J. Phys. Chem. 1992,96,4615-4620 nickel counterparts, but since most of the tungsten atoms are consumed by the formation of the oxides, only a maximum of one-third of the surface carbon can end up bonded to tungsten while the rest must be adsorbed on the nickel surface. The formation of some nonstoichiometric trinary interstitial phases cannot be completely ruled out based exclusively on our data.27 The values for the free energies of the reactions leading to the formation of oxides on the surface are quite negative at low temperatures but change sign below 800 K, and since our experiments are done under conditions far from equilibrium (pressures around 10-Io Torr), these oxides are expected to decompose as the sample temperature is increased. This is indeed the case: between 600 and 700 K the carbon and oxygen atoms on the surface recombine and desorb as carbon monoxide, and most of the surface tungsten is reduced to the metallic state; above 700 K all that is left on the surface is about 20% of a monolayer of tungsten atoms, from which a t least 85% is in a zero valence state. The reduction is not complete, however, since the remaining 15% is in a high oxidation state, somewhere between W+5 and W+6, and a small amount of atomic oxygen is present on the surface as well (0, = 0.10). Finally, tungsten diffuses into the nickel bulk above 1100 K. On the basis of the results presented above, we propose that the steady-state deposition of tungsten films at temperatures below 700 K using tungsten hexacarbonyl could result in the growth of a partially oxidized sample with interstitial carbon incorporated within the film lattice. This hypothesis is consistent with some of the early observations reported in chemical vapor deposition experiments using this precursor. It has been shown that temperatures above 1200 K are required for the preparation of the pure metal at atmospheric pressures unless the carbonyl is fed in a highly diluted mixture with hydrogen, but unfortunately gas-phase nucleation precludes the formation of dense adherent deposits at those high t e m p e r a t ~ r e s . Elemental ~~ analysis of films (45) Vapor Deposition; Powell, C. F., Oxley, J. H., Blocher, J. M., Jr., Eds.; John Wiley and Sons: New York, 1966.
4615
deposited a t low temperatures also indicates that the d e p o s i t e d carbon and oxygen are usually present in about equal a m ~ u n t s . ~ . ~ ~ Even though the chemical nature of the final product is still controversial, our results indicate that the codeposited oxygen forms a mixture of tungsten oxides while the carbon is probably trapped in the metal interstices. We also found that heating the sample to higher temperatures under vacuum results in the desorption of carbon monoxide and in the reduction of the surface tungsten back to its metallic state. These results suggest that cycles where dosing a t low temperatures are followed by periodic annealing to higher temperatures could result in the deposition of cleaner f h with good mechanical properties. The use of reducing agents during the growth is also believed to improve the quality of the final films; we are presently exploring these ideas in more detail.
Conclusions We have studied the thermal chemistry of tungsten hexacarbonyl chemisorbed on Ni( 100) surfaces by using TPD and XPS. Low-temperature adsorption is mostly molecular, but total decarbonylation takes place below 300 K. The carbon monoxide resulting from this decomposition chemisorbs initially onto the clean nickel substrate but later desorbs or dissociates to form carbon and oxygen atoms. A mixture of tungsten oxides forms on the surface between 400 and 600 K together with coadsorbed atomic carbon, but around 700 K about 80% of the carbon and oxygen atoms recombine and desorb as CO, leaving behind metallic tungsten. Tungsten atoms diffuse into the nickel crystal bulk above 1100 K. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to Lam Research Corp. for support of this research. Registry No. W(CO)6, 14040-1 1-0; Ni, 7440-02-0; CO, 630-08-0. (46) Vogt, G. J. J. Vue. Sei. Technol. 1982, 20, 1336. (47) Warson, I. M.; Connor, J. A.; Whyman, R. Polyhedron 1989,8, 1794.
Tunable Diode Laser Measurements of CO Energy Distributions from Acrolein Photodissociation at 193 nm Paul C. Lessardt and Robert N. Rosenfeld*,f Department of Chemistry, University of California, Davis, Davis, California 95616 (Received: January 29,1992)
Rovibrational energy distributions and average translational energies for CO photofragments produced by the 193-nm photolysis of gas-phase acrolein have been measured. A tunable infrared diode laser system was used to probe the CO fragment. Energy release to the CO translational [Pl,o(16)J,rotational (u = 0), and vibrational degrees of freedom can be described by the effective temperatures 1380,2750, and 2230 K, respectively. A simple phase space model, consistent with a two-step mechanism where radical pair formation occurs prior to the formation of free CO, accurately predicts rotational and vibrational energy distributions.
