1329
J. Phys. Chem. 1991, 95, 1329-1333
Surface Chemistry of Dlmethylalumlnum Hydride and Trimethylaluminum on Polycrystalline Aluminum Daniel R. Strongint and Paul B. Comita* IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 (Received: February 26, 1990; In Final Form: July 17, 1990)
The surface chemistry of both dimethylaluminum hydride (DMAH) and trimethylaluminum (TMA) on a polycrystalline aluminum surface has been investigated with temperature-programmed desorption (TPD), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). Under our experimental conditions, DMAH adsorbs on the aluminum surface in its trimeric form and TMA adsorbs primarily in a dimeric form. DMAH desorbs from the atomically clean aluminum surface at 235 K, and TMA desorbs at 200 K. Both DMAH and TMA dissociate readily on the aluminum surface, and analysis by SIMS shows that this decomposition leaves a partially methylated aluminum surface at temperatures up to 500 K. DMAH desorbs from the carbon-contaminated aluminum surface at about 200 K. Both TMA and DMAH are found to produce methane product during the TPD experiments. The TPD experiments also suggest that the surface reactions of DMAH produce TMA as a reaction product. However, the reactions that form methane and TMA product occur in minor yield under our experimental conditions. By AES the surface is comprised of both aluminum and carbon after either DMAH-TPD or TMA-TPD.
introduction The formation of aluminum thin films by chemical vapor deposition (CVD) has been accomplished with aluminum alkylaluminum hyrides? and molecular complexes of aluminum hydride.s The nature of the ligands bonded to the aluminum atom can influence the reaction channels available to the aluminum alkyl and can therefore influence the reaction channels in the gas phase as well as at the gas-solid interface. For example, the surface reactions of triisobutylaluminum have been investigated, and the @-hydrideelimination of the isobutyl group has been found to give rise to a low-energy reaction channel to form aluminum metaL6 Recently, dimethylaluminum hydride (DMAH) has been found to dissociate to form metallic a l u m i n ~ m .In~ contrast to this, the corresponding trialkylaluminum, trimethylaluminum (TMA), does not react to form metallic aluminum but instead forms aluminum carbide.' The surface reactions of DMAH and TMA may thus provide some insight into the reaction channels that are available to hydrides of aluminum alkyls which allow the deposition of pure metal films. The surface chemical reaction channels have been investigated for both TMA and DMAH by using temperature-programmed desorption (TPD), secondary ion mass spectrometry (SIMS), and Auger electron spectroscopy (AES). In our experiments, DMAH is adsorbed on clean polycrystalline aluminum as primarily the trimeric species and TMA as a dimeric species. Methane is detected as a decomposition product of both TMA and DMAH, during TPD. The yield of methane is about 20 times greater when DMAH is the reactant than when TMA is the adsorbed species. The experiments also suggest that the surface reactions of DMAH produce TMA as a reaction product. This reaction is the reverse of a process used for the production of dialkylaluminum hydrides from the trialkylaluminum species, hydrogen, and aluminum metaL8 This reaction may be a mechanism for removing methyl groups from the aluminum surface and the deposition of aluminum. However, both DMAH and TMA show substantial decomposition on the aluminum surface leaving a surface carbon coverage of approximately 0.5, suggesting that methane and TMA product are only minor carbon-removing reactions under these experimental conditions. Experimental Section All the research presented in this paper was performed in a bakeable, stainless steel, ultrahigh-vacuum (UHV) chamber. Through the use of ion and turbomolecular pumps, a working 'Permanent address: Department of Chemistry, State University of New York-Stony Brook, Stony Brook, N Y 11794.
