J . Phys. Chem. 1989, 93, 7681-7688
7681
Surface Photochemistry: 6. CH,Br on Pt(ll1) Z.-M. Liu, S. A. Costello, B. Roop,? S. R. Coon, S. Akhter, and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: November 14, 1988; In Final Form: May 31, 1989)
The surface photochemistry of monolayer and submonolayer CH3Br on Pt( 111) has been studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) to diagnose the results of continuous low-intensity ultraviolet irradiation. For wavelengths of 300 nm or less, and surface temperatures near 100 K, evidence is presented for the nonthermal cleavage of the C-Br bond and the formation of adsorbed Br atoms and CH3 groups. The wavelength dependence is strong and red-shifted compared to the gas-phase electronic excitation spectrum of CH3Br. By quantitative XPS, the maximum photolysis yield of retained Br from one monolayer of CH3Br is 60 5%. While there is some slow thermal desorption, there is no detectable photodesorption of the parent during irradiation. Excitation mechanisms and product formation channels are discussed.
*
1. Introduction
It is becoming clearer that photochemical bond cleavage and rearrangement processes, well-known in the gas phase and expected on most inert insulator substrates, can compete with quenching on metal substrates. This occurs even though the electronic coupling of the adsorbate states with metal substrates is expected to be much stronger than with insulator substrates. Very effective quenching by metals of fluorescence' and phosphorescenceZ is well-known, but bond dissociation can be significantly faster and, therefore, more competitive with quenching. In the background description that follows, we focus on photodissociation and surface bond rearrangement and refer the reader to the literature on nonthermal parent desorption ~ t u d i e s . ~ - ~ The successful experiments on metals have, to date, focused on ketene (CH,CO), H2S, and organic halides. Both continuous and pulsed irradiation methods have been used. The latter focus on the identification and dynamics of species desorbed during, or immediately after, the illumination pulse.'*14 The former focus on identification of species retained at the surface and rely on intermittent or postirradiation surface ana lyse^.^^-^^ On semiconductors, there are a number of interesting studies on Si showing the importance of both band-gap illumination and direct excitation of the adsorbed specie^.^'^^ These studies can be related to the interesting subject of phot~electrochemistry.*~-~~ On insulators there have been numerous studies but little detailed surface s c i e n ~ e . ~ Dynamics ~-~~ of photoprocesses on insulator surfaces is a topic of considerable current i n t e r e ~ t . ~ ~ - ~ ~ The work of Cowin and co-workers is directly related to the work reported here. Using pulsed laser excitation (193 nm) of CH3Br on Ni( 11 1) and bromided Ni( 11 l), they" showed that nonthermal photochemistry occurred, particularly on bromided Ni. For coverages less than or equal to one monolayer on bromided Ni, few photochemically produced Br atoms desorbed; most formed a relatively weakly adsorbed atomic Br species. For coverages above one monolayer, CHI and Br fragments, along with parent molecules, desorbed with each laser pulse. From the kinetic energy distribution of the CH3 fragments, they concluded that the substrate plays a strong role in the dissociation dynamics. More recently, the same group has reported the photolysis of CH3C1on Ni( 11l).'Ic They find good evidence for photochemistry in monolayers and multilayers. At 248 nm, the chemistry is ascribed to photoelectrons or hot electrons produced by substrate excitation. In this paper, we report the details of our work with CH3Br, making quantitative and semiquantitative assessments of the photochemical properties of this system which have been qualitatively described in our earlier communications.18J9 We focus here on TPD and XPS results that describe the products that are retained by the metal substrate. This work complements the 'Present address: Gaithersburg, MD.
