Photochemistry of adsorbed molecules. VII. Ultraviolet photoejection and photodesorption of OCS on LiF(001)

Abstract

The UV photoejection and photodesorption of OCS on LiF(001) at 222 nm are reported. Translational energy and angular distributions were obtained by angle-resolved TOF mass spectrometry. Energetic photoejection (PEJ) product, characterized by a peak translational energy T′p∼0.3 eV, was detected for ≳0.5 ML coverages of OCS(ad). The PEJ angular distribution was sharply peaked around the surface normal, ∼cos 18θ. The initial absorption of energy was into the chromophore of an OCS molecule. Molecular photodesorption (PDES) product, characterized by low translational energy, was detected for coverages ≥10−2 ML. The translational energy distribution P(T′) was found to be a sensitive function of detection angle, adsorbate coverage and laser energy. Peak translational energies and FWHM’s were ∼0.05 and ∼0.20 eV, respectively. Angular distributions were also sensitive to coverage, narrowing from ∼cos θ to cos 11θ with increasing coverage. Photodesorption is thought to involve absorption of radiation by defect centers in the LiF(001), with conversion of the absorbed energy into phonons which, following propagation to the surface, induce desorption of OCS(ad).