Photoinduced charge-transfer reaction at surfaces. II. HBr⋯Nan/LiF(001)+hv(610 nm)→Br−Nan+/LiF(001)+H(g)

Abstract

A sub-monolayer of atomic sodium was deposited on LiF(001) at 40–90 K. The adsorbed sodium atoms and clusters were dosed with HBr, to form HBr⋯Nan/LiF(001) (n=1,2,…) complexes which were then irradiated by 610 nm laser-light to induce charge-transfer reactions. The reaction-product atomic H(g) was observed leaving the surface, by two-color Rydberg-atom TOF spectroscopy. The H-atom translational energy in its “fast” (0.9 eV) component exhibited structure (40±10 meV spacing) attributed to vibration of the NaBr residue at the surface following photoinduced reaction in Na…HBr. The cross-section of the harpooning event was obtained as 7.5×10−19 cm2 for the “fast” H-atom reaction-product. Investigation of the coverage and temperature dependencies of the H-atom signal and of temperature programmed desorption (TPD) and x-ray photoelectron spectra gave an activation barrier for surface diffusion of Na-atoms Ediff<170 meV. High-level ab initio calculations were employed to interpret the TOF and TPD spectra. The reaction precursors and products, Na-HBr, Na+-HBr, Na2-HBr, Na2Br, NaBr, and NaBr-HBr, were investigated in terms of potential energy surfaces, equilibrium structures, binding energies, and vibrational frequencies.