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(A 2 Π, v = 4) + Ar system. Aligned samples of CN(A 2 Π, v = 4, F 1 , j = 1.5 -23.5 e) 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. Stateresolved 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 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 ∆j transitions. Alignment transfer efficiencies ranged from 0.55 ± 0.06 for ∆j = -1, to 0.32 ± 0.08 for ∆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.

Collisions of electronically excited molecules: differential cross-sections for rotationally inelastic scattering of NO(A2Σ+) with Ar and He

“…These vector correlations are particularly informative with regard to testing the details of the calculated potential energy surface (PES) via comparison with semi-classical (QCT) or full quantum scattering (QS) calculations [23]. Excited state vector measurements are challenging, although one approach that has seen considerable recent activity is the laser-based measurement of the removal of rotational angular momentum in a thermal collision bath, and associated theoretical development [24][25][26][27]. The few measurements of differential scattering cross-sections (DCS) for electronically excited species are essentially limited to experiments combining the 'photo-loc' method with optical pumping in a thermal bath to study CN(A 2 Å) þ Ar collisions [28], and our proof-of-concept publication of the technique applied in this current paper [29].…”

Collisional depolarisation in electronically excited radicals

“…Using two lasers in an optical-optical double resonance (OODR) or 1 + 1 LIF technique can give extra rotational state resolution compared to the one-photon LIF method, for which the fluorescence must be dispersed to gain such resolution. Costen et al [18,25] have recently used a variant of OODR on CN(A) in which the probe laser is tuned to stimulated emission on the A ← X band. In their experiments, the probe laser is frequency modulated, giving greater sensitivity.…”

“…Costen and co-workers have recently studied elastic depolarisation and RET in CN(A 2 Π, v = 4) + Ar, in references [18,25] and reviewed in reference [24]. The differential cross sections for selected inelastic transitions in CN(A) + Ar have also been recently measured [79].…”

Parity-Dependent Rotational Energy Transfer in CN(A2Π, ν = 4, j F1ε) + N2, O2, and CO2 Collisions

“…The experimental apparatus has been described in detail previously. 10 − 12 , 15 , 16 , 32 The experiments were performed in a 2 m longitudinal vacuum chamber, within a region shielded by a μ-metal cylinder to isolate the sample from stray magnetic fields. 33 A 10 sccm mass flow controller (Aera) provided a slow flow of collider gas (N 2 or O 2 research grade >99.9995%, CO 2 research grade >99.999%, BOC Gases), which carried ICN (Acros Organics, 98%) into the vacuum chamber.…”

“…The CN(A 2 Π) + Ar system has been one for which RET has been intensively studied by both experiment and QS theory. 12 , 17 − 19 We have recently collaborated with Dagdigian, Alexander, and co-workers in a combined experimental and theoretical study. 10 , 11 Using the same basic experimental approach as that used to obtain the results presented in this paper, CN(A 2 Π, ν = 4, j Fε) was prepared by optical pumping in a thermal (296 K) bath of Ar.…”