Rate coefficients for rotational energy transfer from the levels OH(X 2Π3/2, v = 1, ji = 1.5, 3.5–8.5) in collisions with He, Ar, N2 and HNO3

Abstract

Using an infrared-ultraviolet double resonance method, we have measured rate coefficients at room temperature for the transfer of OH radicals from rotational levels between j_i = 1.5 (N = 1) and j_i = 8.5 (N = 8) in the X ²Π, Ω = 3/2; v = 1 vibronic state in collisions with He, Ar, N₂ and HNO₃. OH radicals were generated by 266 nm pulsed laser photolysis of HNO₃ and promoted to selected j_i using a pulsed infrared laser tuned to an appropriate line in the (1,0) infrared fundamental band of OH. The evolution of population in selected levels was observed using time-delayed laser-induced fluorescence in the (1,1) band of the A ²Σ⁺–X ²Π system. The results of two kinds of measurement are reported. For j_i = 3.5 and 6.5, a single Λ-doublet component of the selected rotational level was excited and the evolution of the populations in both Λ-doublet components was observed. The decay of the sum of the two individual populations then yields rate coefficients for the transfer of population from j_i, free of complications arising from the transfer of population between the two Λ-doublet levels. For a wider range of levels, including j_i = 6.5, we have carried out simpler measurements in which rate coefficients for transfer from the initially excited level j_i are inferred by monitoring only the change in population in the Λ-doublet level that is directly populated by the pump laser. Measurements of both kinds have been carried out for j_i = 6.5 and the rate coefficients derived from the two sets of measurements are in good agreement. The measured rate coefficients for rotational relaxation (k_RET) show a significant dependence on both the collision partner, with k_RET(He) ≈ k_RET(Ar) < k_RET(N₂) < k_RET(HNO₃), and on the rotational level with the values of k_RET generally decreasing as j_i is increased beyond j_i = 3.5.