0022-3654/9l/2095-l329$02.50/0
pressure of 5 x Torr was achieved. The chamber was equipped with a differentially pumped mass spectrometer (Extrel, Model (2-50)to analyze the residual gas in the chamber and to perform temperature-programmed desorption. An ion gun (PHI, Model CO4-300)was used to clean the sample and to provide an ion beam to perform secondary ion mass spectrometry. Auger electron spectroscopy was performed with a single-pass cylindrical mirror analyzer. A differentially pumped chamber connected to the main chamber was equipped with a molecular beam nozzle and a skimmer which was aligned with the sample. This doser produced a collimated effusive beam, and typically there was only a minor perturbation in the pressure of the main chamber while gases were adsorbed on the sample. The aluminum sample used in this study was obtained from Aldrich (99.99%), and it was cleaned by repeated 2000-eV Ar+ or Ne+ ion sputter and anneal cycles. The aluminum surface was considered clean when the intensity of the 67-eV Auger transition was at least IO times greater than the carbon 272-eV intensity and the oxygen 510-eV intensity was within background. The surface carbon concentration on aluminum, denoted by Oc throughout this paper, was determined by using the relative sensitivity factors for carbon and aluminum, which are given in ref 9. The sample was mechanically held on tantalum supports which were attached to '/4-in. copper feedthroughs. The copper feedthroughs were bored out to the end, and a '/s-in. Teflon tube was inserted to this point. Liquid nitrogen could be passed through the Teflon tubing so that efficient cooling to 90 K could be routinely achieved. The sample was heated resistively by passing current through the copper feedthroughs, and the temperature of the sample was measured with a chromel-alumel thermocouple ~~
( I ) Bent, B. E.; Nuzzo, R. G.;Dubois, L. H. Meter. Res. Soc. Symp. Proc. 1988, 101, 177. ( 2 ) Cacouris, T.; Scelsi, G.;Shaw, P.; Scarmozzino, R.; Osgood, R. M.; Krchnavek, R. R. App. Phys. Lett. 1988,52, 1865. (3) Green, M. L.; Levy, R. A.; Nuzzo, R. G.; Coleman, E. Thin Solid Films 1984, 114, 367. (4) Pierson, H. 0. Thin Solid Films 1977, 45, 257. ( 5 ) Baum, T. H.; Larson, C. E.; Jackson, R. L. Mater. Res. SOC.Symp. Proc., in press. ( 6 ) Bent, B. E.; Nuzzo, R. G.; Dubois, L. H. J. Am. Chem. Soc. 1989, 111.
I.
(7) Rytz-Froidevauz, Y.; Salathe, R. P.. Gilnen. - H. H. Phvs. Lett. 1981. 84A, 216. (8) Ziegler, K.; Kroll, W. R.; Larbig. W.; Steudel, 0. W. Ann. Chem. 1960,629, 5 3 . (9) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbmk of Auger Electron Spectroscopy;Physical Electronics Division, Perkin-Elmer Corp.: Eden Prairie, MN. 1976.
0 1991 American Chemical Society
1330 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
Strongin and Comita
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Figure 1. Mass spectral fragmentation of dimethylaluminum hydride (DMAH) at an electron energy of 30 eV.
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Figure 2. Mass spectral fragmentation pattern of trimethylaluminum (TMA) at an electron energy of 30 eV.
which was attached to the bottom of the foil. Secondary ion mass spectrometry was performed with an ion beam consisting of 2000-eV Ne+ ions. The base pressure in the UHV chamber rose to 5 X Torr when Ne was introduced into the ionization chamber in the ion gun. The ion current was extremely low ( < l o nA), and the intensity of the species removed during the S l M S experiment did not vary over at least I O min. Thus, the experiment was carried out in the static mode and the surface was not changed on the time scale of the SlMS experiments. Temperature-programmed SlMS experiments (TPSIMS) were also performed, in which sputtered surface ions were monitored as the sample was heated at a rate of 8 K/s. Figures 1 and 2 display the cracking pattern of DMAH and TMA, respectively, from ionization by electron impact at an ionization energy of 30 eV. To obtain these spectra, the TMA and DMAH beam was introduced directly into the mass spectrometer, allowing us to determine the nature of the cracking pattern of the alkylaluminum compounds used to dose the aluminum surface during our experiments. Data from previous work indicate that with our experimental configuration DMAH exists primarily in a trimeric form in the gas phase, with hydrogen bridge bonds linking the DMAH monomers,lO$ll and TMA is a dimer with methyl bridgesI2 in the gas phase. The cracking patterns thus obtained have used an experimental configuration with very short residence times in the gas phase and can thus be assigned as the DMAH trimer (parent ion, m / e 174) and TMA dimer (parent ion, m / e 144). TMA and DMAH was obtained from Alfa (electronic grade), and the reagents had to be purified (liquid N2 freeze-thaw cycles) (IO) Wartik, T.; Schlesinger, H. 1. J . Am. Chem. SOC.1953, 75, 835. ( I I ) Anderson, G . A,: Almenningen, A,; Forgaard, F. R.; Haaland, A. Chem. Commun. 1971, 480. (12) Smith, M . B. J . Organomef. Chem. 1972, 46, 31.
