Dynamics of harpooning studied by transition state spectroscopy Na•••FH

This paper presents experimental results, for the first time, on photoinduced charge transfer in Na ... FH complexes, and relates these results to an extensive new ab initio study of the same process performed here. The complexes were obtained by crossing a beam of sodium with the expansion region of a supersonic jet of HF in excess helium. Photodepletion of these complexes occurred through excitation of the sodium chromophore followed by charge-transfer to the hydrogen halide, i.e. Na ... FH + h nu(1) --> [Na*... FH]double dagger --> [Na+... FH-]double dagger --> NaF + H (or Na + FH). Measurement of the photodepletion cross-section as a function of the excitation wavelength provided an approach to the study of these harpooning reactions by 'transition state spectroscopy', starting in selected transition-state geometries. The observed action spectrum for photodepletion consisted of two broad peaks with vibrational structure; a small peak centered at 780 nm and a larger one at 650 nm. Extensive ab initio calculations of ground and excited state potential-energy surfaces of NaFH indicated the presence of electronically excited states in the wavelength region corresponding to the observed peaks. These excited states correspond to Na in the 3p P-2 state perturbed by HF. Ab initio calculation revealed an avoided crossing between the first excited state and the ground state which could lead to reaction (--> NaF + H) from a restricted range of transition states.