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

The depolarisation of selected OH (X-2 Pi(3/2) v = 0, J = 1.5 and 4.5, e) levels in collisions with the molecular partners N-2 and O-2 at room temperature (nominally 298 K) has been studied using the polarisation spectroscopy (PS) technique. We obtain total depolarisation rate constants, k(PS)((K)), 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. N-2 causes more rapid decay of PS signals than O-2. There are no clear dependences of k(PS)((K)) on J for either partner. The K-dependence for N-2 mirrors that determined previously for the noble gases, but is less regular for O-2, warranting further investigation. Comparison with independent line-broadening data suggests that there may be an additional, pure-elastic-dephasing contribution to collisional broadening for N-2 that is not apparent for O-2. The presence of an independently established deeper HO-OO attractive minimum at shorter range clearly does not outweigh other factors that favour k(PS)((K)) for N-2. The most obvious explanation is stronger, longer-range attractive interactions due to the larger quadrupole moment of N-2. However, this appears to be contradicted by the rigorous ab initio calculations currently available on OH-O-2.