Photoinduced charge transfer reaction at surfaces. III. (HF)2...Na n/LiF(001)+hnu(640 nm)-->HFF-Na n +/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...Na(n) (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 approximately 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)x10(-19) cm(2). 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 Na(n)...(HF)(m) (n,m=1,2) and reaction products Na(n)F(m)H(m-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 Na(n)...(HF)(2) (n=1,2) species through cleavage of the NaF-HF and Na-Na(HF)(2) bonds.