THERMAL AND PHOTOINDUCED DESORPTION, DISSOCIATION, REACTIONS OF H2S ADSORBED ON SI(111)7X7

The initial stage of H2S adsorption on Si(111)7 X 7 at 68 K is complet ely dissociative. Adsorption in molecular form takes place only after the surface is saturated with decomposition products: H, HS and S. Che misorbed H,S desorb at 101 K (99 K for D2S); ice multitayers sublimate at 89 K. Annealing to room temperature results in coverage composed o f chemisorbed hydrogen and sulphur atoms. Annealing to T greater than or equal to 460 K results in formation of SiS2. Electronic EEL spectra of adsorbed H,S show a red shift of the absorption maximum of Delta E = 0.65 eV compared to gas phase. Under photon irradiation H2S decompo ses and desorb, yielding H-2 and H2S with HS fragments remaining on th e surface. Coverage and photon wavelength dependences reveal two mecha nisms for the photo-induced processes. The first, valid for high cover ages, involves direct excitation of H2S, leading to dissociation and d esorption. The second, characteristic for submonolayer coverages, invo lves a substrate mediated mechanism.