Thermal and photo-induced desorption, dissociation, reactions of H2S adsorbed on Si(111) 7 × 7

The initial stage of H2S adsorption on Si(111) 7×7 at 68 K is completely dissociative. Adsorption in molecular form takes place only after the surface is saturated with decomposition products: H, HS and S. Chemisorbed H2S desorb at 101 K (99 K for D2S); ice multilayers sublimate at 89 K. Annealing to room temperature results in coverage composed of chemisorbed hydrogen and sulphur atoms. Annealing to T ≥ 460 K results in formation of SiS2. Electronic EEL spectra of adsorbed H2S show a red shift of the absorption maximum of ΔE = 0.65 eV compared to gas phase. Under photon irradiation H2S decomposes and desorb, yielding H2 and H2S with HS fragments remaining on the surface. Coverage and photon wavelength dependences reveal two mechanisms for the photo-induced processes. The first, valid for high coverages, involves direct excitation of H2S, leading to dissociation and desorption. The second, characteristic for submonolayer coverages, involves a substrate mediated mechanism.