National Institute of Standards and Technology,
0022-365418912093-768 1$01.50/0
elegant dynamics work that focuses on species desorbed during irradiation." (1) Drexhage, K. H.; Fleck, M.; Kuhn, H.; Schafer, F. P.; Sperling, W. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 1179. (2) Campion, A.; Gallo, A. R.; Harris, C. B.; Robota, H. J.; Whitmore, P. M. Chem. Phys. Lett. 1980, 73,447. (3) Budde, F.; Hamza, A. V.; Ertl, G.; Weide, D.; Anderson, P.; Freund, H.-J. Phys. Rev. Lett. 1988, 60, 1518. (4) Natzle, W. C.; Padonitz, D.; Sibener, S. J. J . Chem. Phys. 1988, 88, 7975. (5) Burns, A. R.; Stechel, E. B.; Jennison, D. R. Phys. Rev. Lett. 1987, 58, 250. (6) Weide, D.; Andresen, P.; Freund, H.-J. Chem. Phys. Lett. 1987, 136, 1 nh.
(7) Burgess, D.; Mantell, D. A,; Cavanaugh, R. R.; King, D. S.J. Chem. Phys. 1986, 85, 3123. (8) Chuang, T. J. Surf. Sci. Rep. 1983, 3, 1. (9) Desorption Induced by Electronic Transitions-DIET II; Brenig, W., Menzel. D.. Eds.: Sorineer-Verlac Berlin. 1985. (IO) Marsh, E. P.;Tibares, F.-L.; Schneider, M.R.; Cowin, J. P. J Vac. Sci. Technol. 1987, A5, 519. (1 1) (a) Marsh, E. P.; Tabares, F. L., Schneider, M. R.; Cowin, J. P. In Chemically Modified Surfaces; Leyden, D., Ed.; Gorden and Breach: New York, 1988. (b) Marsh, E. P.; Schneider, M. R.: Gilton. T. L.; Tabares. F. L.; Meier, W.; Cowin, J. P. Phys. Rev. Lett. 1988,60, 2551. (c) Marsh, E. P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. Rev. Lett. 1988, 61, 2725. (12) Celii, F. G.; Whitmore, P. M.; Janda, K. C. Chem. Phys. Lett. 1987, -178 - -, -257 - .. (13) Domen, K.; Chuang, T. J. Phys. Rev. Lett. 1987, 59, 1484. (14) Domen, K.; Chuang, T. J. J . Chem. Phys., submitted for publication. (15) Grassian, V. H.; Pimentel, G. C. J. Chem. Phys. 1988, 88, 4478. 1161 Grassian. V. H.: Pimentel. G. C. J. Chem. Phvs. 1988. 88. 4484. (17j Roop, B.; Costello, S. A.; Greenlief, C. M.; Whit; J. M. Chem. Phys. Lett. 1988, 143, 38. (18) Costello, S . A,; Roop, B.; Liu, Z.-M.; White, J. M. J . Phys. Chem. 1988, 92, 1019. (19) Zhou, Y.; Feng, W. M.; Henderson, M. A.; Roop, B.; White, J. M. J. Am. Chem. SOC.1988. 110. 4441. (20) Roop, B.; Costello, S. A,; Liu, Z.-M.; White, J. M. Springer Ser. Surf. Sci. 1988, 14, 343. (21) Roop, B.; Lloyd, K. G.; Costello, S.A,; Campion, A.; White, J. M. J . Chem. Phys., in press. (22) Liu, Z.-M.; Akhter, S.;Roop, B.; White, J. M. J . Am. Chem. Sac. 1988, 110, 8708. (23) Lloyd, K. G.; Roop, B.; Campion, A.; White, J. M. Surf. Sci. 1989, 214. 227. (24) Ying, Z.; Ho, W. Phys. Rev. Lett. 1988, 60, 57. (25) Swanson, J. R.; Friend, C. M.; Chabal, Y. J. J . Chem. Phys. 1987, 87, 5028. (26) Creighton, J. R. J . Vac. Sci. Technol. 1986, A4, 669. (27) Ho, W. Comments Condens. Matter Phys. 1988, 13, 293. (28) Bard, A. J. Science 1980, 207, 139. (29) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72, 83. (30) Krautler, B.; Bard, A. J. J . Am. Chem. Sac. 1978, 100, 5958. (31) Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson Ginley, D. S.J. Am. Chem. Sac. 1976, 98, 2774. (32) Darsillo, M. S.; Gafney, H. D.; Paquette, M. S. J . Am. Chem. Sac. 1987, 109, 3275. (33) Bourdon, E. B. D.; Das, P.; Harrison, I.; Polanyi, J. C.; Segner, J.; Stanners, C. D.; Williams, R. J.; Young, P. A. Faraday Discuss. Chem. Sac. 1986, 82, 343. (34) Bourdon, E. B. D.; Cowin, J. P.; Harrison, I.; Polanyi, J. C.; Segner, J.; Stanners, C. D.; Young, P. A. J . Phys. Chem. 1984, 88, 6100.