I. Introduction The study of molecular photofragmentation dynamics has prop& in recent yeam to a level where detailed physical models can be compared to soph&icatd experimental rsults.1 One g a l Present address: Aqua Terra Technologies, Inc. 3950 Bushkirk Ave., Walnut Creek, CA 94596. *&went address: tbd Analysis, Inc., 2261 Federal Ave., Los Angeles, CA 90064. *To whom correspondence should be addressed.
of these studies is to understand the forces that control the dynamics Of a chemical reaction. At this point in time, the theory of photodissociation is sufficiently advanced that comparison with experiment can often be used to improve our understanding of potential energy surfaces (PESs) and excited-state dynamic^.^.^ (1) Levine, R. D.; Bernstein, R. B. Molecular Reaction Dynamics and Chemical Reactivity; Oxford University Press: London, 1987. ( 2 ) Reisler, H.; Wittig, C. Annu. Rev. Phys. Chem. 1986, 37, 307.
0022-3654/92/2096-4615%03.00/00 1992 American Chemical Society
Lessard and Rosenfeld
4616 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
In this paper, we present a model for the mechanism of the photodissociation of acrolein based on partitioning of excess energy into product translational, vibrational, and rotational degrees of freedom. Simple unimolecular decompositions (A B + C) serve as prototypes for testing, modifying, and reformulating dynamical models in chemistry. For small polyatomic reactants (three to five atoms), it often possible to compute the product state distributions for both fragments based on energy and angular momentum con~ervation.~Small triatomic (ICN, H 2 0 , H2S)4-6 systems offer the advantage of simplifying potential energy calculations, whereas tetratomic (e.g., NCNO, H202)7-8 and larger (e.g., CH,CO, CH3CH3C0)9J0systems introduce more degrees of freedom that limit computational accuracy. Various models have been developed for comparison with experimental results. Phase space theory (PST) has successfully described nascent product state distributions when dissociation into free radicals proceeds via a loose transition state with little appreciable barrier other than the endoergicity."J2 A variation of this theory, the separate statistical ensembles (SSE) method,2J3 allows some restriction on energy flow between parent vibrations and rotations. We recently developed early phase space theory13 (EPST) where vibrational energy release occurs early in the exit channel taking into account steep exit potential effects, whereas rotational and translational energy release occurs later in the exit channel (applicable to ion-molecule reactions or reactions with highly polarizable fragments). A RRKM statistical model, developed by Marcus and Klippen~tein,'~ predicts moderately greater vibrational excitation than the PST model. Impul~ive'~ and modified impulsive models16 are sometimes applicable when a dissociation occurs via a repulsive excited-state surface and little vibrational excitation is observed. Recently, researchers have used a model based on the rotational reflection principle" to map out rotational distributions in small molecular systems where the PES is known. Here, we develop a phase space model for the description of a two-step photodissociation reaction. Several researchers have investigated acrolein photodissociation dynamics at 193 nm. Shinohara and Nishi18measured an average translational energy of the HCO radical in a pulsed supersonic molecular beam using a time-of-flight system. Lin and co-workers measured acrolein's UV absorption spectrum, photochemical yields, and CO vibrational energy distributions at photolysis wavelengths of 1190 and 193 nm.19*24 In this paper we report nascent CO translational and rovibrational energy distributions obtained from acrolein (CH2CHCHO) photolyzed at 193 nm. We find that a phase space model
