Depolarisation of rotational orientation and alignment of OH (X2Π) in collisions with molecular partners: N2 and O2

Abstract

The depolarisation of selected OH (X²Π_3/2v = 0, J = 1.5 and 4.5, e) levels in collisions with the molecular partners N₂ and O₂ at room temperature (nominally 298 K) has been studied using the polarisation spectroscopy (PS) technique. We obtain total depolarisation rate constants, k^(K)_PS, which are the combination of population transfer out of the initial level and elastic depolarisation of the tensor moment of respective rank K = 1 (orientation) or K = 2 (alignment) of its angular momentum distribution. N₂ causes more rapid decay of PS signals than O₂. There are no clear dependences of k^(K)_PS on J for either partner. The K-dependence for N₂ mirrors that determined previously for the noble gases, but is less regular for O₂, warranting further investigation. Comparison with independent line-broadening data suggests that there may be an additional, pure-elastic-dephasing contribution to collisional broadening for N₂ that is not apparent for O₂. The presence of an independently established deeper HO–OO attractive minimum at shorter range clearly does not outweigh other factors that favour k^(K)_PS for N₂.The most obvious explanation is stronger, longer-range attractive interactions due to the larger quadrupole moment of N₂. However, this appears to be contradicted by the rigorous ab initio calculations currently available on OH–O₂.