P. L. Chapovsky

Description of light-induced drift in terms of transport mean paths

Light-induced drift is often described in terms of a change Delta nu in the collision rate (or of the transport collision rate) caused by optical excitation of atoms or molecules immersed in a buffer gas. One assumes (at least tacitly) that the collision rates are independent of velocity, though this is true only for heavy particles in a light buffer gas. The recently observed anomalous light-induced drift of molecular gases suggests using a speed-dependent Delta nu which, however, is difficult to justify. The formulation in terms of speed-dependent transport mean paths, proposed here, is free of such ambiguities. For molecules a Wang-Chang-Uhlenbeck-de Boer multilevel formalism leads to extended transport mean paths, whereupon anomalous light-induced drift can be interpreted in a plausible way.

Intermolecular versus intramolecular interactions in nuclear spin conversion: Experiments on 13CH3F–O2

Ortho–para conversion for gaseous 13CH3F is measured in mixtures with O2. As collision partner, O2 is found to be 4 times less efficient for conversion that CH3F itself. This demonstrates that intramolecular rather than intermolecular magnetic interactions provide the main pathway leading to nuclear spin conversion for such molecules.

Influence of molecular rotation on light-induced drift of CH3F

Experimental results on light-induced drift of (ro)vibrationally excited CH3F immersed in the noble buffer gas Kr or in the polar buffer gas CH3Cl are presented. For pure vibrational excitation, the relative change in collision rate is found to be essentially velocity independent. For rovibrational excitation, this quantity can have a significant velocity dependence, as can be concluded from the detuning behaviour of light-induced drift for two transitions of CH3F immersed in Kr. In combination with earlier observations of anomalous light-induced drift in C2H4, these results demonstrate that a sizable velocity dependence of the change in collision rate caused by rovibrational excitation is a general feature for molecular systems. Additionally, while the transport collision rate generally increases with vibrational quantum number, the data suggest that it decreases with increasing rotational quantum number. Finally, the data for CH3F in CH3Cl indicate that rotational-state-changing collisions are accompanied by a significant velocity change.

Intermolecular potential for vibrationally excited CH3F measured by light‐induced drift

From changes in the diffusion coefficient upon vibrational excitation of CH3F in He, Ne, Ar, Kr, and Xe, as measured by light‐induced drift, the intermolecular potential between two vibrationally excited CH3F molecules is determined on the basis of simple combination rules. The main effect of vibrational excitation is found to be an increase in potential well depth by 3%.

Molecular rotation and kinetic collisions: a systematic study of CH3F in the nu 4 band

In order to determine the effect of molecular rotation on gas kinetic collisions, light-induced drift measurements were performed on CH3F in the nu 4 rovibrational band. Data were taken for a large set of transitions of the type v=0 to 1, Delta J=1, Delta K=1 over a range of values of J or K. Systematic trends in the J- and K-dependence of the kinetic collision rate have been observed. For Kr and CH3Cl as collision partners the K-dependence is found to be large and nearly linear in K, whereas the J-dependence is found to be small. We present a theoretical model to describe the dependence on the rotational quantum numbers of the relative change in collision rate upon excitation; this model is in satisfying agreement with the experimental results.

Rovibrational-state-dependent collisions of C2H4-Kr at low temperatures

Light-induced drift experiments at temperatures between 130 and 300 K have been performed on C2H4 With Kr as a collision partner. The experimental results for various rovibrational transitions in the nu 7 fundamental vibrational band are expressed in terms of the relative change in collision rate upon excitation, Delta nu / nu , as a function of the mean relative speed grms between the collision partners. For low grms, the collision rate nu is found to depend strongly on the rotational states involved. For high grms, the rotational dependence is relatively weak, and the values of Delta nu / nu converge to what is interpreted as the pure vibrational contribution.