SHENG-GUI HE, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100080, P.R.C.; DENNIS J. CLOUTHIER, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.

The A 2A1 - X 2B1 electronic transition of the jet-cooled AsH2 free radical has been studied by laser-induced fluorescence (LIF), wavelength-resolved emission, and fluorescence lifetime measurements. The radical was produced by a pulsed electric discharge through a mixture of arsine (AsH3) and high-pressure argon at the exit of a pulsed valve. Nine vibronic bands were identified by LIF spectroscopy in the 505-400 nm region, including a long progression in the bending mode and two bands (101 and 101201) involving the excited state As-H symmetric stretch. Single vibronic level emission spectra showed similar activity in the bending and symmetric stretching frequencies of the ground state. High- resolution spectra of the 000 band exhibited large spin-splittings and small, resolved arsenic hyperfine splittings, due to a substantial Fermi contact interaction in the excited state. The rotational constants obtained in the analysis gave effective molecular structures of r0'' = 1.5183(1) Å, theta0'' = 90.75(1)o and r0' = 1.4830(1) Å, theta0' = 123.10(2)o. The excited state fluorescence lifetimes vary dramatically with rovibronic state, from a single value of 1.4 µs to many with lifetimes less than 10 ns, behavior which we interpret as signaling the onset of a predissociative process in the excited state.