Acc. Chem. Res. 1986,19, 287-292
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SIMS as a Probe of Surface Chemical Kinetics P.L. RADLOFF and J. M. WHITE* Department of Chemistry, University of Texas, Austin, Texas 78712 Received March 14, 1986 (Revised Manuscript Received J u l y 22, 1986)
Elucidating the kinetics of surface chemical reactions on low-area substrates is a difficult problem, particularly when the products remain adsorbed. The difficulty stems in large part from a paucity of techniques that both are sensitive to chemical environment and possess a dynamic range sufficiently large to permit data acquisition on the time scale of reaction. Temperature-programmed desorption (TPD) has provided a majority of the available kinetic data, but is limited to the study of species which desorb from the substrate and can provide information about surface kinetics only by inference. Laser-induced desorption (LID)lv2 overcomes the shortcomings of conventional desorption methods. In this technique an extremely rapid laser-generated temperature jump induces desorption of all adsorbed species, often in high yield, despite the existence of competing reaction channels. A variety of other techniques, for example, ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), extended X-ray absorption fine structure (EXAFS),infrared measurements (IR), and electron energy loss spectroscopy (EELS), provide a direct probe of the surface chemical environment. Due to their generally poor sensitivity, however, it has been possible thus far to implement only a few of the~e-EELS,~v~ IR,5 and angle-resolved photoemission6-in in a configuration having acceptable time resolution. Here we focus on another chemically sensitive technique, secondary ion mass spectrometry (SIMS), which has been used in this and other laboratories to monitor surface kinetic^.^-^^ SIMS is an extremely sensitive probe, with the ability to detect many surface species a t concentrations well below the detection threshold of Auger electron (AES) and other surface spectroscopies. As a consequence of its huge dynamic range, rather short data collection periods (100-200 ms) are required in SIMS experiments. By monitoring secondary ion yields during the course of reaction, information about surface chemical kinetics can be obtained. The extreme chemical sensitivity of SIMS, while permitting its use as a dynamical probe, also is the greatest complication in its employment. SIMS cross sections are known to vary by as much as 5 orders of Patricia L. Radloff was born in Cleveland, OH, on April 26, 1955. She received a B.S. from the University of Virginia in 1977 and a Ph.D. in chemistry at the University of Chicago in 1984. Currently she is a Postdoctoral Research Associate with Professor J. M. White at the University of Texas at Austin. Her major research interests center on the chemistry and physics of surfaces and clusters. John M. White was born November 26, 1938, in Danville, IL. He received a B.S. from Hardlng College in 1960 and a Ph.D. in chemistry from the University of Illinois in Urbana in 1966. He joined the faculty at the University of Texas at Austin in 1968 and currently holds the Norman Hackerman Professorship in Chemistry. White is a principal editor with the Journal of Materlals Research and is on The Journal of Physical Chemistry advisory board. He has been a vislting staff member at Los Aiamos National Laboratory since 1967. His research interests include surface chemistry, the dynamics of surface reactions, and photoassisted catalytic reactions.
0001-4842/86/0119-0287$01.50/0
magnitude as the chemical environment and work function are changed.26 Other difficulties in interpretation arise since SIMS cracking patterns often differ from their gas-phase, electron-impact ionization analogues. The monitoring of cluster ions requires particular care, as clusters may be formed both on and above the surface as a consequence of ion bombardment.26 In kinetic applications, it is also necessary to verify that the primary ion beam is neither sputtering the surface at a great rate nor inducing chemical reaction. Given the difficulties inherent in the technique, it is clear that SIMS must be used in conjunction with other methods in the characterization of surface kinetics. This paper describes the utilization of SIMS to help characterize the formation, decomposition, and isotope exchange reactions of alkylidynes, the formation and decomposition of methoxy, and the reaction of hydrogen and oxygen to form water, all on Pt(ll1). In each case, kinetic parameters are calculated from the time and/or temperature dependence of calibrated SIMS (1) Hall, R. B.; DeSantolo, A. M.; Bares, S. J. Surf. Sci. 1985, 161, L533. (2) (a) Cowin, J. P.; Auerbach, D. J.; Becker, C.; Wharton, L. Surf. Sci. 1978, 78,545. (b) Wedler, G.; Ruhmann, H. Surf. Sci. 1982,121, 464. (c)
