Depolarisation of rotational orientation and alignment in OH (X2Π) + Xe collisions

Abstract

We have applied the polarisation spectroscopy (PS) technique to the collisional depolarisation of selected OH (X²Π, v = 0, F₁, J = 1.5 and 4.5, e) levels by Xe at room temperature (nominally 298 K). The measured total depolarisation rate constants, k^(K)_PS, 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. Neither k^(K)_PS is strongly J-dependent. k⁽²⁾_PS is consistently larger than k⁽¹⁾_PS, as expected for |J,m_J〉→ |J,m′_J〉 propensities that decline with |Δm_J|. We have predicted the population transfer rate constants, k_POP, via quantum scattering calculations on a recent ab initio OH(X)–Xe potential energy surface. Elastic depolarisation rate constants, k^(K)_DEP, have been inferred by difference, k^(K)_DEP = k^(K)_PS−k_POP. The results imply that elastic depolarisation is not substantially more rapid for Xe than for Ar, despite the corresponding increase from He to Ar. The dominant effect of the deeper attractive potential for OH(X)–Xe appears to be enhanced Λ-doublet transfer. This may speculatively be explained by the respective changes in odd and even terms in Legendre expansions of the potentials.