Rotational relaxation of high-N states of OH (X²Π, v=1–3) by O₂

We have studied the relaxation of OH(X(2) Pi, v, N) produced by the reaction O(D-1) + H-2 --> OH* + H. Infrared emission measurements of the fundamental vibration-rotation band of OH were acquired at a temperature of 100 K in a large cryogenic chamber. Trace amounts of ozone were added to mixtures of H-2, O-2, and Ar at low (less than or equal to 38 mT) pressures. The ozone was photolyzed to produce O(D-1). Rapid reaction with H-2 produced OH*. Spectrally and temporally resolved emissions from levels up to the exothermic limit were observed with sufficient spectral resolution to permit kinetic analysis of individual level populations. The production rate was observed to scale as the rotational quantum number except at the highest levels populated. At early times we observed inverted rotational state distributions that subsequently relaxed to form thermal distributions in each vibrational level. Under these conditions, rotational relaxation was rapid in comparison with vibrational relaxation. Rotational relaxation within a given vibrational state could be represented by single quantum collisional exchange at near-gas-kinetic rates. The rotational level dependence of the deduced relaxation was determined.