Photochemistry of adsorbed molecules. IX. Ultraviolet photodissociation and photoreaction of HBr on LiF(001)

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Abstract
The dynamics of the UV photochemistry of HBr on LiF(001) has been studied by angle-resolved time-of-flight mass spectrometry in ultra-high vacuum. Single-photon photodissociation of adsorbed HBr at 193 nm resulted in photofragment translational energy distributions that differed from those produced in the gas-phase photolysis. Angular distributions of the fast H-atom photofragments peaked at 55±5° to the surface normal, consistent with a preferentially oriented adsorbate geometry. The angular distribution of atomic H obtained from photodissociation of HBr(ad) using polarized light indicated that a substantial fraction of the H atoms collided with the surface before leaving it. Two types of photoreactions in the adsorbed state were observed. Molecular hydrogen was formed in the photoinitiated abstraction reaction, H+HBr(ad)→H2(g)+Br, and its markedly non-Boltzmann translational energy distribution was found to have less energy than would be consistent with gas-phase experiments (performed elsewhere). Photoproducts from the bimolecular reaction 2HX(ad)→H2+X2, X=Cl, Br were also observed in the present study. This photoreaction, which did not depend on prior photodissociation of HX(ad), is thought to proceed through electronic excitation of an HX dimer in the adsorbed state.