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 ji = 1.5 (N = 1) and ji = 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 ji 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 ji = 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 ji, free of complications arising from the transfer of population between the two Λ-doublet levels. For a wider range of levels, including ji = 6.5, we have carried out simpler measurements in which rate coefficients for transfer from the initially excited level ji 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 ji = 6.5 and the rate coefficients derived from the two sets of measurements are in good agreement. The measured rate coefficients for rotational relaxation (kRET) show a significant dependence on both the collision partner, with kRET(He) ≈ kRET(Ar) < kRET(N2) < kRET(HNO3), and on the rotational level with the values of kRET generally decreasing as ji is increased beyond ji = 3.5.
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