Photodissociation, photoreaction and photodesorption of adsorbed species: Part 2 - CH3Br and H2S on LiF(001)

Abstract

Submonolayers of CH3Br and H2S, on an LiF(001) surface, were irradiated by pulsed ultraviolet (u.v.) in ultrahigh vacuum. Translational energies and angular distributions were obtained for photofragments, photoreaction products and photodesorbed molecular species by angularly resolved time-of-flight to a mass spectrometer. Single-photon photolysis of the adsorbates led to photofragment energy and angular distributions that, in each case, differed from those for gas-phase photolysis. For CH3 from CH3Br photolysis at 222 nm the peak of the translational energy distribution was 1.7 eV (f.w.h.m. = 0.5 eV), shifted to lower energy from the gas phase. For the Br photofragment at 222 nm the energy distribution extended to energies 0.6 eV in excess of that observed in the gas. The translational energy distribution for H from H2S, at 222 and 193 nm, gave evidence of increased vibration in the HS fragment as compared with the gas phase. For H2S coverages approaching one monolayer, H2 was formed in a yield that increased with coverage; this was attributed to the exchange reaction H→ + H2S(ads) → H2(g) + HS, in which the H came from an adjacent H2S(ads). This type of surface-aligned photoreaction (SAP) is modelled here for H→ + BrH(ads) by a classical trajectory computation. SAP may assist in explaining the emergence of more complex molecules from simpler ones in prebiotic synthesis. Absorption of u.v. by crystalline LiF was found to desorb efficiently every adsorbate with which we have experimented (CH3Br, H2S, H2O, HBr and Xe). We surmise that this involves excitation of colour centres (seen to fluoresce) which give rise to an optoacoustic shock.