Electronic quenching of OH a ²Σ⁺ (ν′ = 0,) in complexes with hydrogen and nitrogen

The quenching of electronically excited OH A ²Σ⁺+ radicals has been investigated in complexes of OH with molecular hydrogen, deuterium, and nitrogen and through complementary theoretical calculations. Many of the intermolecular vibrational levels supported by the OH A ²Σ⁺ (ν′ = 0, 1) + H₂, D₂, and N₂ potentials have been characterized by laser-induced fluorescence and fluorescence depletion measurements of the complexes in the OH A ²Σ⁺-X²Π 1-0 and 0-0 spectral regions. Homogeneous line broadening of the spectral features yields picosecond lifetimes for complexes prepared in levels derived from OH A ²Σ⁺ (ν′ = 0) as a result of electronic quenching and/or chemical reaction. More extensive line broadening is observed for complexes excited to levels correlating with OH A ²Σ⁺ (ν′ = 1). The corresponding decay rates are 10-75 times faster than obtained for ν′= 0 due to the opening of the vibrational predissociation channel and/or enhancement of the quenching/reaction processes upon OH vibrational excitation. Ab initio calculations of the OH (A ²Σ⁺, X ²Π) + H₂ and N₂ potential energy surfaces reveal the minimum energy configurations, T-shaped O-H-H₂ and linear O-H-N≡N, and the large increases in interaction energy upon electronic excitation of OH. The theoretical calculations also identify specific orientations, T-shaped H-O-H₂ and linear H-O-N≡N, that lead to conical intersections between the ground- and excited-state surfaces and give rise to quenching of OH A ²Σ⁺ by hydrogen and nitrogen.