Photon-exposure-dependent photon-stimulated desorption for obtaining photolysis cross section of molecules adsorbed on surface by monochromatic soft x-ray photons

Photon-exposure-dependent positive- and negative-ion photon-stimulated desorption (PSD) was proposed to study the photoreactions and obtain the photolysis cross sections of molecules adsorbed on a single-crystal surface by monochromatic soft x-ray photons with energy near the core level of adsorbate. The changes in the F⁺ and F- PSD ion yields were measured from CF₃Cl molecules adsorbed on Si (111) -7×7 at 30 K (CF ₃Cl dose=0.3× 10¹⁵ molecules cm², ∼0.75 monolayer) during irradiation of monochromatic soft x-ray photons near the F (1s) edge. The PSD ion yield data show the following characteristics: (a) The dissociation of adsorbed CF₃Cl molecules is due to a combination of direct photodissociation via excitation of F (1s) core level and substrate-mediated dissociation [dissociative attachment and dipolar dissociation induced by the photoelectrons emitting from the silicon substrate]. (b) the F⁺ ion desorption is associated with the bond breaking of the surface CF₃Cl, C F2 Cl, CFCl, and SiF species. (c) the F- yield is mainly due to DA and DD of the adsorbed CF₃Cl molecules. (d) The surface SiF is formed by reaction of the surface Si atom with the neutral fluorine atom, F⁺, or F- ion produced by scission of C-F bond of CF₃Cl, C F2 Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of this submonolayer CF₃Cl -covered surface. Based on this model and the variation rates of the F⁺ F- signals during fixed-energy monochromatic photon bombardment at 690.2 and 692.6 eV [near the F (1s) edge], the photolysis cross section was deduced as a function of energy.