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(3) Koplitz, B.; Xu, Z.; Baugh, D.; Buelow, S.; Hausler, D.; Rice, J.; Reisler, H.; Qian, C. X. W.; Noble, M.; Wittig, C. Faraday Discuss. Chem. Soc. 1986, 82, 125. (4) Black, J. F.; Waldeck, J. R.; Zare, R. N. J. Chem. Phys. 1990, 92, 3519. Joswig, H.; O'Halloran, M. A.; Zare, R. N.; Child, M. S. Faraday Discuss. Chem. SOC.1986, 82, 79. (5) Weiner, B. R.; Levene, H. B.; Valentini, J. J.; Baronavski, A. P. J. Chem. Phys. 1989, 90, 1403. (6) Engel, V.;Schinke, R.; Staemmler, B. J . Chem. Phys. 1988,88, 129. (7) Qian, C. X. W.; Noble, M.; Nadler, I.; Reisler, H.; Wittig, C. J. Chem. Phys. 1985,83, 5573. (8) Scherer, N. F.; Zewail, A. H. J. Chem. Phys. 1987, 87, 97. Butler, L. J.; Ticich, T. M.; Likar, M. D.; Crim, F. F. J. Chem. Phys. 1986,85,2332. (9) Nesbitt, D. J.; Petek, H.; Foltz, M. F.; Filseth, S. V.;Bamford, D. J.; Moore, C. B. J. Chem. Phys. 1985,83, 223. (10) Trentelman, K. A.; Kable, S. H.; Moss, D. B.; Houston, P. L. J: Chem. Phys. 1989, 91, 7498. (1 1) Pechukas, P.; Light, J. C. J. Chem. Phys. 1965,42,3281. Pechukas, P. R.; Rankin, R.; Light, J. C. J. Chem. Phys. 1966, 44, 794. Klotz, C. J. Phys. Chem. 1971, 75, 1526. (12) Holland, J. P.; Rosenfeld, R. N. J. Chem. Phys. 1988, 89, 7217. (13) Wittig, C.; Nadler, I.; Reisler, H.; Noble, M.; Catazarite, J.; Radhakrishnan, G. J. Chem. Phys. 1985,83, 5581. (14) Klippenstein, S. J.; Marcus, R. A. J. Chem. Phys. 1989, 91, 2280. (15) Busch, G. E.; Wilson, K. R. J. Chem. Phys. 1972, 56, 3626. (16) Levene, R. D.; Valentini, J. J. J. Chem. Phys. 1987, 87, 2594. (17) Schinke, R. J . Chem. Phys. 1990, 92, 2397 and references therein. (18) Shinohara, H.; Nishi, N. J. Chem. Phys. 1982, 77, 234. (19) Fujimoto, G. T.; Umstead, M. E.; Lin, M. C. J. Chem. Phys. 1985, 82, 3042.
HrNc Alipmrnt Laser
1
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Figure 1. Instrumentation used to perform time-resolved infrared absorption experiments.
41
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Relative CO Absorption Intensity
Figure 2. Relative CO [P1,2(16)] absorption intensity versus UV photolysis fluence at 193 nm.
is in accord with a simple, two-step free radical intermediate dissociation mechanism. 11. Experimental Section The photolysis source is a fiber-optically triggered Lambda Physik EMG 101 excimer laser operated at 193 nm (ArF*). The beam was mildly focused (Optics for Research, f = 10 m) to accommodate the extended beam distance (ca. 6 m). A temperature/current controlled tunable diode laser (Laser Photonics, L5622-1990) provides infrared coverage from 2050 to 2150 cm-l. To ensure single-mode operation of the tunable diode laser, a 0.5-cm-l band-pass monochromator was used. The UV beam (ca. 1 cm2) overlaps the IR beam (ca. 0.7 cm2) as the two beams are coaxially propagated through a 1.4-mPyrex cell with CaF, windows (see Figure 1). A ZnS window diverts ca. 10% of the IR beam into a reference cell for continuous frequency verification. The IR beam is focused on a HgCdTe detector with a f / l KCl lens, and the UV is diverted through the cell via a dichroic matter (Acton Research). A high-voltage dc discharge in a N,/He/CO mixture provided reference frequencies for CO hot band transitions. Additionally, a solid Ge etalon provided relative frequency markers. Typically, 50 mTorr of acrolein was flowed through the cell. When the CO vibrational distribution was measured, 1 Torr of nitrogen buffer gas was used to relax the rotational distribution. Care was taken while monitoring thermally populated levels to avoid any excessive CO buildup, which could attenuate signal. Fluences were 5-6 mJ/cm2 at 193 nm. UV fluence was varied for a power dependence study by placing screens of various mesh size in the laser beam path at the output of the excimer beam path. Following photolysis, transient absorption of the IR beam was monitored with a 1-ps risetime HgCdTe detector (InfraRed Associates, Inc.). These signals were amplified, digitized, and averaged (LeCroy TR8837F transient recorder). Experiments
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4617
Acrolein Photodissociation at 193 nm
-C
-.- ,
-3.5 0
1
2
0
3
Vibrational Quantum Number
Figure 3. Vibrational energy distribution of CO following the 193-nm photolysis of acrolein. Mixing 50 mTorr of acrolein with 1 Torr of N, yielded an effective vibrational temperature of 2230 f 310 K.
