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

Abstract

Rate constants for collisional loss and transfer of population and rotational angular momentum alignment have been determined for the CN(A²Π, v = 4)+Ar system. Aligned samples of CN(A²Π, v = 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 v = 3 level. Rapid elastic depolarization of rotational alignment was observed for j = 1.5–6.5, with an average rate constant of 1.1 × 10⁻¹⁰ cm³ 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 Δj transitions. Alignment transfer efficiencies ranged from 0.55 ± 0.06 for Δj = −1, to 0.32 ± 0.08 for Δj = +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.

Keywords:

• alignment
• polarization
• inelastic collisions
• rotational energy transfer
• rate constant

Acknowledgements

We thank Prof. P.J. Dagdigian for the communication of the state-to-state rate constants included in Figure 8. We thank the Research Councils UK for an Academic Fellowship for M.L.C. and the Leverhulme Trust for a project grant.