0 1989 American Chemical Society
7682 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 We conclude that when a monolayer of CH3Br is photolyzed, the maximum photolysis yield of retained Br is 60% and there is some thermal desorption, but no photodesorption, of the parent molecule. We also find that the initial photolysis rate is linear with the incident flux of photons and that products accumulate with time as expected for a decreasing concentration of parent molecules. There is a strong wavelength dependence that we can address only semiquantitatively; the yield drops as the wavelength increases but extends to longer wavelengths than in the gas phase. In the later stages of photolysis, the dissociation rate drops sharply. Tentatively, we attribute this to a decreased coupling of parent molecules to the metal as products (Br and CH,) accumulate. The photochemistry presented here is undergirded by previously reported surface science measurements from our laboratory on the CH,Br/Pt( 1 1 1) system.39 This work involved careful examination of TPD for three methyl halides, CH3Cl, CH3Br, and CH31, specular and off-specular high-resolution electron energy loss spectroscopy (HREELS) for these molecules, and, in the case of CH,I, the thermal decomposition products, including those derived from CD31. For the work reported here, five points of major importance were established: (1) a t 100 K, adsorption of CH3Br proceeded molecularly with no detectable dissociation; (2) in TPD, thermal decomposition was negligible and never exceeded 0.025 monolayer (ML); (3) the CH,Br adsorbed with the Br attached to the Pt; (4) methyl iodide underwent significant C-I bond cleavage during TPD to form adsorbed CH,, identified by HREELS, and I; and (5) in TPD of CH31, CH4 desorbed at 290 K. Methane formation was ascribed to surface reaction of adsorbed CH3 groups with adsorbed H atoms. The latter were formed by background adsorption of H2 and thermal decomposition of CH,. 2. Experimental Section The experiments were carried out in two separate ultrahighvacuum chambers. One was equipped with TPD and Auger electron spectroscopy (AES) facilities and has been described previously.'@ The second was a Kratos Series 800 X-ray photoelectron spectrometer to which was added a UTI mass spectrometer and an ion gun. The Pt( 1 1 1) surfaces were cleaned by either Ar+ ion sputtering or by oxidation and high-temperature annealing, or combinations of the two methods. Cleanliness was confirmed by either XPS or AES. The samples were cooled to between 100 and 106 K by contact with a liquid nitrogen cooled reservoir. They were heated resistively, and temperatures were monitored with a chromelalumel thermocouple spot-welded to the crystal. For the XPS experiments, CH,Br was dosed through a 3mm4.d. tube positioned 2.5 cm away from the crystal. In the other chamber, the CH3Br was back-filled. For both, no detectable impurities were found by mass spectrometric analysis. Many of the experimental results depend on reproducibly forming a saturated monolayer of CH3Br. The procedure is described in section 3. The UV light source was a 100-W high-pressure Hg arc lamp. The light (0.09-1 .O W cm-*) passed through a 1-2-cm-diameter aperture and then a quartz window at the vacuum chamber wall. In the Kratos machine, the incident angle was approximately 60° off the sample normal, whereas in the other machine it was at normal incidence. In separate experiments, using a third machine, we have established that these two geometries give qualitatively the same results.23 Using the optical constants of Pt, we show elsewhere that, at UV wavelengths, relatively large electric fields exist at the surface for normal incidence.*l This is unlike the (35) Tabara, F.L.;Marsh, E.P.;Bach, G. A.; Cowin, J. P. J . Chem. Phys. 1981, 86, 138.
