Surface Light-induced Drift Research Articles

Light-induced kinetic effects in molecular gases

Abstract Velocity-selective excitation can be achieved by tuning a narrow-band laser within the Doppler-broadened absorption profile of a gas. If the excitation modifies the kinetic properties of the molecules (e.g., the kinetic cross-sections or the molecule-surface interactions), the Maxwell velocity distribution will be distorted. This gives rise to a new class of kinetic effects that do not require external gradients to be imposed on the system; rather, the laser intensity and its gradient can be considered as thermodynamic forces. Examples are light-induced drift, surface light-induced drift and light-induced viscous flow. A phenomenological overview is given of the various light-induced kinetic effects which can arise in pure gases or mixtures. This includes effects resulting from velocity-selective excitation immediately followed by collisional de-excitation (‘velocity-selective heating’). The theoretical description of the various effects is briefly reviewed. Experiments are described which invest...

Surface light-induced drift affected by chemical reactions.

Surface light-induced drift caused by backscattering from rough surfaces can be strongly affected by surface-enhanced state-specific chemical reactions. Expressions for the drift fluxes of the incident resonant-gas component and of the reaction product are presented. A resulting asymmetry in the spatial distributions of the components in a T-shaped enclosure is discussed, as a possible sensitive tool for studying the state specificity of the scattering and reaction processes

Gas-kinetic effects resulting from velocity-selective excitation

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.

Experiments on surface light-induced drift for various systems

Abstract 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 CH 3 F and OCS, rovibrationally excited by a CO 2 laser, interacting with various surfaces. For the R(4, 3) transition of 13 CH 3 F, 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.

Surface light-induced drift caused by roughness.

A possible mechanism of surface light-induced drift (SLID) under velocity-selective excitation arising from the roughness of the cell wall surface is discussed here both qualitatively and quantitatively. Due to the backscattering from the facets of roughness SLID can arise even when scattering is completely locally diffusive. It provides significant widening of the scope of real physical situations where SLID can occur. This mechanism can basically explain the unexpected sign of SLID observed in recent experiments [Phys. Rev. Lett. 59, 447 (1987); Phys. Rev. A 42, 6471 (1990)]. As a method to study gas-surface interactions, SLID should be combined with some technique providing study of the surface topography.

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 molecular beam data on the system NO/Pt(111) studied by Jacobs et al. [J. Chem. Phys. 91, 3182 (1989)]. A possible explanation of the observations is given in terms of the role of rotational (de-)excitation in the accommodation process.

Unified description of rotating-molecule-surface interactions: Comparison with experiment.

From previous experiments on surface light-induced drift it was found that the coefficient for tangential momentum accommodation in molecule-surface collisions can increase with increasing angular momentum of the molecule. By contrast, various molecular-beam-type experiments yielded the opposite result. These seemingly conflicting data are reconciled in a unified picture describing various phenomena related to molecule-surface interaction for rotating molecules. Using physical arguments it is demonstrated that the accommodation as a function of rotational quantum number should peak around the thermal value, decreasing both for lower and higher rotational quantum numbers of the incoming molecule.

Surface light-induced drift of CH3F.

Surface light-induced drift arises under velocity-selective excitation of a low-pressure gas if the accommodation coefficient \ensuremath{\alpha} for tangential momentum transfer to the surface depends on the state of the gas particles. A kinetic description of this effect is presented. Experiments were performed for rovibrationally excited ${\mathrm{CH}}_{3}$F colliding with a quartz surface at 300 K. It is found that \ensuremath{\alpha} depends on the rotational state (J,K) of the molecule much more strongly than on the vibrational state. For the transition (J,K)=(4,3)\ensuremath{\rightarrow}(5,3), \ensuremath{\alpha} is found to increase by \ensuremath{\delta}\ensuremath{\alpha}=1.9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$. This result is shown to be primarily due to the change in magnitude of the rotational angular momentum J, while its orientation with respect to either the molecular figure axis or the surface normal plays a negligible role.

Slip of a gas in a field of optical radiation

A new mechanism of surface light-induced drift associated with the difference between the collision frequencies of excited and unexcited atoms is predicted.

Observation of surface light-induced drift.

In a one-component low-pressure gas, velocity-selective excitation can produce a drift due to state-dependent molecule-surface interaction. The first observation of this effect is reported for rovibrationally excited ${\mathrm{CH}}_{3}$F colliding with quartz. Experiments involving different rotational sublevels within the ${\ensuremath{\nu}}_{3}$ vibrational band indicate a strong dependence of the effect upon rotational quantum number.

Surface light-induced drift of a rarefied gas

A new effect, the drift of a single-component gas due to its interaction with cell walls at the velocity-selective excitation, is predicted and described theoretically. Flows of particles and energy, the effective temperature of the gas, and stationary particle concentration gradients are calculated. This effect can be employed to study the interaction with surfaces of the gas particles in the ground and excited states.