Elastic depolarization and polarization transfer in CN(A2Π, v = 4)+Ar collisions

Rate constants for collisional loss and transfer of population and rotational angular momentum alignment have been determined for the CN(A(2)Pi, nu = 4) + Ar system. Aligned samples of CN(A(2)Pi , nu = 4, F-1, j = 1.5-23.5e) were prepared by optical pumping on the A-X(4,0) band. Their evolution was observed using Doppler-resolved frequency-modulated spectroscopy in stimulated emission on the A-X(4,2) band. State-resolved total population removal rate constants, and state-to-state rotational energy transfer (RET) rate constants, are found to be in excellent agreement with previous experimental measurements and theoretical predictions for the nu = 3 level. Rapid elastic depolarization of rotational alignment was observed for j = 1.5-6.5, with an average rate constant of 1.1 x 10(-10) cm(3) s(-1). This declines with increasing j, reaching zero within experimental error for j = 23.5. The polarization transfer efficiency of the initially created alignment in state-to-state RET was also determined for the selected initial state j = 6.5, F-1, e. Substantial depolarization of the alignment was observed for small Dj transitions. Alignment transfer efficiencies ranged from 0.55 +/- 0.06 for Delta j = -1, to 0.32 +/- 0.08 for Delta j = +3. These measurements are discussed with reference to recent experimental and theoretical advances on collisional depolarization of related open-shell species. We suggest that the surprisingly efficient collisional depolarization observed may be the result of the multiple potential energy surfaces involved in this system.