(36)Ehrlich, D. J.; Osgood, R. M. Chem. Phys. Lett. 1981, 79, 3 8 1 . ( 3 7 ) Chen. C.J.; Osgood, R. M. Chem. Phys. Lett. 1983, 98, 363. (38)Higashi, G.S.; Rothgerg, L. J. J. Vac. Sci. Technol. 1985,83, 1460. (39)Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987,184, L325. (40) Mullins, D. R.; Rmp, B.; Costello, S. A,; White, J. M. Surf. Sci. 1987, 186, 67.
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TEMPERATURE (K)
Figure 1. TPD spectra for multilayer CHoBr dosed on P t ( l l 1 ) at 106 K: (a) annealed to 130 K and recooled to 106 K before TPD and (b) not annealed. Spectrum b defines a monolayer in this work.
situation for infrared frequencies where, for normal incidence, electric fields are strongly damped by the response of the metal. At UV wavelengths, there is a variation of the total field strength with angle of incidence; we would expect this to be reflected in quantitative initial rates as a function of incident angle, if the adsorbate-substrate complex is optically excited. For TPD, the substrate was heated resistively at 5 K s-l from 100 K to as high as 1050 K. AES was done in the derivative mode with a single-pass cylindrical mirror analyzer. XPS spectra were obtained with Mg Ka radiation (1253.6 eV). The current and the voltage settings were 20 mA and 12 kV (240 W), respectively. To gain sensitivity, a low-resolution setting ( E p = 80 eV) and the fixed analyzer transmission (FAT) mode were employed in the data acquisition. All the XPS spectra reported here were measured in terms of the binding energy (BE). A typical XPS measurement took 30 min scanning from 187 to 177 eV BE for the Br(3p3,J peak. The measured Pt(4f7,J BE was 70.9 eV throughout these experiments.
3. Results This section is organized into two major subsections, one dealing with TPD data and the other with XPS data. Wherever possible, the two sets of data are correlated. 3.1. Temperature-Programmed Desorption Experiments. These experiments were undertaken to determine how postirradiation TPD differed from TPD with no irradiation. Because earlier work showed that no more than 5% of a monolayer of CH3Br thermally decomposes on Pt( 11l)?9 establishing the role of photochemistry is particularly simple in this system. Once a photochemical, as opposed to a thermal, effect is established, TPD also provides for testing the dependence of the results on wavelength, light intensity, illumination time, and coverage. 3.1.1. Monolayer Preparation Procedure. For the work reported here, involving two different vacuum chambers, it is very important to establish a reproducible method for producing a standard coverage of CH3Br on Pt( 111). We chose to standardize by adsorbing a multilayer of CH3Br at 100-1 10 K ( =2-langmuir dose) followed by quick heating to 130 K (and immediately recooling) to remove the multilayer and leave the monolayer.39 Figure 1 shows the parent (94 amu) TPD of as-dosed (b) and heated (a) overlayers. Clearly, in both cases desorption occurs only at very low temperatures. The sharp spike at 125 K reflects the multilayer, while the peak at 165 K is the monolayer de-
Surface Photochemistry of CH3Br on Pt(ll1)
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7683
7 33.2
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T E M P E R A T U R E (K) Figure 2. Methane (16 amu signal) TPD for monolayer CH,Br adsorbed on R ( l 1 1 ) at 106 K: (a) no irradiation, (b) irradiation with full arc for 1700 s, (c) irradiation through a 420-nm cutoff filter for 1700 s, and (d) irradiation through 300-nm cutoff filter for 1700 s. Bulk temperature during irradiation rose to about 113 K in (b-d).
