Collisional Transfer between Λ-Doublet Levels of OH(X2Π3/2, v = 1, j = 3.5, and 6.5) in Collisions with He, Ar, N2, and HNO3

Using an infrared−ultraviolet double resonance method, we have measured rate coefficients at room temperature for the transfer of OH radicals between the Λ-doublet levels associated with the j = 3.5 and j = 6.5 rotational levels of the X 2 Π, Ω = 3 / 2, v = 1 vibronic state in collisions with He, Ar, N 2, and HNO 3 . OH radicals were generated by 266 nm pulsed laser photolysis of HNO 3 and promoted to selected Λ-doublet levels in j = 3.5 and j = 6.5 using a pulsed infrared laser tuned to the appropriate line in the (1, 0) infrared fundamental band of OH. The evolution of population in each Λ-doublet component of the selected rotational line was then observed using time-delayed laser-induced fluorescence in the (1, 1) band of the A 2 Σ + −X 2 Π system. The measured rate coefficients ( k Λ ) show a strong dependence on collision partner with k Λ (He) < k Λ (Ar) < k Λ (N 2 ) ≪ k Λ (HNO 3 ) and a significant dependence on rotational level, with the values of k Λ larger for j = 3.5 than for j = 6.5, especially for HNO 3 as the collision partner.