Figure 3. DMAH-TPD spectra as a function of adsorbate coverage. The low-temperaturepeak observed at coverages near 1 ML is due to DMAH desorption from carbon-contaminated aluminum.
from methane impurity which was constantly being produced in the reagent bottles, presumably from decomposition of the alkylaluminum compounds. Temperature-programmed desorption was performed by dosing the sample with either TMA or DMAH at about 90 K and then heating the sample at a rate of 8 K/s. With this experimental configuration, extraneous desorption of molecules from the. supports is minimized, since the copper feedthroughs are in direct contact with liquid nitrogen and do not heat substantially in the TPD experiment. In addition, the doser is directional and adsorption of compound on the supports is minimized. The mass spectrometer was controlled by CAMAC D/A modules and could be multiplexed so that up to I O masses could be monitored during a single TPD experiment, thus allowing a cracking pattern identification of the desorbing species. During the DMAH and TMA-TPD experiments presented later, I O cracking fragments were monitored for each run. The cracking patterns determined during the DMAH and TMA-TPD experiments matched those shown in Figures 1 and 2, respectively. Generally, reference to DMAH and TMA in the text implicity refers to their respective trimeric and dimeric forms. Determination of the adsorbate coverage is difficult, because the doser was directional and a pressure rise in the chamber was not observed. In order to estimate the coverages of both TMA and DMAH, a series of SlMS experiments were performed in which the increase in m / e 42 (AI(CH,+)) was monitored as a function of dose time for DMAH and TMA while the sample was held at 90 K. In the case of DMAH, the m / e 42 signal rises rapidly and saturates after 20 s. When the same experiment is performed for TMA adsorption, saturation of the m / e 42 signal intensity occurs at IO s. The difference is dosing times to reach saturation is related to coverage on the surface and sticking coefficients, as well as the vapor pressure of the reactant in the dosing chamber. The point at which the plateau in the S l M S signal occurs is referred to throughout this paper as 1 monolayer (ML). The surface coverage of DMAH and TMA is estimated by a linear function of the dose time, which is only an approximation to the true adsorbate coverage. Results and Discussion Temperature-Programmed Desorption. Figure 3 shows TPD spectra for DMAH adsorbed on atomically clean aluminum as a function of surface coverage. At DMAH coverages between 0. I and 0.5 M L only one desorption peak is evident, shifting in temperature from 198 to 220 K . This rapid increase in the temperature at peak maximum at low DMAH coverages suggest that there are strong attractive interactions between neighboring DMAH molecules on the aluminum surface even at relatively low coverage. At higher DMAH coverages ( 1 ML) an additional desorption state at about 200 K appears during TPD. The origin of this low-temperature peak was examined by accomplishing successive 2-ML DMAH-TPD spectra off an aluminum surface which has
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1331
Surface Chemistry of DMAH and TMA on A1
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Figure 4. Effect of carbon deposition on the DMAH desorption profile. The carbon surface concentration, e,, is determined by AES.
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Figure 6. AES spectra of aluminum cleaned by ion sputtering and the aluminum surface after DMAH-TPD and TMA-TPD experiments.
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Figure 5. TMA-TPD as a function of adsorbate coverage.
