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

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(E_T) 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.