Photochemistry of adsorbed molecules. IV. Photodissociation, photoreaction, photoejection, and photodesorption of H2S on LiF(001)

Publisher Website
Google Scholar
Abstract
The UV surface‐aligned photochemistry (SAP) of submonolayers of H2S physisorbed on LiF(001) has been examined. Translational energy and angular distributions for photodissociation products and for H2S molecules leaving the surface after pulsed laser irradiation at 193 and 222 nm were measured by angularly resolved TOF to a quadrupole mass spectrometer. Single photon surface‐aligned photodissociation (PDIS) of adsorbed H2S produced H with structured translational energy distributions P(ET) indicative of vibrational excitation within the complementary SH fragments. The SH vibrational distribution was bimodal and varied markedly with H2S coverage. Photoreaction (PRXN) within the adsorbate layer occurred as the H2S coverage increased beyond ∼0.1 monolayer. Molecular hydrogen was produced by PRXN of H with adjacent H2S(ad) molecules. The product H2 translational energy distribution showed evidence of both direct and indirect PRXN dynamics. At coverages greater than one monolayer, photoejection (PEJ) of H2S molecules with translational energies up to several eV was observed; PEJ was thought to be due to interadsorbate quenching of electronically excited H2S. At all the coverages examined, absorption of UV by the LiF substrate was found to photodesorb (PDES) H2S molecules with low translational energies (0–0.5 eV). The PDES was ascribed to an acoustic wave produced by laser excitation of color centers in the LiF, which were seen to fluoresce.