not been cleaned between the adsorption-desorption cycles (see Figure 4). The surface carbon concentration (8,) was determined by AES, before DMAH adsorption for that particular run (see Figure 4). The data clearly show that after each TPD experiment there is an increase in the surface carbon concentration, and this is associated with an increased amount of DMAH desorption from the low-temperature state. After four TPD experiments the carbon concentration reaches a steady-state value of 0.46 f 0.1, and there is no further change in the appearance of the TPD traces. These results suggest that the evolution of the low-temperature DMAH desorption peak is due to the deposition of a carbonaceous layer on the aluminum surface and that DMAH is more weakly adsorbed to this carbonaceous layer than on clean aluminum. Temperature-programmed desorption of TMA is shown in Figure 5 . TMA exhibits molecular desorption at about 200 K. The behavior of the TPD spectra with increasing coverage is indicative of TMA desorption from its condensed state. TMA adsorption on the aluminum surface, as is the case with DMAH, leads to the development of a carbonaceous overlayer. Figure 6 exhibits AES spectra of the "clean" aluminum surface and the aluminum surface after four successive DMAH or TMA-TPD cycles. Further adsorption and desorption of DMAH or TMA does not alter the AES spectra shown in Figure 6. Within experimental error, the carbon concentration after multiple DMAH or TMA-TPD is about 0.5 f 0.1. A reaction probability for the decomposition of DMAH or TMA could not be determined, since the flux of impinging reactant is unknown and AES is unable to differentiate between the substrate and deposited aluminum. The AES spectrum for a cleaned AI surface shows that a small amount of surface carbon is present (approximately 5%). Further cleaning of the surface is difficult because the filament used to generate the electrons for the AES experiments outgasses small amounts of the alkylaluminum during the AES experiments that further contaminate the aluminum surface.
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Figure 7. Static SIMS of aluminum cleaned by ion sputtering, with I-ML DMAH/AI, and after DMAH-TPD.
Secondary Ion Mass Spectroscopy. Static SlMS was used to characterize the chemical nature of the carbonaceous overlayer. Figure 7 shows static SlMS data for the clean aluminum, I-ML DMAH, and post-DMAH-TPD surfaces. The m / e 54 intensity represents the AI2+ fragment, and the m / e 39 and 40 peaks in the clean AI spectrum are likely due to the potassium and calcium present in the aluminum sample. However, the K and Ca surface concentration was too low to be detected by AES. The adsorption of DMAH results in the growth of the m / e 42,43, and 57 peaks which are assigned to the CH3AI+, CH3AIH+,and (CH3)2AI+ fragments, respectively. Peak intensities at m / e 129 and 159 are not shown, but they are present, barely above background. This result suggests that the cracking fragments from the I-ML DMAH/AI spectrum are, at least in part, due to the Ar+-induced cracking of the trimeric DMAH species. The last spectrum is a representative SlMS spectrum of the surface after a TPD experiment in which the surface was ramped from 90 to 473 K. The spectrum shows a decrease in m / e 57 and 43 and an increase in
1432 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 I
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Figure 8. Temperature-programmed SIMS (dashed lines) and TPD (solid lines) of (a) 2-ML DMAH and (b) 2-ML TMA.
m / e 42. The m / e 129 and 159 peaks are no longer present, indicating that molecular DMAH left on the aluminum surface is not detectable. These results indicate that the post-TPD SIMS profile is representative of the DMAH decomposition fragments and indicates that the aluminum surface is methylated. Previous research has shown that methyl groups are not stable on aluminum at temperatures above 150 K,” suggesting that the aluminum which binds the methyl groups in these experiments has not been fully incorporated into the aluminum surface at temperatures close to 500 K. Temperature-programmed SIMS (TPSIMS) experiments were performed in which either TMA or DMAH was adsorbed at 90 K, and m / e 57 was monitored continuously as the aluminum temperature was increased to 500 K. Figure 8a shows TPSIMS experiments (dashed lines and reference TPD data is shown with solid lines) for 2-ML DMAH/AI and 2-ML DMAH/AI with a BC of 0.46, and Figure 8b shows TPSIMS of a 2-ML TMA/AI surface. Both the DMAH and TMA experiments show prominent features in their respective spectra. The DMAH surfaces show a sharp drop in m / e 57 intensity at about 150 K and then a relatively stable m / e 57 intensity up to about 500 K. The signal decrease occurs well before DMAH desorption from the surface (or any other product desorption), suggesting that this feature is probably due to a DMAH decomposition reaction. This feature does not show any change in intensity or shape beyond 2-ML DMAH and is also present in TPSIMS spectra with DMAH surface coverages down to 0.3 ML, suggesting that the SIMS signal is due to DMAH which is directly bound to the aluminum surface. The surface reaction that causes this TPSIMS feature is not obvious, but the absence of this same feature in the TMA spectrum would implicate a surface reaction channel which is only open to DMAH. as for example AI-H bond cleavage. Further heating of the surface leads to no noticeable change in the DMAH spectrum, indicating that the SIMS signal from the DMAH decomposition fragments is much higher than the signal which is due to the molecular DMAH that desorbs during the TPSIMS experiment. The most apparent features in the TMA-TPSIMS spectrum is the gradual rise and then the drop in m / e 57 intensity at about 225 K. Firstly, the rise in the m / e 57 intensity coincides with the desorption of molecular TMA, suggesting that this TPSlMS feature is due to the uncovering of surface species which give a large secondary ion signal. Furthermore, submonolayer concentrations of TMA fail to give an increasing m / e 57 signal intensity during TPSIMS. The fall in intensity beginning at about 225 K is likely to be due to methyl decomposition or its removal from the surface as methane, as will be discussed in the next section. Produci Formation from DMAH and TMA. Figure 9, a and b, shows reaction products detected during TPD experiments for (13) Chen, J. G.; kebe, J. P.; Crowell, J. E.;Yates Jr., J. T. J . Am. Chem. SOC..1987.109, 1726.
