Photoinduced charge transfer reaction at surfaces. III. (HF)2⋯Nan/LiF(001)+hν(640 nm)→HFF−Nan+/LiF(001)+H(g)

A sub-monolayer of atomic sodium was deposited on a LiF(001) surface at 40 K. The adsorbed sodium exists at the surface as single atoms and clusters. The surface was dosed with 1 L of HF, to form adsorbed (HF)2⋯Nan (n=1,2,3,…) complexes, which were then irradiated by 640 nm laser light, to induce charge-transfer reaction. The reaction-product atomic H(g) was observed leaving the surface by two-color Rydberg-atom time-of-flight (TOF) spectroscopy. The TOF spectrum of the desorbed H atoms contained two components; a “fast” component with a maximum at ≈0.85 eV, and a “slow” component with a maximum at 0.45 eV. These two components were attributed to photoreaction on adsorbed single atoms and clusters of sodium, respectively. The fast component exhibited a structure (48±17 meV spacing) near the high-energy end of spectrum. This structure was attributed to vibration of NaFHF photoproduct residing on the surface. The cross section of the harpooning event in the Na⋯(HF)2 adsorbed complex was determined as (9.1±2.0)×10−19 cm2. To interpret the experimental vibrational structure and the relative energies of the fast and slow components of the TOF spectrum, high-level ab initio calculations were performed for reactants Nan⋯(HF)m (n,m=1,2) and reaction products NanFmHm−1. The calculated NaF–HF and Na–Na(HF)2 bond dissociation energies indicated that photoexcitation of the precursor complexes led not only to ejection of H atoms, but also to dissociation of the Nan⋯(HF)2 (n=1,2) species through cleavage of the NaF–HF and Na–Na(HF)2 bonds.