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J. Phys. Chem. 1991,95,8385-8387
ArF Excimer Laser Ablation of Ca(OH), Films: Observation of Unusual Rotational OH Populations Subbash Deshmukh,? Erbard W. Rothe,* and Gene P. Reek* Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48202 (Received: February 5, 1991)
Laser induced fluorescence is used to analyze for the distribution of rotational states of OH (v" = 0) that are emitted from Ca(OH)2films after interaction with 193-nm light. The results depend upon fluence: At large values, a Boltzmann distribution applies reasonably well. At smaller fluence, we observe alternations of population with increasing rotational quantum numbers.
Introduction
There are now bases for the understanding of state-to-state photodissociation from simple molecules. Andmen and Schinke'J have discussbd the fine details of the OH that originates from state selected H20. The population distributions in the X doublets are of particular interest here. Following ref 1, we call these a+ and r-. They correspond physically to the cases where the unpaired p r orbital tends either to lie in the plane of rotation of the OH or to be perpendicular to it, respectively. Reference 1 has results of calculations and of experiments. These show the distributions of OH states to be very sensitive to the rotational state of the H 2 0 and to be very non-Boltzmann. For example, their Figure 7 has interesting product distributions that are produced from the rotational ground state (Ow) of vibrationally excited (0,0,1) water. The present paper describes observations of rotational distributions that arise from the interaction of 193-nm light with Ca(OH)2films. This is the first time that drastic deviations from Boltzmann behavior, similar to those from H20, have been reported from a solid. Our experiment has an advantage over that with H 2 0in that no initial rotational state selection is needed to observe such effects. Unfortunately, we cannot do the types of detailed calculations that were possible with isolated H 2 0 molecules.
Experimental Section Figure 1 is a schematic diagram of the experiment. Excimer laser light, at 193 nm, ablates molecules from a Ca(OH)2 film. A plasma plume results that is analyzed, by laser induced fluorescence (LIF), for various OH rotational levels in the or'= 0 level of the X state. The probe light is in the 306.8-307.4-nm range, and the detection is usually near 309 nm. The Ca(OH)2is maintained at Torr. The plume is visible to the eye. It becomes dimmer at lower fluence. The target is prepared by coating a glass substrate with a Ca(OH)2-methanol slurry and then baking at 110 OC in air. The resulting film, which is -0.2 mm thick, is kept under vacuum for =10 h before it is ablated. The film can be moved relative to the excimer beam so that a fresh surface can be maintained. The ablation laser, a Lambda-Physik EMG IOZMSC, has an -1-m wavelength spread and is randomly polarized. The target fluence is adjusted with an aperture and lenses. The probe light is generated by frequency doubling (in KDP) the output of an excimer-pumped dye laser (Lambda-Physik, Models 200E and FL2002) that operates with sulforhodamine-B. The probe beam has a diameter of 4 . 8 mm, a wavelength spread of -0.02 nm, and an energy of 50.2 mJ/pulse. It passes 15 mm from the Ca(OH)2. The energy is measured with a photodiode. The probe light is adjusted to come 4.2 ps after the ablation light because that yields the maximum LIF signal. In order to localize the light emission, we also measured inside a 5m"miameter baffle tube. Use of the baffle did not alter the observed times. Department of Chemistry, Tulane University, New Orleans, LA 701 18. 'Department of Chemistry, Wayne State University, Detroit, MI 48202.
The OH transitions, which are in the 0-0 band, were identified with the use of tables in Dieke and Cros~white.~We also use their notation (except that we substitute N for their K ) : e.g., Q,(N"), where i = 1 or 2 for the spin components of rotational level N", and J" = NN ' I 2and J N = N"respectively. Q and R branches refer to changes in N. A grating monochromator (1/4-mJarrel-Ash), with a photomultiplier output, accepts fluorescence light that lies in a small solid angle centered on a line that passes through the intersection of the laser beams and is perpendicular to them. The electric vector of the linearly polarized probe light is parallel to that line. The monochromator is set to have a 3-nm pass-band. It serves to remove light from the excimer laser and from transitions outside the 0-0 band. It is also sometimes adjusted to exclude scattered light from the probe laser. The photomultiplier output goes to one input of a boxcar that is triggered by the probe laser. The effect of scattered probe light is removed with the boxcar's gate. The photodiode signal goes to a second boxcar input. The ratio of these two signals represents a fluorescence yield normalized to laser intensity. These LIF signals, for each transition, are averaged over 32 laser pulses. Corrections to the raw data are made with known' Einstein B and A factors for excitation and for all allowed fluorescence transitions (whose wavelengths are within the pass-band of the monochromator), respectively. We measured both R and Q transitions. These originate from the a+ and r- A doublet states, respectively. The polarized probe light creates an anisotropy of excited states, in addition to any OH alignments that may be intrinsic to the ablation. That means the fraction of the R-branch-induced light that reaches the monochromator is different from that from Q-branch excitation. The grating efficiency is also polarization dependent. Accordingly, we can not deduce absolute population ratios, 4, for a+ and rstates.