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Figure 9. Product formation from (a) DMAH surface reactions (the m/e I5 and 129 correspond to methane and TMA products, respectively) and (b) TMA surface reactions.
both DMAH and TMA reactants, respectively. In the case of DMAH, both methane ( m / e 15) and a product with a cracking fragment at m / e 129 are observed. The contribution to m / e 129 by DMAH has been deconvoluted from the spectrum. To rule out the possibility that the m / e 129 intensity was the result of the cracking of an alkylaluminum compound with a molecular weight higher than trimeric DMAH (MW 174), TPD experiments were carried out in which m / e increments of I5 and 27 higher than m / e 174 (as well as others) were monitored, and no additional products were found. This result suggests that the m / e 129 intensity results from the cracking of a compound with a molecular weight less than DMAH. This desorption product was detected at m / e values consistent with those found for TMA (see Figure 2). A complete cracking pattern of the product could not be achieved because of a low desorption yield and interference by the cracking pattern of the reactant. However, the source of the TMA detected was not likely to be an impurity in the DMAH used to dose the aluminum surface, because the position of the product state is different than that found in the TMA desorption experiments. Furthermore, we found in separate experiments that the coadsorption of DMAH and TMA on the aluminum surface did not change their individual desorption spectrum; Le., TMA desorbing from an aluminum surface with coadsorbed DMAH showed the same reactant desorption state present in the TMATPD spectrum (Figure 5). These results suggest that the additional desorption state at 213 K resulting from the dissociation of DMAH is due to the formation of TMA during the surface heating. Recent worki4 has found that the surface reactions of TMA on Ru metal produces DMAH as reaction product. Our research finds no evidence for DMAH production from TMA, but instead we observe TMA production from the surface reactions of DMAH on AI. The production of TMA as a DMAH reaction product is just the reverse reaction of a synthetic route to DMAH from TMA and h y d r ~ g e n ’ ~ DMAH = 2/,TMA + Y2H2+ y3Al and the production of TMA also supplies a reaction channel for the removal of carbon and the production of pure aluminum from DMAH, which has been found under certain experimental conditions.I6 In our TPD experiments, the reaction channel to form TMA is minor, since the carbon coverage found in our experiments is 0.5, similar to the carbon concentration found after TMA-TPD. Quantification of the yield of TMA from DMAH surface reaction has not been determined, since this calculation would require the relative cracking efficiencies of both DMAH and TMA, and this (14) Zhou, Y.; Henderson, M . A.; White, J . M. Surf. Sci. 1989,221, 160. ( 1 5) Coates, G . E. I n Organometallic Compounds, Methuen’s Mono-
graphs on ChemicalSubjects; Emeleus, G.J., Style, D. W.G . , Eds.; Methuen and Co.: London, 1956; p 72. (16)Oliver, J. P In Aduances in Organometallic Chemistry; Stone, F. G . A., West, R., Eds.; Academic Press: New York, 1977; p 1 I I .
