Close coupling calculations for rotational relaxation of CO in argon: Accuracy of energy corrected sudden scaling procedures and comparison with experimental data

Abstract

Fully quantal scattering calculations are carried out for CO in argon using both the close coupling (CC) and coupled states (CS) methods. CC and CS cross sections σ(j→j′) generally agree to within 15% or less with the exception of those corresponding to low Δj=|j′−j| values at low j where differences may reach 50%. The discrepancy arises mostly from efficient collisions with large orbital angular momentum, where the rotation of the quantization axis can no longer be neglected. Then, the CC calculations were used to test a scaling procedure based on the energy corrected sudden (ECS) approximation: given a set of basic cross-section σ(j→0), is it possible to predict the entire σo(j→j′) relaxation matrix? The ECS procedure yields reasonable agreement, on average, at the 13% level. However it fails at reproducing the Δj=1 cross sections in cases where the concept of a mean adiabaticity factor losses its physical meaning since the duration of the efficient collisions varies too much with the orbital angular momentum. On that basis, we have examined another question: the validity of an inversion procedure, based on the ECS scheme. Is it possible to determine the basic cross sections σ(j→0) starting from the knowledge of the easily measurable diagonal elements σo(j→j)? The ECS inverted basic rates agree with the CC ones to within about 20% up to j=15 and strongly diverge for higher j while the inversion leads to an overestimation of the mean duration of the efficient collisions. Then, using a high resolution Raman spectrometer, we recorded the Q-branch head of the fundamental band of CO in mixture with Ar at three temperatures, 87, 195, and 300 K, and total pressures up to 1.25 bar. Line-mixing effects in experimental Raman profiles are compared with CC theoretical predictions. Finally the close coupling results are also used to predict rotational relaxation times measured in free jets.