel to the ejection axis to deflect any ions that are formed during the vaporization process. A 225-227-nm tunable laser, which is used to effect MPI, is generated by frequency-summing the output of a frequency-doubled Nd:YAG pumped dye laser with 1064-nm radiation. An unfocused, homogeneous, 2-mm portion of the UV beam is selected with an iris, and its energy is maintained at 25 pJ/pulse. Time delays of 210 or 100 ps are selected to probe NO molecular velocities of 905 and 1900 m/s (0.14- or 0.56-eV kinetic energy, respectively). An electron multiplier held at -2.3-kV bias is placed 1.3 cm above the axis of the probe region to collect the positive ions produced by the resonant MPI. The MPI signal is integriited over 3-5 laser pulses by using a boxcar. The TOF behavior and the quantity of desorbed molecules are monitored throughout the wavelength scan by means of a quadrupole mass spectrometer (QMS) located in the third differentially pumped chamber, which is 55 cm downstream from the vaporization window. For calibration, the MPI spectrum of room-temperature NO is measured by use of the same probe laser configuration as in the experiments. Using a backfill of 1.33 X lo4 Pa (1 33 Pa = 1 Torr) of NO in the chamber, we find that the intensity of the P22(13.5) + Q12(13.5) transition, which also contains -20% contribution from the overlapping R2,(2.5) transition, varies as the square of the laser fluence up to twice the laser fluence used in the experiment. This indicates that unsaturated 1 + 1 MPI is the main source of ionization. Line positions are calculated by using the Hill/Van Vleck formulas16 and the molecular constants reported by Engleman and Rouse.17 Analysis of the rotational lines yields a calibrant rotational temperature of 295 f (16) Herzberg, G. In Spectra of Diatomic Molecules; D. Van Nostrand: Princeton, 1950; Vol. I, p 232. (17) Engleman, R.; Rouse, P. E. J. Mol. Spectrosc. 1971, 37, 240.
Letters K I N E T I C ENERGY ( e V ) 1.0 0.25
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Figure 2. Representative TOF traces of NO molecules vaporized from a transparent MgF2 window held at 30 K. The overall translational energy is increased by increasing the film thickness or laser fluence.
30 K, using the formulas for rotational line strengths given by Bennett.I8 Thus, the data are analyzed assuming unsaturated excitation of the rotational lines. The analysis ignores the small deviations associated with alignment effects in the intermediate state in the 1 1 p r o c e ~ s . ' ~For the laser vaporization experiments, the individual rotational lines are normalized by the square of the laser fluence and by the amplitude of the QMS signal corresponding to the selected NO velocity.
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Results and Discussion The average kinetic energy of ejected molecules increases with an increase in laser fluence or deposition rate. This phenomenon is discussed in detail in other publicati~ns.l-~.~~~ Two representative TOF traces obtained with the QMS are shown in Figure 2. The laser vaporization conditions have been adjusted so that the average energies in these traces are 0.4 and 0.09 eV. The average energies are obtained by using the appropriate Jacobian transformation. The TOF trace (a) is obtained by using a deposition rate of 4.5 X 10'' molecules/(cm2 s), a laser repetition rate of 3 Hz, and an incident laser fluence of 20 mJ/cm2; the TOF trace (b) is obtained by using a deposition rate of 3 X 10l6molecules/(cm2 s), a laser repetition rate of 3 Hz, and a laser fluence of 25 mJ/cm2. The arrows on the figure designate the velocities at which the internal states of NO are probed. To obtain relative yields, the velocity-weighted TOF distributions are integrated. To estimate absolute yields, the mass spectrometer currents are calibrated with thermal velocity NO gas, accounting for the l / u ionizer efficiency. A 45" conical angular distribution is assumed for the vaporization to estimate the geometrical factor. In this way, we estimate that approximately 2 X lOI5 molecules/cm2 are desorbed per laser pulse for Figure 2a (-4 monolayers/pulse or ~ 2 of % the incoming deposition flux) and 0.5 X lOI5 molecules/cm2 for Figure 2b (-1 monolayer/pulse or -16% of the deposition flux). The desorbed flux is subject to large error because the actual angular distribution is not ascertained. The percentages may indicate that a steady state is achieved if the sticking probability is less than unity; however, the actual film thickness is not known. If the photon energy were distributed uniformly throughout the film, then for 20 mJ/cm2, approximately 1.2 X lOI3 photons are absorbed per layer (-2% of the molecules excited with 6.4 eV/photon), assuming an absorption coefficient of 1 X cm2. An unnormalized rotational spectrum of the NO molecules with ET = 0.14 eV is shown in Figure 3a. The majority of the population is collapsed into the bandheads. For comparison, a simulation of a 160 K ( ( E R ) = 0.014 eV) spectrum is displayed in Figure 3b. The low rotational energy is immediately apparent. (18) Bennett, R. J. M. Mon. Not. R . Astron. SOC.1970, 147, 35. (19) Jacobs, D. C.; Madix, R. J.; Zare, R. N. J . Chem. Phys. 1986.80, 5469.