sorption. Figure l a was defined as 1 ML and was used as a standard reference throughout this work. Lower coverages were then established by determining the parent TPD peak area relative to this standard. Two other points should be made about Figure 1: (1) The spectrum tails toward high temperatures, reflecting the difficulty of pumping CH3Br, and (2) the leading edge of the monolayer desorption extends to fairly low temperature ( r 1 2 0 K). The latter point indicates that slow thermal desorption of the parent should occur during illumination as the sample temperature increases slightly. This expectation is supported by XPS and TPD results discussed below. 3.1.2. Photolysis, TPD, and AES. By use of the above preparation procedure, monolayers of CH3Br were prepared and photolyzed under a variety of conditions. In postirradiation TPD (multiplexed), the only desorbing products were CH3Br, CH4, Br, and tiny amounts of H2. Figures 2-4 summarize the results of one blank and three photo experiments. Methane desorption (Figure 2) was monitored by following 15 and 16 amu in postirradiation TPD. For an unirradiated monolayer, there is no CHI peak (Figure 2a), even though we waited 1700 s before doing TPD. This demonstrates that slow thermal or background processes, not searched for in earlier work,39 do not lead to methane formation. Irradiation with the full arc (0.9 W cmS2 at the sample) for 1700 s leads to the TPD of Figure 2b. There is a strong CH4TPD signal, peaking at 275 f 5 K. This is within 15 K of that observed for the thermal decomposition of CH31.39When a 420-nm cutoff filter was inserted and the lamp output increased to achieve 0.9 W cm-* at the sample, irradiation for 1700 s gave no detectable CH4 signal (Figure 2c). Using a filter that cut off at 300 nm, again adjusting the total power to 0.9 W cm-2, we observe CH4 at 275 K (Figure 2d), but significantly less than with the full arc. In other experiments involving CD3Br coadsorbed with CH3CI, we find that the onset for photochemical C-Br bond dissociation lies near 360 nme2I The Br (79 amu) and CH3Br (94 amu) results are summarized in Figures 3 and 4. There is a low-temperature 79 amu peak in every case, and it tracks the parent molecule desorption (cracking fragment in the ion source of the quadrupole). The important result is the small broad peak near 800 K which appears only in those cases where CHI also desorbed (Figure 3b,d). We attribute
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T E M P E R A T U R E (K) Figure 3. Atomic bromine (79 amu signal) TPD for monolayer CHIBr adsorbed on P t ( l l 1 ) at 106 K. Conditions of each of the curves match those of Figure 2. The low-temperature signal is a cracking fragment of the parent CH3Br. Atomic Br desorbs from P t ( l l 1 ) near 800 K.
-f
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T E M P E R A T U R E (K)
Figure 4. CH,Br (94 amu signal) TPD for monolayer CH,Br adsorbed on P t ( l l 1 ) at 106 K. Conditions are identical with those of Figure 2.
this desorption to atomic Br as no signal was detected at 158 amu (Br2). While the intensity of the atomic Br signal is weak, it does correlate with the CH4 TPD peak area. Turning to the parent molecule TPD (94 amu), it is important to keep in mind that, for Figure 4a where there was no illumination, the dosed substrate was held under vacuum before TPD for the same period as a normal photolysis, 1700 s. The area of this peak for unirradiated samples did not change with waiting time over this period. The spectra of Figure 4 reflect the observations of Figures 2 and 3. When there is evidence for photochemistry, the TPD peak area of CH3Br goes down (Figure 4b,d).
