PHOTOCHEMISTRY OF ADSORBED MOLECULES .6. ULTRAVIOLET PHOTOREACTION OF OCS ON LIF(001)

The surface-aligned photoreaction (PRXN) of OCS physisorbed on LiF(001) was investigated at 222 nm. Ultraviolet PRXN within the adsorbate was detected over a wide range of coverages. Diatomic sulphur was produced by reaction of photolytically generated sulphur atoms with a coadsorbed OCS molecule: S*, S + OCS(ad) →S2*, S2(g) + CO(ad/g). Translational energy and angular distributions of the S2 reaction product were obtained by angle-resolved time-of-flight mass spectrometry. The dynamics of PRXN were found to vary with adsorbate coverage. At low coverages (<10-2 ML), reaction was attributed to a "direct" abstraction mechanism, leading to energetic S2. As the coverage was increased above 10-2 ML, both "direct" and "indirect" PRXN dynamics were observed. Possible origins of these differing photoreaction dynamics are discussed. © 1990 American Institute of Physics.