G. J. van der Meer

Role of rotational alignment in molecule–surface interaction for CH3F and OCS

The influence of rotational alignment on molecule–surface interaction is studied for CH3F and OCS colliding with a glass surface. Experiments were performed at 285 K with the technique of surface light‐induced drift using a flat channel, the alignment being produced through excitation by linearly polarized light. For molecules having rotational energy well below thermal, it is found that the accommodation coefficient for parallel momentum α, which can be related to the trapping/desorption probability, is larger if the angular momentum J is parallel to the surface (‘‘cartwheeling motion’’) than if perpendicular (‘‘helicopters’’). For CH3F the experiments indicate that this difference decreases strongly with increasing K, denoting the component of J along the principal molecular axis. Experiments on OCS confirm this behavior. For molecules having rotational energy well above thermal, however, the reverse behavior is found, viz., α is larger for helicopters than for cartwheels. This is consistent with molecu...

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.

Experiments on surface light-induced drift for various systems

Surface light-induced drift of a low-pressure gas results from velocity-selective optical excitation combined with state-dependent molecule-surface interaction. This effect has been studied for CH3F and OCS, rovibrationally excited by a CO2 laser, interacting with various surfaces. For the R(4, 3) transition of 13CH3F, an increase in tangential momentum accommodation coefficient α upon excitation is found ranging from δα = 1.0 × 10-3 for stainless steel to δα = 14 × 10-3 for LiF(100) at 295 K. For this transition, the effect was also studied as a function of temperature for a quartz surface, resulting in a 40% decrease of δα as the temperature is raised from 300 to 700 K. Experiments for the P(5) transition of OCS yield δα = -0.53 × 10-3 on a quartz surface at 295 K.

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%.

Gas-kinetic effects resulting from velocity-selective excitation

Abstract Various kinetic effects are discussed which arise from velocity-selective rovibrational excitation in molecular gases. Prerequisites for such effects are state-dependent collisional interactions or rapid collisional de-excitation. The emphasis is on the first type, with the state-dependent interaction being either with a foreign gas (giving rise to light-induced drift) or with a surface (giving rise to surface light-induced drift). Experiments investigating these phenomena are briefly described and their results discussed. Applications include determination of vibrational-state-dependent kinetic cross sections, isotope separation, and the study of the rotational-state-dependence of molecule-surface interactions.