+
Results and Discussion In contrast to the photodissociation of isolated molecules, the interaction of UV lasers with surfaces can yield a number of phenomena whose relative importance depends upon laser fluence Several groups- have studied such laser ablation. In general,
(e.
(1) Andresen, P.; Schinlie, R. In Molecular Phorodissociarion Dynamics; Baggott, J. E.; Ashfold, M. N. R., Eds; Royal Society of Chemistry: London, 1987; Chapter 3. Set also: VanderWaal, R. L.; Scott, J. L.; Crim, F. F.J. Chem. Phys. 1991,94, 1859-1867. (2) Schinke, R. Annu. Rev. Phys. Chem. 1988, 39, 39-68. (3) Diekc, G. H.; Crosswhite, H. M. J. Quanr.Specrrarc. Radar. Transfer
1962, 2,97-199. (4) Goldman, A.; Callis, J. R. J . Quanr. Specrrarc. Radiat. Transfer 1981, 25, 11 1-135. ( 5 ) Dreyfus, R. W.; Kelly, R.; Walkup. R. E. Appl. Phys. Lcrr. 1%, 49, 1478-1 480. (6) Deshmukh, S.; Rothe, E. W.; Reck, G.P.; Kushida, T.; Xu, 2. G. Supercond. Sci. Technol. 1989, I , 319-323; Appl. Phys. Lctr. 1988, 53, 2698-2700. Deshmukh, S.; Rothe, E. W.; Reck, G. P. J . Appl. Phys. 1989, 66, 1370-1374.
0022-365419112095-8385$02.50/0 0 1991 American Chemical Society
8386 The Journal of Physical Chemistry, Vol. 95, No. 21, 1991
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Figure 1. Experimental arrangement of (top) overall view and (bottom) detail of interaction geometry, with probe laser beam 15 mm from sample surface. 2000
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Rgwe 4. Data for Ql(N”)and Rl(N’?, shown by circles and squares, respectively, at a fluence of 250 mJ/cm2. Here J” = N” + l/2.
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Figwe 2. Plot of measured R,(N”)intensities, divided by degeneracy U N + 1, versus N”. Here N” is the angular momentum without spin and Jrr = Nr’ + The circles and squares are for fluences of 250 and 500 mJ/cm2, respectively.
the process is initiated by a combination of thermal and photochemical bond breaking. At small values of F, it is expected that individual bonds are broken. Ablation at large F, which has practical uses for photochemical etchmg, also produces “explosive* (7) Tabares, F. L.; Marsh, E.P.;Bach, G. A.; Cowin, J. P.J. Chem. Phys. 3981.86, 738-744. (8) Kutmer, J.; Lindeke, G.; Welge, K.H.;Feldmann, D.J . Chem. Phys. 1989,90,548-555. (9) Cousins, L. M.;Leone, S . R. J . Murer. Res. 1988, 3, 1158-1168. Cousins, L. M.;Levis, R. J.; Leone, S. R. J. Chem. Phys. 1989, 91, 5731-5142.
desorptions that lead to plasma formation. A measurement of the intensity of the RA5) line, as a function of the delay between the two lasersand at F = 250 mJ/cm2, shows that the translational energies of OH are mainly in the range 1.0-1.8 eV. A fit to the distribution has a maximum near 1.3 eV . The intensity ratio R2(2)/R2(3) is different at F = 90, 130, and 190 rd/cm2. The LIF signals at those fluences were too small to obtain reliable data over a range of N”. Accordingly, all data cited below are at either F = 250 or 500 mJ/cm2. These are called low F o r high F, respectively. Figure 2 shows an OH distribution obtained via the RI(N”) lines at these two fluences. The high F data are fairly Boltzmann-like, and yield a rotational temperature of =720 K. In contrast, the low F data are very non-Boltzmann. Figure 3 shows similar results for the R2(N’9lines. The high F data are significantly more Boltzmann-like than those at low
F. We infer that non-Boltzmann behavior occurs at very small F and that much of this character is retained at the measured low F. At high F,the distribution appears substantially more thermal, either because of (a) collisions within the plasma plume or (b) a high-temperature surface desorption. The former is more likely because 720 K is too small to generate thermal desorption of OH from Ca(OH)2. In any case, it is the low F data that are of primary interest here. However, the nearly Boltzmann-like high F data do show that the unusual distributions that are observed at low F a r e not
J. Phys. Chem. 1991,95,8387-8393
caused by errors in the LIF method or in the subsequent data analysis, Figure 4 shows data, at low F, for Q,(N”) and R,(N”). The unusual structure of the RI branch has already been noted. In contrast, the QI branch is reasonably Boltzmann-like and yields a temperature of 4 8 0 K. As was pointed out in the experimental section, we cannot deduce absolute A doublet population ratios (r-/r+) = 4 from our data. However, there is a big decrease in 4 at larger N”. Because it is likely, particularly at high F, that the OH that have larger ”’are thermalized, it seems plausible to assume that 4 = 1 there. If this is correct, it implies a preponderance of the r- state in nascent OH at low N”. We have similar results and conclusions with Q2(N19and R2(N’? at low F. However, because of line overlaps in the Q2 branch, we feel more secure with Q1(N”)and RI(N”).