distribution model relative vibrational distributions, N(u)
0 1
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2
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1
0.247 0.041 statistical
vibrational rotational (u = 0) translational P,,O(W p I ,0(8)
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obsd temp 2230 (h310) 2750 (f280) 1380 (f220) 1200 (h310) 1050 (f310)
av energy
model temp
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1955 3070
I
.
200
1
2500 2000
the P2,,(10) CO photofragment transition (in 1 Torr of N2) versus photolysis laser fluence 2.5 ps after photodissociation. The 2.5-ps time interval was chosen since the absorption amplitude is close to maximum and little decay (molecules translating out of the probe beam volume and/or vibrational relaxation) has occurred. A linear dependence on the photolysis fluence up to 6 mJ/cm2 is observed, suggesting a single-photon photolysis process. This is an important consideration since Nishi et al.'* observed CH and CO via a two-photon mechanism at 193 nm. Vibrational Energy Distribution. In order to determine nascent vibrational energy distributions, 1 Torr of N2buffer gas was mixed with the sample. These conditions ensure complete rotational relaxation while preserving the vibrational populations (within a5 ps).*O Observed populations (Table I) did not significantly change if the buffer gas pressure was increased to 2 Torr of N2. The relative populations were determined using21 the equation
where J is the rotational quantum number, u,,, is the highest vibrational level observed, and N ( D )is the normalized signal intensity. The maximum observable vibrational level, u,,, = 2 [for the PdJ3(J) rovibrational transition, P3,2(8)],was limited by signal-to-noise. The observed populations remained essentially the same (within one standard deviation) when calculated at 0.5-ps intervals from 1.5 to 3.0 ps following photolysis, indicating slow (20) Brkhignac, Ph.; Picard-Berselli, A.; Charneau, R.; Laudy, J. M. Chem. Phys. 1980,53, 165. (21) Houston, P. L.; Moore, C. B. J . Chem. Phys. 1976, 65, 757.
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.
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,
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.
J(J+l) Figure 4. CO rotational distribution measured at 5 fis following the 193-nm photolysis of acrolein. The slope of this line corresponds to an effective rotational temperature, T, = 1100 f 280 K. Pressure of acrolein = 50 mTorr.
TABLE I: Comparison of Experimental CO Temperatures (K) and Energies (kcal/mol) with Statistical Model Values obsd statistical U
I
100
15 (
I
I
20
25
pried
Figure 5. Rotational relaxation of CO(u=O) as a function of time. The rotational temperature exponentially decays with time. The total pressure is 50 mTorr.
(on our time scale) vibrational relaxation. A Boltzmann plot, In N(u) versus vibrational quantum number, is used to represent the data in Figure 3. The slope, determined by least-squares regression, yields an effective vibrational energy temperature, T, = 2230 f 310 K. Since energy conservation limits the upper vibrational state population, in this case to u = 6, it should be pointed out that this is not a true thermodynamic temperature. Rotational Energy Distributions. CO(u=O;J=7-24) relative intensities were measured for acrolein flowing neat at 50 mTorr. Not all rotational transitions could be observed due to TDL tuning limitations. The rotational energy distribution at 5 ps following photolysis is represented by a Boltzmann plot of In [signal/(2J l ) ] versus J(J 1) in Figure 4. As with vibrational measurements, the slope of the best fit line yields an effective rotational temperature. In Figure 5 rotational temperatures at various times following photolysis are shown. Note that the early time data (
