Optical pumping of molecules II. Relaxation studies

Abstract

The (ν″ = 3, J″ = 43) level of Na₂ has been optically aligned using the 4880‐Å line of a cw argon ion laser as a light source. The relaxation of this alignment is measured upon addition of foreign gas. In a low‐pressure regime, where the mean free path exceeds the diameter of the light beam which both pumps and samples the alignment, the relaxation of the alignment versus pressure shows a ``dog‐leg shape'' consisting of two linear regions. The first linear region corresponds to both elastic (velocity‐changing) and inelastic (primarily rotational transfer) collisional relaxation whereas the second corresponds purely to inelastic collisional relaxation. The former process is dependent on the mode structure of the laser and shows saturation with increased pressure. In this low‐pressure regime, the measured cross sections are shown to be lower bounds to the true cross sections and excitation by a multimode laser is shown to be inequivalent to excitation by a white light source when the width of the holes in the velocity distribution of the absorber molecules are nonoverlapping.