Laser probing of alignment effects in collisional energy transfer: the anisotropy of the interaction potential

Abstract

Recently there has been considerable experimental and theoretical interest in the study of orientation and alignment effects as a probe of geometry-dependent aspects of potential energy surfaces. In this contribution we report experiments in which the collisional alignment of the electronic orbital angular momentum of excited Ca atoms and of the rotational angular momentum of N 2 + ions are investigated. The effect of orbital alignment on the slightly exothermic energy transfer process from Ca ( 4 s 5 p 1 P 1 ) to Ca ( 4 s 5 p 3 P J ) in collisions with different rare gas atoms is studied in a crossed beam experiment. Initial orbital alignment is introduced by exciting Ca with a linearly polarized, pulsed ultraviolet laser. The relative inelastic cross section as a function of the initial alignment is measured by time-gated total fluorescence detection. The transition probability depends on the configuration of approach: for He and Ne enhancement is observed for the perpendicular vs the parallel approach while Xe leads to an opposite effect. In a flowing afterglow experiment the rotational alignment of N 2 + ions drifted in He buffer gas is studied. The extent of alignment is determined as a function of field strength and total angular momentum by measuring the degree of polarization of laser-induced fluorescence vs the angle of polarization of the exciting laser. Since the probed (J, M J ) distribution is the result of many collisions, the alignment can be related to the rate coefficients for specific (ΔJ, ΔM J ) transitions and therefore reveals information on the anisotropy of the N 2 + buffer gas interaction.