Photolysis of CF3Cl adsorbed on Si(111)(7x7) surface by monochromatic synchrotron radiation

We present an analysis of the photolysis of an adsorbed molecule by monochr omatic synchrotron radiation. The system studied was CF3Cl adsorbed on Si(1 11)(7x7) at 30 K, and the techniques employed were photoemission spectrosco py (PES) and photon-stimulated desorption (PSD). The valence-level photoemi ssion spectra, under various photon exposures, show that the photolysis eff ect induced by the incident monochromatic synchrotron radiation photons (h nu=98 or 110 eV) occurs on this adsorbed gas-solid system. Variations of th e F- and F+ PSD ion yields were measured as a function of monochromatic (11 0 eV) photon exposure at three adsorbate coverages (the lowest dose=0.3x10( 15), the medium dose=0.8x10(15), and the highest dose=2.2x10(15) mol/cm(2)) . For the lowest CF3Cl-dosed surface, the photon-exposure dependencies of t he F- and F+ yields show the characteristics: (a) at early stages of photol ysis, the desorption of F- yields is mainly due to dissociative attachment (DA) and dipolar dissociation (DD) of the adsorbed CF3Cl molecules induced by the photoelectrons emitting from the silicon substrate, while at high ph oton exposure the F- formation by electron capture of the F+ ion is the dom inant mechanism; (b) the F+ ion desorption is associated with the bond brea king of the surface SiF which is formed by reaction of the surface Si atom with the neutral fluorine atom or F- ion produced by scission of the C-F bo nd of CF3Cl, CF2Cl, or CFCl species. A kinetic model was proposed for the e xplanation of the photolysis of the lowest CF3Cl-dosed surface. Based on th is model and the variation rates of the F-/F+ signals during 110 eV photon bombardment, a photolysis cross section of similar to 1.9x10(-17) cm(2) was determined. The photolysis cross section as a function of incident photon energy in the range 98-110 eV, near Si(2p), was also obtained, which shows a threshold at around 100 eV, corresponding to the same rise as the total e lectron yield (TEY) spectrum of the clean silicon substrate. This edge-jump indicates an enhancement of the photolysis efficiency by indirect excitati on of the substrate core-level [Si(2p)] electrons, and confirms the substra te-mediated excitation as the photolysis mechanism for the lowest-dosed sur face. However, for the medium- and the highest-dosed surfaces both the dire ct photodissociation and the substrate-mediated dissociation of the adsorbe d CF3Cl occur at early stages of photolysis, while only the substrate-media ted dissociation was observed at higher photon exposure. For photolysis of all three surfaces the SiF is the only fluorosilyl product observed on the surface. (C) 2000 American Institute of Physics. [S0021-9606(00)70919-6].