EVIDENCE FOR PHOTOCHEMICAL QUENCHING BY A SURFACE - LASER-INDUCED DECOMPOSITION OF W(CO)6 ON SI(111)-(7 X 7)

The ultraviolet laser induced decomposition of W(CO)6 adsorbed on Si(111)-(7 × 7) was investigated under ultrahigh vacuum conditions with temperature programmed desorption/reaction, multiple internal reflection Fourier transform infrared, laser induced desorption and Auger electron spectroscopies. Ultraviolet photodecomposition of W(CO)6 proceeds in a one photon process. Different photochemical activity is observed for multilayer W(CO)6 as compared to W(CO)6 directly in contact with the surface. At multilayer coverages, photodecomposition proceeds via a single step, and the yield of generated CO strongly depends on photolysis wavelength. This dependence is consistent with direct electronic excitation of the molecule being the primary mechanism leading to decomposition. At lower coverages, the photoprocess is significantly quenched and near constant levels of CO are produced during photolysis at all ultraviolet wavelengths. At multilayer coverages, coadsorbed CO contributes to the CO laser induced desorption signal generated during irradiation of W(CO)6 and interferes with the analysis of the resulting data. This coadsorbed CO is most likely introduced into the vacuum chamber by W(CO)6 decomposing in the doser. Ultraviolet photolysis generates adsorbed partially decarbonylated tungsten carbonyl fragments, which decompose further in postphotolysis temperature programmed reaction. The photoproducts, in multilayers, do not react with coadsorbed benzene or 13CO and, when isolated, are insensitive to additional ultraviolet photolysis. Auger electron spectroscopy is shown to have limited usefulness for obtaining quantitative details regarding surface stoichiometries for W(CO)6 and its photoproducts. © 1990.