Depolarisation in Na-Ar optical collisions

Abstract

The depolarisation of the redistributed Na D1 and D2 line fluorescence caused by Na-Ar collisions after far-wing excitation was investigated. In the limit of zero Ar density the J=1/2 orientation and J=3/2 alignment reflect the distribution among Zeeman sublevels of Na 32PJ produced after absorption of a photon into one or several excited Na-Ar molecular states at small internuclear separation, followed by a mixing between these states during dissociation. Measurements of these quantities as functions of the detuning up to Delta omega J=50 cm-1 in the red wings are compared with quantum-mechanical coupled-channel (CC) calculations using realistic molecular potentials and thermal averaging, and with predictions from the factorisation approximation (FA) of the theory of redistribution. Fair agreement with CC calculations and FA predictions was obtained within the impact region, proving the validity of the factorisation assumption there. Outside this region the overall depolarisation is primarily due to reorientation of the Na 3P electronic angular momentum and is reproduced qualitatively by the CC calculations but not by the FA theory. Additional depolarisation connected with the fine-structure coupling was observed.