Collisional depolarisation of rotational angular momentum in the OH Radical

Abstract

Polarisation Spectroscopy (PS) has been used as a novel approach for measuring the collisional removal of bulk rotational orientation and alignment in OH (X 2Π3/2, v = 0, j = 1.5 – 6.5, e) and OH (A 2Σ+, v = 1, N = 1 – 5, f1). Both one-colour degenerate PS (OCPS) and two-colour PS (TCPS) have been exploited. TCPS provides a route to measuring thermal rate constants (298 K) for the collisional evolution of rotational polarisations in unique rotational quantum states. For OH (X), the dependence on the decay of the PS signal was investigated with a number of collision partners: He, Ar, Xe N2 and O2. The ability to remove PS signals increases across the series of noble gases He < Ar < Xe. In all cases the measured rate constant for loss of alignment is larger than that for orientation. This provides conclusive evidence that elastic depolarisation (the elastic redistribution of mj-sublevels) contributes to the loss of PS signals. The efficiency of this contribution is found to be modest for He, but significant for Ar and Xe. Comparison of the PS measurements with quantum scattering calculations assists in the evaluation of the magnitude of elastic depolarisation for the atomic partners and also provides a rigorous test of the potential energy surfaces describing their interaction. Intriguing differences are found between the kinematically similar N2 and O2. A detailed cross comparison between colliders, comparison of the measurements to the potential energy surface and independent theoretical work suggest that the weaker long-range attractive forces play a significant role in elastic depolarisation. The dynamics of OH (A) was measured with He and Ar only. PS experiments were compared to independent experimental and theoretical work carried out in parallel to this research. Distinctively different j-dependent trends for the removal of bulk rotational polarisations are observed for OH (X) + Ar and OH (A) + Ar. This comparative study emphasises the role of the attractive limbs of the potential energy surface in mediating elastic depolarisation.