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Figure 3. (a) A rotational spectrum of NO molecules with translational energy of 0.14 eV obtained by 1 + 1 MPI. The NO molecules are generated by 193-nm laser vaporization of a 30 K NO film. The spectrum is not normalized for small fluctuations in laser !hence or desorbed flux. (b) A simulated spectrum of NO molecules at 160 K rotational temperature ( ( E R )= 0.014 eV) for comparison.
After normalization, the lines of both the 111/2 and I I 3 / 2 states may be fit to a Boltzmann rotational temperature of 160 f 20 K;within the experimental error, the relative populations of the two spin-orbit states (F2/FI= 0.35 f 0.15) are also characterized by this temperature. This rotational energy (0.014 eV) is substantially lower than the =O. 1-eV width of the translational energy distribution, Le., the energy spread in the direction of the flow. We observe similar results for ET = 0.56 eV NO molecules, which can be characterized by a rotational temperature of 180 f 20 K ( ( ER) = 0.017 eV). The spin-orbit "temperature", however, is higher, T 500 K (F2/F1= 0.7 f 0.20). The translational energy width in this case is -0.4 eV, which is again much greater than the spread in rotational energy. The results most likely represent a convolution of initial desorption dynamics and postcollisional cooling; they do not appear to indicate a local equilibrium between translation and rotation and, for molecules with ET = 0.56 eV, the spin-orbit states as well. These points will be addressed in more detail in a future publication.20 Several groups have reported laser-induced desorption experiments that produce rotationally and translationally hot NO molecules from well-characterized surface^.^*^'^^^ There are also several examples of a translational-rotational disequilibrium occurring in NO molecules for laser desorption from near-monolayer coverages of NO on Ni23and €'to.%For example, Burgess et alez3observe rotationally cold non-Boltzmann distributions which can be described by 425 K in rotation and 1200 K in translation (ET = 0.2 eV). Similarly, Budde et al.24observe 625 K in rotation and 0.2 eV in translation. Here, we observe 160 and 180 K in rotation, for 0.14 and 0.56 eV in translation, respectively. Thus,
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(20) Cousins, L. M.; Levis, R. J.; Leone, S. R. J . Chem. Phys., submitted for publication. (21) Weide, D.; Andresen, P.; Freund, H. J. Chem. Phys. Lett. 1987,136, 106. (22) Buntin, S. A.; Richter, L. J.; Cavanagh, R. R.; King, D. S . Phys. Rev. Lett. 1988, 61, 1321. (23) Burgess, D.; Cavanagh, R. R.; King, D. S . J . Chem. Phys. 1988.88, 6556
(24) Budde, F.; Hamza, A. V.;Ferm, P. M.; Ertl, G.; Weide, D.; Andresen, P.; Freund, H.-J. Phys. Rev. Lett. 1988, 60, 15 18.
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the ejection of fast N O molecules can result in a substantial disequilibrium between the rotational and translational energy, with a factor of 3-30 less energy in rotation compared to the translational energy. These rotational energies are much lower than those observed in the laser ablation of PMMA’O or sapphire,25 although a disequilibrium has been observed in these experiments as well. The production of high yields of translationally fast molecules, with rotational excitations that are characterized by such low temperatures, is suggestive that collisional cooling after the desorption (adiabatic expansion) may be partly responsible for the mechanism. The result may be a common feature in photoejection phenomena and is consistent with the calculations of Noorbatcha
et al.13-15There is also the interesting possibility that a greater amount of energy is initially transferred into translation than rotation during the desorption, as might be the case if short-range impulsive collisions dominate the ejection on-axis. A dynamic mechanism such as an electronic or vibrational-to-translational energy transfer would be consistent with the observation that the velocity widths are narrower than those reported in Noorbatcha’s calculations. It is also possible that localized pockets of excited molecules produce highly directed “jets” of gas which undergo stronger adiabatic expansions. A full study is under way to address the relative contributions of thermal heating, hydrodynamic cooling, and dynamical mechanisms which can influence the rotational and translational energy contents of the laser-vaporized fast molecules.20
(25) Dreyfus, R. W.; Kelley, R.; Walkup, R. E. Appl. Phys. Lett. 1986, 2, 1478.