7684 The Journal of Physical Chemistry, Vol. 93, No. 22, I989
There is another important point to be made concerning the data of Figure 4. The peak area of Figure 4c is measurably less than that of Figure 4a. Thus, even when there is no evidence for photodissociation, there is still some loss of intensity in the postirradiation TPD (Figure 4c). We return to this issue in section 3.2, where XPS is used in order to avoid the CH3Br pumping speed problems. There is one final point to be made about Figure 4. When there is evidence for photochemistry, there is a hint of a small desorption peak for the parent at about the same temperature where CH4 desorbs (275 K). Its origin is not known, but it might indicate some recombination of Br and CH3. Qualitatively, Figures 2-4 indicate that nonthermal photochemical cleavage of the C-Br bond occurs. In separate HREELS experiments, we have identified adsorbed CH, as a major product.Ig This bond cleavage process is well-known in the gas-phase photochemistry of CH3Br.41*42Our results underscore the fact that C-Br bond cleavage competes with quenching by Pt. Only one other TPD product was observed, H2, which varied but never amounted to more than 7% of the H2 TPD observed after a saturation dose of H2 on the clean Pt(ll1) surface at 100 K. This hydrogen comes from background adsorption and CH, decomposition. We ascribe the CH, production to the reaction of this H(a) with CH3(a). A small and variable C(KVV) AES signal was detected after TPD. The extent of CH, decomposition, as reflected by this C(KVV) AES signal, is lowered by preadsorbing H. Unfortunately, we have been unable to quantitatively assess the amount of CH, decomposition. 3.1.3. Wavelength Dependence. The wavelength dependence is interesting. It is clear from Figures 2-4 that the photodissociation probability drops to negligible values somewhere between 300 and 420 nm and that eliminating radiation below 300 nm (there is a strong pressure broadened transition at 254 nm in the spectrum of the high-pressure Hg arc) causes a strong decline in the rate. While the gas-phase optical absorption spectrum of CH,Br follows this same trend,42,43we would find no measurable photoeffect at, and above, 300 nm if an extrapolated gas-phase were valid. Thus, absorption cross section ( - 5 X lo-*, as observed by Grassian and Pimentel'5,16for dichloroethylene on Pt(l1 l), the wavelength dependence is significantly red-shifted with respect to gas-phase CH3Br. We return to a discussion of the origin of this red shift in section 4. A second interesting aspect of the wavelength dependence is that it confirms the presence of a photochemical, as opposed to a thermal, effect. The absence of CH, and Br in TPD after irradiating the sample with wavelengths in excess of 420 nm underscores this point. Finally, the observation of photochemistry at wavelengths longer than those found in the gas shows that the products are not the result of photodissociation of gas-phase molecules followed by chemisorption of radical products. 3.1.4. Coverage Dependence. To qualitatively establish the sensitivity of the results to CH,Br coverage, we used an exposure giving 0.05 ML (based on 1 ML in Figure 1) of adsorbed CH3Br in two separate experiments, one irradiated and one not. The results are summarized in Figures 5 (94 amu) and 6 (16 amu). Dosing and waiting for 1800 s without irradiation leads to no detectable products in TPD (Figure 6). Irradiation with the full Hg arc for 1800 s leaves little parent (Figure 5 ) and produces a signifcant amount of CH4 (Figure 6). At least 95% of the initial coverage is converted to products. Comparing high and low coverages of CH3Br (Figures 2 and 5 ) indicates that a much larger fraction of the initial coverage is reacted or desorbed for a given photolysis time when the initial coverage is small. The adsorbed CH3Br layer is optically thin so that thefraction of the initial coverage removed in a given time should be independent of the initial coverage if all the adsorbed molecules participate equally. This is clearly not the case, in~
(41) Calvert, J. C.: Pitts, J. N. Photochemistry; Wiley: New York, 1966. (42) van Veen, G . N. A.; Bailer, T.: de Vries, A. E. Chem. Phys. 1985,92, 59. (43) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978: p 301.
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TEMPERATURE (K)
Figure 5. CH3Br (94amu signal) T P D for 0.05 ML of CH,Br adsorbed on P t ( l l 1 ) at 106 K: (a) without irradiation and (b) irradiation with full arc for 1800 s.
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Figure 6. C H 4 (16 amu signal) T P D for 0.05 ML of CH3Br adsorbed on Pt( 11 1) at 106 K. Conditions are identical with those of Figure 5.