Burgess, D., Jr.; Hussla, I.; Stair, P. C.; Viswanathan, R.; Weitz, E. J. Chem. Phys. 1983, 79, 5200. (d) Sherman, M. G.; Kingsley, J. R.; Dahlgren, D. A.; Hemminger, J. C.; McIver, R. T., Jr. Surf. Sci. 1985,148, L25. (e) Chuang, T. J.; Seki, H.; Hussla, I. Surf. Sci. 1985, 158, 525. (3) Richter. L. J.: Ho. W. J. Chem. Phvs. 1985. 83. 2569. (4) DuBois; L. H:; Ellis, T. H.; Kevan, S . D. J. 'Vac. Sci. Technol. A 1984. - - - - , 3. -, 1643. ~ - - (5) Burrows, V. A.; Sundaresan, S.; Chabal, Y. J.; Christman, S. B. Surf. Sci. 1985, 160, 122. (6) Haight, R.; Bokor, J.; Stark, J.; Storz, R. H.; Freeman, R. R.; Bucksbaum, P. H. Phys. Rev. Lett. 1985,54, 1302. (7) DeLouise, L. A.; Winograd, N. Surf. Sci. 1985,159,199; 1985,154, 79. (8) Benninghoven, A.; Beckmann, P.; Griefendorf, D.; Schemmer, M. Appl. Surf. Sci. 1980, 6, 288. (9) (a) Mohri, M.; Hashiba, M.; Watanabe, K.; Yamashina, T. 2.Phys. Chem. (Munich) 1978.109.S233. (b) Mohri. M.: Kakibavashi. H.: Watanabe, K.; Yamashina, T. AppE. Surf. Sci. 1978, 1, 170.(10) Fogel, Ya. M. Int. J. Mass Spectrom. Ion Phys. 1972, 9, 109. (11) Ogle, K. M.; Creighton, J. R.; Akhter, S.; White, J. M. Surf. Sci.,
in press. (12) Creighton, J. R.; White, J. M. Surf. Sci. 1983, 129, 327. (13) Creighton, J. R.; Ogle, K. M.; White, J. M. Surf. Sci. 1984, 138, L137. (14) (15) (16) (17) (18)
Mitchell, G. E.; Akhter, S.; White, J. M. Surf. Sci., in press. Ogle, K. M.; White, J. M. Surf. Sci. 1986, 165, 234. Ogle, K. M.; White, J. M. Surf. Sci. 1984, 139, 43. Creighton, J. R.; White, J. M. Surf. Sci. 1984, 136, 449. Ogle, K. M.; Creighton, J. R.; Luftman, H. S.; White, J. M. J. Chem. Phys. 1983, 78, 5839. (19) Creighton, J. R.; White, J. M. J. Vac. Sci. Technol. A 1983, 1,
1225. (20) (21) (22) (23) 5172. (24)
Creighton, J. R.; White, J. M. Chem. Phys. Lett. 1982, 92, 435. Creighton, J. R.; White, J. M. Surf. Sci. 1982, 122, L648. Akhter, S.; White, J. M. Surf. Sci., in press. Belton, D. N.; Sun, Y.-M.; White, J. M. J. Phys. Chem. 1984,88,
Sun, Y.-M.; Belton, D. N.; White, J. M. In Catalyst Characterization Science; Deviney, M. L., Gland, J. L., Eds.; ACS Symposium Series 288; American Chemical Society, Washington, DC, 1985; p 80. (25) Benninghoven, A. Surf. Sci. 1975, 53, 596. (26) (a) Winograd, N.; Garrison, B. J.; Harrison, D. E., Jr. J . Chem. Phys. 1980, 73, 3473. (b) Garrison, B. J.; Winograd, N.; Harrison, D. E., Jr. J. Chem. Phys. 1978, 69, 1440.
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signals. Other techniques are used to identify the surface reactants and products. While the SIMS ions detected must be (and are) consistent with these reactions and products, in no case is SIMS used to identify them.
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Experimental Considerations In this laboratory SIMS is currently being utilized in conjunction with TPD and AES. In other systems being completed, SIMS will be combined with EELS and pulsed laser desorption. These are all ultrahigh vacuum systems capable of reaching 1 X T~rr.'l-~~ Surface cleanliness is monitored with both AES and SIMS. TPD involves a standard e-beam ionizer supplied with the SIMS quadrupole. It is important to recognize the resolution limitations of these quadrupole L systems. A particularly noteworthy example for the mle systems discussed here occurs a t m / e 28 where CO+, Figure 1. SSIMS fragmentation pattern for ethylene in the C1 Si+, and C2H4+cannot be resolved. region ( m / e 12, 13, 14, 15) for various temperatures. Ion beam Molecules are adsorbed either by backfilling the flux was 3 nA f cm2. chamber (gives more reproducible dosing) or by directing the flux from a capillary array onto the sample (minimizes background). Programmed temperatures between 100 and 1800 K are realized by combining liquid nitrogen cooling with resistive heating. SIMS is done in a static mode (SSIMS) using Ar ions (600-1000 eV and typically