Conclusions The main point here is to show the existence of these unusual population distributions and to encourage further work. The present ‘low F results do not represent nascent OH, because the population ratios R2(2)/R2(3) are still changing at even lower F.
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There is no reason why better experiments cannot be designed that (a) use smaller F, (b) operate at closer probetarget distances, and (c) use a detection system whose response to polarized light is known, so that absolute A doublet populations can be deduced. Another interesting study would be the correlation of rotational states with translational energies. Finally, once the experimental facts are established, an explanation should be sought for the origin of such fine structure in the distributions. Because this depended so sensitively upon initial conditions in H20, we did not attempt even a qualitative interpretation. Acknowledgment. E.W.R. was Richard Bernstein’s first graduate student in the molecular beam area (1955-59). He gratefully acknowledges the privilege of having worked with Bernstein as his student, as a colleague, and as a friend. Bernstein was a continuing inspiration, and he is sorely missed. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the WSU Institute for Manufacturing Research for partial support of this work. Registry No. Ca(OH)2, 1305-62-0; OH, 3352-57-6.
Effect of Surface Temperature on Coiilsion- Induced Dissociation of I-C3F7N0Scattered from MgO(lOO), GaAs(100), and Ag(lll)+ P. S. Powers,: E. Kolodney,t L. Hodgson,ll G. Ziegler, H. Reisler,* and C. Wittig* Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482 (Received: March 26,1991;In Final Form: June 7, 1991)
The surface temperature (T,)dependence of hyperthermal molecule-surface collision-induced dissociation (CID) was studied at incident kinetic energies of 1-5 eV and T,= 300-760 K. i-C3F7N0was impulsively scattered from Mg0(100), GaAs(100), and Ag( 11 1) single crystals under ultrahigh vacuum conditions. CID yields, as well as internal energy distributions, were obtained for the product NO fragment, employing state-selectivelaser ionization detection. CID yields were found to increase strongly with T,,whereas NO rotational and spin-orbit excitations were relatively insensitive to T,.The CID yield was largest for scattering i-C3F7N0from MgO( 100)and smallest for Ag( 11 1). It is shown experimentally that surface morphology and stoichiometry do not change significantly with T, in the range studied here. The experimental results are rationalized in terms of coupling of the translational motion of the incident molecule to surface phonons.
I. Introduction The direct inelastic scattering of a molecule from a singlecrystal surface constitutes one of the most basic processes in gassurface interactions and is quite often the initial entrance channel step for more complicated reactive processes. Understanding the dynamical details of the mechanisms involved, as well as their dependence upon initial conditions, is prerequisite to obtaining a detailed understanding of more complex processes whose evolution is influenced by the molecule-surface excitations and deformations brought about by the impulsive collision. Recently, considerable attention has been devoted to molecule-surface scattering, with emphasis on energy transfer from the translational energy of the incoming molecule into crystal vibrations and internal degrca of freedom of the scattered molecule. In particular, several important direct processes have been observed for polyatomic molecules scattered from MgO(100)’ and diamond( 1 1 1),24 including intramolecular excitations, collision-induced dissociation ‘Research supported by AFOSR and the ARO Center for the Study of Fast Transient Processes. !Department of Education Predoctoral Fellow. 8 Present address: Technion Institute of Technology, Haifa, Israel. I NSF Predoctoral Fellow.
0022-3654/91/2095-8387302.50/0 , I
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(CID) where one of the fragments is an atom (Le., I) or an open-shell molecule (i.e., NO), ionization, dissociative ionization, and energy transfer to the crystal. By their nature, the time scales for impulsive particle-surface interactions are very short, with memory of the incident momentum fully or partially retained. Quite often the surface, as seen by the rapidly approaching molecule, appears to be “vibrationally frozen” on the collision time scale; hence, some of the observables are not expected to be sensitive to the surface temperature. However, it should be pointed out that the effect of surface temperature in direct inelastic processes depends upon the nature and properties of the surface as well. For metallic surfaces, bulk temperature controls the density of charge carriers, and moleculesurface electronic interactions are expected to be sensitive to these changes and exhibit correspondingdependences. (1) Kolodney, E.; Powers, P. S.;Hodgaon, L.; Reisler, H.; Wittig, C. J. Chem. Phys. 1991, 94, 2330. (2) Danon, A.; Kolodney, E.; Amirav, A. Surf. Sct. 1988, 193, 132. (3) (a) Danon, A.; Amirav, A. J. Phys. Chem. 1989,93.5549. (b) Amirav, A. Comments At. Mol. Phys., submitted. (c) Danon, A,; Amirav, A. Isr. J. Chem. 1989. 29. 443. (4) Danon, A.;Amirav, A.; Silberstein, J.; Salman, Y.;Levine, R. D. J . Phys. Chem. 1989, 93,49.
0 1991 American Chemical Society