dicating that there is a coverage dependence in the extent of photolysis which favors low coverages. One aspect of this problem is discussed further in section 3.2. 3.1.5. Intensity and Time Dependence. Initial rates, as assessed by postirradiation CH4 TPD peak areas, were linear with intensity variations of the full arc from 0.3 to 1.0 W cm-2. With longer irradiation times, the products accumulated as expected for a system where the amount of parent decreases; the rate decreased as the parent molecule was consumed. This is also discussed in more detail in section 3.2. 3.1.6. Control Experiments. We now turn to a series of control experiments designed to test for sources of products other than
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7685
Surface Photochemistry of CH3Br on Pt( 11 1)
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B i n d i n g Energy (eV) Figure 7. The XPS B r ( 3 ~ , , ~spectrum ) of the monolayer CH,Br in cases (a) without irradiation, (b) with temperaturechanged resistively to mimic that during illumination,(c) with 420-nm cutoff filter, and (d) with full
arc irradiation. photochemistry a t the Pt( 11 1) surface. (i) Based on shifts of the secondary electron threshold measured with a low-energy electron beam, the adsorption of CH3Br lowers the work function of the substrate-adsorbate system by at least 0.5 eV. Preliminary measurements, using XPS thresholds, show that the work function change for 1 ML of CH,Br/Pt( 111) is no more than 1.6 eV. In other more quantitative work using UV photoelectron spectroscopy, the work function of Ag(ll1) is lowered by 1 eV upon adsorption of 1 ML of CH3Br.QI Thus, for CH,Br/Pt( 11 l ) , the work function could be as low as 4.2 eV. This corresponds to a wavelength threshold of 295 nm, below the photochemical threshold we find here. This rules out photoelectron-induced chemistry at 300 nm and longer wavelengths. As a separate test, we examined the effect of electrons by biasing the crystal (+10.4 V) to collect electrons from an ion gauge filament. With a current of ~ 1 0 nA 0 for 10 min, measured at the sample, no changes were observed in either the illuminated or nonilluminated TPD spectra. This test does not rule out chemistry due to hot electrons formed in the substrate (energies between the Fermi level and the vacuum level) or to photoelectrons with energies just above the vacuum level." (ii) To test for the possibility that irradiation of surfaces other than the sample contributed to the results, the following blank test was performed. Immediately after dosing CH3Br, the sample was heated (TPD) and recooled. Then the sample was irradiated with the full arc for 1700 s. With this procedure, the sample should be relatively free of active species, while other surfaces should remain populated with them. In postirradiation TPD there was no evidence for CH4, CH,Br, or Br. Only background amounts of H2 and CO were found. Thus, we conclude that the observed photochemistry is attributable entirely to the adsorbate-Pt(ll1) system. (iii) Finally, we purposefully exposed the substrate to a 100langmuir dose of CH4 at 100 K. As expected, no CH4 was detected in subsequent TPD, demonstrating that the source of CH4 in the these TPD experiments was a reaction-limited process that we attribute to adsorbed CHI. 3.2. X-ray Photoelectron Spectroscopy. Complementary experiments using XPS were undertaken for several reasons. First, we expect45a significant BE shift to lower values (~0.6-1.0 eV) of the Br core levels if the C-Br bond is cleaved and the Br becomes strongly bonded to the Pt. This is an initial state effect; the negative charge on the Br increases when it switches from C-Br to Pt-Br bonding. Second, this shift can be detected without heating the substrate after irradiation. Thus, one possible ambiguity in the postirradiation TPD work is removed. Third, XPS peak areas for Br can be made quantitative. The pumping speed difficulties associated with CH,Br TPD are avoided. Fourth, comparing the results of XPS and TPD provides a self-consistency check. (44) Zhou, X.-L.; White, J. M. To be published. (45) Grunze, M.; Dowben, P.A. Appl. Surf. Sci. 1982, 10, 209.