Acknowledgment. This work was generously supported by the U S . Army Research Office.
Infrared Photodissociation of Ar,’ P. G. Lethbridge, C. A. Woodward, R. Hallett, J. E. Upham, and A. J. Stace* School of Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ. U.K. (Received: April 24, 1989)
A supersonic nozzle has been used to generate Ar2+in the ion source of a mass spectrometer via the electron impact ionization of neutral dimers and clusters. Following mass selection, the dimer ion has been photodissociated with infrared radiation from a COz laser in a crossed-beam configuration. By changing the plane of polarization of the laser radiation, two electronic transitions have been identified. One transition corresponds to excitation to a repulsive state, a process that has been established from previous photodissociation studies using visible radiation. It is proposed that the second transition is associated with excitation to quasibound levels in a shallow ion-induced-dipole well, situated in the long-range region of an otherwise repulsive electronic state.
Introduction Considerable attention has been devoted to studying the electronic spectroscopy of dimer ions of the inert gases, and of Ar2+ in p a r t i ~ u 1 a r . l ~Much of this work has been directed toward investigating the role that the electronic excitation of these ions may play in excimer lasers, where it has been proposed that such a process could influence the absorption properties of the laser medium.2 More recently, attention has again focused on the spectroscopic properties of Arz+,5but this time in conjunction with the proposal that the ion might act as the central core of an argon ion To date all the spectroscopic studies, both on Ar2+and on argon ion clusters, have been concerned with electronic transitions promoted by either visible or UV radiation.I4 The purpose of this letter is to report the results of some preliminary experiments where electronic transitions have been induced through the interaction of Ar2+ with infrared radiation.
Experimental Section The apparatus consists of a combination of a supersonic nozzle and a modified double-focusing “reverse geometry” VG ZAB-E (1) Miller, T.M.; Ling, J. H.; Saxon, R. P.; Moseley, J. T. Phys. Reu. A 1976, 13, 2171. (2) Moseley, J. T.; Saxon, R. P.; Huber, B. A.; Cosby, P. C.; Abouaf, R.; Tadjeddine, M. J. Chem. Phys. 1977, 67, 1659. (3) Lee, L. C.; Smith, G. P.; Miller, T. M.; Cosby, P. C. Phys. Rev. A 1978, 17, 2005. (4) Lee, L. C.; Smith, G. P. Phys. Rev. A 1979, 19, 2329. (5) Levinger, N. E.; Ray, D.; Alexander, M. L.; Lineberger, W. C. J. Chem. Phys. 1988, 89, 5654. (6) Stace, A. J.; Moore, C. Chem. Phys. Lett. 1983, 96, 80. (7) Haberland, H. Surf.Sci. 1985, 156, 305. (8) Stace, A. J. Chem. Phys. Lett. 1985, 113, 355.
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mass spectrometer. Details of the apparatus have been given previo~sly;~ the only additional feature being a line-tunable COz infrared laser (Edinburgh Instruments PL4). Neutral dimers and clusters were generated by the adiabatic expansion of argon through a 150-pm-diameter nozzle and collimated by a l-mmdiameter skimmer situated 5 cm from the nozzle. A further 30 cm downstream from the nozzle, the dimers and clusters entered the ion source where they were ionized by electron impact ( ~ 7 0 eV). Following extraction from the ion source, Ar2+ was mass selected using the magnet. The ion beam was then crossed at right angles with IR radiation from the laser and any resultant photofragmentation detected via an electric sector which was tuned to transmit Ar’. By scanning the electric sector voltage, the kinetic energy spread of Ar+ in the laboratory frame could be measured. The laser beam was modulated to eliminate interference from any Ar+ background, which could have originated either from the predissociation of metastable Arz+ generated in the ion source or from collision-induced dissociation. A background pressure of 6 X 10+ mbar in the flight tube of the mass spectrometer ensured that interference. from the latter process was kept to a minimum.’*
Results and Discussion Buck and Meyer” have shown that, following electron impact ionization, approximately 40% of the neutral dimers become Ar2+. Obviously, a further major source of Arz+ is from the breakdown of larger ion clusters. In contrast to the present experiments, the Arz+in previous spectroscopic studies came from three-body ion association rea~ti0ns.l~ Therefore, ions from the two sources could (9) Lethbridge, P. G.; Stace, A. J. J. Chem. Phys. 1988.89, 4062. (10) Stace, A. J.; Lethbridge, P. G.; Upham, J. E.; Wright, M. J . Chem. Phys. 1988, 88, 483. ( 1 1 ) Buck, U.; Meyer, H. J . Chem. Phys. 1986, 84, 4854.
0 1989 American Chemical Society