J . Phys. Chem. 1991, 95, 1333-1338 has not been determined in our investigation. The production of methane product ( m / e 15) is also found from the surface reactions of both DMAH and TMA as shown in Figure 9a,b. To verify that the product was methane, m / e 14 and 16 were also monitored and compared to the methane cracking pattern. Good agreement was found between the m / e 14, 15, and 16 intensities during TPD and those obtained from the cracking of methane. The yield of methane is approximately 20 times greater when DMAH is the reacting surface species than when TMA is the adsorbed species on the aluminum surface.” The temperature at which methane desorbs in the TMA experiment corresponds to the same temperature range in which the m / e 57 intensity drop occurs in TPSIMS (see Figure 8b). Thus, this feature in the TMA-TPSIMS data might correspond to the removal of methyl groups as methane. The possibility of methyl radical desorption also has been investigated, since previous researchI8*l9has shown that methyl is the sole product formed when TMA reacts with aluminum, copper, and gallium arsenide surfaces at temperatures between 398 and 838 K. However, we find that the intensities of mle 15 and 16 correspond to the cracking pattern of methane, and we are not able to deconvolute a methyl radical contribution. The presence of hydrogen bound to aluminum in DMAH might account for its much higher yield of methane than that found for TMA dissociation. Previous work13 has shown that methyl groups deposited on clean aluminum by methyl halides were not stable ( I 7) Methane as a surface product is not unexpected, because it is always being produced in the dimethylaluminum hydride and trimethylaluminum at room temperature in the metal cylinders containing the reagents. (18) Squire, D. W.; Dulcey, C. S.; Lin, M. C. J . Vac. Sci. Technol. B 1985, 3(5), 1513. (19) Squire, D. W.; Dulcey, C. S.; Lin, M. C. Chem. Phys. Len. 1985, 116, 525.
1333
beyond 150 K. In this research we have observed the production of methane between 200 and 300 K, possibly implicating the presence of methyl groups bound to the surface in some manner at temperatures higher than 150 K. This is further supported by our static SIMS results which find methylated aluminum species at temperatures up to at least 500 K. This observation suggests that methyl groups are stabilized in the DMAH and TMA molecule or in their decomposition fragments on the aluminum surface. Even though we observe more methane production from DMAH, there is no reduction in the surface carbon concentration relative to the TMA experiments. Although the production of methane, and the production of TMA, are only minor reaction channels for DMAH under these experimental conditions, these reaction channels which remove carbon from the aluminum surface may become significant at higher temperatures and pressures and may provide a low-energy reaction channel for aluminum deposition from DMAH.6 Summary Temperature-programmed desorption and secondary ion mass spectroscopic experiments have been presented for the DMAH/AI and TMA/A1 systems. The results of this research are summarized as follows: ( I ) DMAH is found to desorb from an aluminum surface in its trimeric form and TMA in its dimeric structure during TPD. (2) Both DMAH and TMA leave residual carbon (surface coverage of about 0.5) on the surface after the TPD experiments. (3) Surface adsorbed DMAH is found to produce about 20 times as much methane product as TMA. (4) A surface reaction channel of DMAH produces TMA as a reaction product, and this pathway may offer a pathway for the surface deposition of aluminum. Registry No. TMA, 75-24-1; DMAH, 865-37-2; AI, 7429-90-5; CH,, 74-82-8.
Photochemistry of Adsorbed Molecules. 8. Photodissociation, Photoelimination, and Photoreaction in Vinyl Chloride on LiF(OO1) St. J. Dixon-Warren, M. S. Matyjaszczyk,+J. C. Polanyi,* H. Rieley,*and J. C. Shapter Department of Chemistry, University of Toronto, Toronto, Ontario, Canada MSS I AI (Received: April 25, 1990; In Final Form: July 25, 1990)
Photochemical processes that occur in submonolayers of vinyl chloride adsorbed on a single crystal of LiF have been investigated. Four different photofragments, CI, C2H3,HCI, and C2H2.as well as intact parent molecules, C2H3CI,were monitored by means of angularly resolved time-of-flight mass spectrometry following irradiation at 193 nm. The translational energy distributions of the fragments differ markedly from those observed in the gas phase. CI atoms were formed through two photodissociation (PDIS) pathways at all surface coverages studied (