I 187
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(eV)
Figure 8. The XPS B r ( 3 ~ , , ~spectrum ) of monolayer CH3Br and the
residual Br atoms: (a) monolayer CH,Br without irradiation, (b) the residual Br atoms produced by full arc illumination and heating to remove remaining CH3Br, and (c) the residual signal after irradiation with 420-nm cutoff filter and heating to remove remaining CH3Br. The TPD results described above were reproduced qualitatively in the Kratos system. Without illumination, a negligible amount of thermal decomposition occurred. With the full arc, CH4 desorbed at 275 f 5 K in TPD. No products appeared in TPD when the sample was irradiated with use of the 420-nm cutoff filter. 3.2.1. Wavelength Dependence. Figure 7 shows the Br core level region for four different XPS experiments involving monolayer CH3Br: (a) without irradiation, (b) without irradiation but with the surface temperature changed with the same time profile as measured during illumination, (c) illumination for 1 h through a 420-nm cutoff filter, and (d) irradiated with the full arc for 1 h. For both experiments involving UV illumination, the power was 0.09 f 0.01 W cm-2. With the exception of Figure 7d, the peak position of Br(3p3/J is constant at 182.5 f 0.2 eV. There is a shoulder at 181.1 f 0.2 eV in Figure 7d. Clearly, irradiation with the full arc changes the Br signal in a way consistent with some C-Br bond cleavage. This, as expected, does not occur when wavelengths longer than 420 nm are used (Figure 7c). In all of these experiments, the Pt(4f7/J peak was a t 70.9 eV BE. Figure 8a shows the XPS Br(3p3/J spectrum of a monolayer of CH3Br without irradiation. Figure 8b shows the XPS Br(3p3 2) spectrum of the Br atoms left on the surface after irradiation #or 1 h with the full arc and postirradiation flashing to 350 K to remove residual parent molecules and CH4 (see Figures 1 and 2). Figure 8c shows the XPS Br(3p3 2) spectrum of the residual Br atoms after illumination for 1 with a 420-nm cutoff filter followed by postirradation flashing to 350 K. The peak position of B r ( 3 ~ , / ~in) Figure 8b is at 181.1 f 0.2 eV Gust as in Figure 7), indicating a 1.4 eV BE decrease upon bond cleavage. As expected, there is no peak in Figure 8c, confirming that there is no photochemistry at these wavelengths and that there is no subsequent thermal chemistry during flashing to 350 K. It is quite clear that XPS confirms the TPD conclusion: monolayer CH3Br is photodissociated by UV irradiation. We also examined the C(1s) signal. Unfortunately, it was relatively weak, and the poor signal-to-noise ratio in the postirradiation measurements, along with some background accumulation, precluded a quantitative assessment of the desorption of carbon-containing species during photolysis." 3.2.2. Time Dependence. We now examine the quantitative time dependence of the Br XPS signal (Figure 9). The photolysis yield of retained Br atoms was calculated as the ratio of the residual Br atom XPS peak area (parent removed as in Figure 8b) divided by the monolayer CH3Br XPS Br peak area (as in Figure 8a). With increasing irradiation time, the yield increased relatively rapidly, but with decreasing slope, to about 60% after 1.5 h. For longer times, the yield grew very slowly. The qualitative shape of this curve is discussed below. In separate experiments, the total loss of Br was followed by XPS during irradiation of a monolayer of CH3Br (Figure 10). There is significant removal of Br from the surface. To test whether this was a thermal (the temperature rise during illu-
h
7686 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 0
occurs in monolayers and submonolayers of CH3Br molecularly adsorbed on Pt( 1 1l), we turn now to a discussion of the product formation channels and excitation mechanisms that may participate. We also discuss product retention, the wavelength dependence, and the coverage dependence. 4.1. Product Formation Channels. In the gas phase, the UV photochemistry of CH3Br is very simple. The absorption spectrum is continuous, extending from about 280 to well below 200 nm, and the quantum yield for the formation of CH3 and Br is unit ~ . All ~ the ~ evidence , ~ ~ we18-22and others11*33*34 have indicates the same kind of bond breaking occurs (the excitation mechanism at the surface is discussed below) in adsorbed CH3Br(a). To form the observed TPD products, CH4, Br, and H2, we need only assume the following set of reactions:
El 0
Yield of Relained Bromine Aloms
Irradialion
Liu et al.
Timelmin.
Figure 9. The photolysis yield (with a constant light power) versus illumination time for monolayer CH3Br dosed on Pt(l11). The full H g
CH3Br(a) + hv
arc was used.
CH3(a)
9 II 0
Loss of
Bromine during Irradiation
-
2H(a)
CH3Br(a)
100
irradiation
150
200
Timelmin.
Figure 10. The loss of Br(3p3,*) signal during illumination of monolayer CH3Br with the full Hg arc. TABLE I: Percentage of Bromine Retained after Irradiation condition full arc 420-nm cutoff resistive heat (no light)
irradiation time 30 min 60 min 93 88 92 89 92 90
mination was
