Electronic spectroscopy and quenching dynamics of OH-H₂/D₂ pre-reactive complexes

Binary complexes of OH X ²∏ and H₂/D₂ have been stabilized in the entrance valley to the hydrogen abstraction reaction and identified in the OH A ²∑⁺-X ²∏ 0-0 spectra region. Nearly all of the intermolecular vibrational levels supported by the OH A ²∑⁺ (v′=0)+H₂/D₂ potential energy surface have been observed in fluorescence depletion experiments. Rapid electronic quenching precludes the observation of OH-H₂/D₂ prepared in these levels by laser-induced fluorescence. A sharp onset of laser-induced fluorescence occurs at the OH A ²∑⁺ (v′=0)+H₂/D₂ dissociation limit. The binding energies for OH-H₂/D₂ in the ground state correlating with OH X ²∏ (v″=0)+H₂/D₂ have been determined to be 54 cm^-1 and more than 66 cm^-1, respectively. The OH A ²∑⁺ (v′=0)+H₂/D₂ excited state is found to be at least 577 cm^-1 (H₂) and 639 cm^-1 (D₂) more strongly bound than the ground state. The positions of observed features are compared with the corresponding intermolecular levels observed by laser-induced fluorescence in the OH A-X 1-0 region as well as theoretical predictions of the transition energies based on ab initio potentials for the ground and excited electronic states. The OH-H₂/D₂ intermolecular levels correlating with OH A ²∑⁺ (v′=0)+H₂/D₂ have lifetimes of 3.2-4.5 ps, deduced from homogeneous linewidths, due to quenching and/or chemical reaction.