Surface Light-induced Drift Research Articles

Surface light-induced drift, including number flux and heat flux, in flat-plate and circular-cylindrical geometries

With standard forms of the approximate Maxwell–Boltzmann rate equations, some of the author’s previous work is used to obtain exact results for the gas particle velocity distribution functions for the general case of laser excitation function, and, for use in calculations of the fluxes, a very simple effective distribution function is derived. The work is confined to the limit of low radiation absorption by the gas and to the limit of large Knudsen number. Where possible, the exact results for the fluxes are compared with recent corresponding approximate results of Chermyaninov, Chernyak, Khinkina, and Vilisova (CCKV), with interesting conclusions. The results allow useful assessment of approximation schemes such as those in CCKV.

Theory of surface light-induced drift with forced specularity

Waxman has recently proposed new mechanisms for surface light-induced drift (SLID), which he called “SLID with forced specularity” (christened SLIDWFS here). SLIDWFS was proposed for its potential use in modelling several important physical processes; examples are selective control of spatial distributions of gases, including isotope separation, probing the molecule-surface interaction potential in gas-surface scattering, heterogeneous chemical reactions in catalysis, and laser cooling. Waxman’s cell geometry consisted of a single flat plate; the usual approximations were made in the analysis, and substantial effects (larger than in ordinary SLID) were reported. In the present contribution, we study the more realistic double-flat-plate and circular-cylindrical geometries. We get exact analytical solutions to the model rate equations, with results quite different from Waxman’s. For example, for circular-cylindrical geometry, which is clearly preferable in ordinary SLID, we predict a negligible SLIDWFS effect.PACS Nos.: 05.60.–k, 34.35.+a, 34.50.Rk

Theory of light-induced drift. VI. Roles of accommodation of normal and tangential momenta in surface light-induced drift with positive spontaneous relaxation parameter

Surface light-induced drift of a rarefied gas in cells with flat-plate and circular-cylindrical geometries is studied, and exact analytical solutions to the model rate equations are obtained in the limit of large Knudsen number. The work is a continuation of part V of this series of papers, in which the model rate equations were tailored specifically in order to study the roles played by both the tangential and normal momentum molecule-surface accommodation coefficients. In part V, the ``spontaneous decay'' parameter, $\ensuremath{\gamma}$, was set equal to zero, and the case of general ``laser excitation'' parameter, $q$, was studied. In the present paper, we get exact analytical results for the case of arbitrary $\ensuremath{\gamma}$ in the limit of small $q$; we also get accurate numerical results for arbitrary $\ensuremath{\gamma}$ and $q$. In passing, one serious, and other less serious, typographical errors in part V are corrected.

Theory of light-induced drift. V. Roles of accommodation of normal and tangential momenta in surface light-induced drift

Surface light-induced drift (SLID) of a rarefied gas in cells with flat-plate and circular-cylindrical geometries is studied, and exact analytical solutions to the model rate equations are obtained in the limit of large Knudsen number. The model rate equations, which have not appeared before, have been tailored specifically in order to study the roles played in SLID, in a physically realistic setting, by both the tangential momentum and the normal momentum molecule-surface accommodation coefficients. Because the model equations are new (to the best of our knowledge), the results are generally different from those of previous work, although emphasis is placed on obtaining relations between the present and previous results; applications to experiments may be made by means of those relations.

Theory of light-induced drift. III. Models of surface and bulk light-induced drift in one dimension

Light-induced drift (LID) of a rarefied gas in a cell is studied, and exact analytical closed-form solutions to the model rate equations, which model the gas motion in one dimension, are obtained for cases of both surface LID (SLID) and bulk LID (BLID); the special case of the limit of low radiation absorption by the gas is given particular attention. Similarities and differences among the results for SLID and BLID are discussed. This is part III of a series of papers, parts I and II having studied LID, but concentrating on SLID, with flat-plate and circular-cylindrical cell geometries, respectively [F. O. Goodman, Phys. Rev. A 65, 064309 (2002); 65, 064310 (2002)].

Theory of light-induced drift. IV. Models of bulk light-induced drift in three dimensions

Light-induced drift (LID) of a rarefied gas in a cell is studied, and exact analytical closed-form solutions to the model rate equations are obtained for bulk LID (BLID) in three dimensions, emphasis being placed on the limit of low radiation absorption by the gas. Comparisons are made with an existing model of BLID in one dimension [F. O. Goodman, preceding paper, Phys. Rev. A 67, 013410 (2003)], and we examine whether there are any conditions under which that model is satisfactory. Comparisons are also made with the author's models of surface LID in one dimension (above reference) and in three dimensions [F. O. Goodman, Phys. Rev. A 65, 063409 (2002); 65, 063410 (2002)].

Flux measurements of alkali atoms diffusing via multiple surface scattering in asilanated glass tube

Alkali atoms that are diffusing away from a source via multiple surface scatteringin a nonstick coated glass tube are monitored by laser fluorescence. Densitiesare measured by fluorescence intensity and weak-field absorption, andsmall asymmetries in the line shape provide a measurement of the flux.Average velocities as small as 0.3% of the thermal velocity can be resolvedusing the technique presented. Model calculations are presented and fitto the data, providing approximate values for the sticking probability,and the probability of specular scattering versus diffuse scattering(related to momentum accommodation). For the silane-based coatingSurfaSilTM,we find that the scattering is mostly diffuse, and that the sticking probability is ofthe order of 10−4 or 10−3. With refinement, the technique can be used to measurethe variation in the sticking probability along the length of the tube, which couldbe useful for studying and improving gas separation columns. We presentnumerical calculations of surface light-induced drift (SLID) effects, but noevidence for SLID is observed experimentally.

Theory of light-induced drift. II. Circular-cylindrical geometry

Light-induced drift (LID) of a rarefied gas in a cell with circular-cylindrical geometry is studied, and exact solutions to the model rate equations are obtained, with exact analytical solutions for the case of surface LID (SLID); the special case of the limit of low radiation absorption by the gas in SLID is given particular attention. Many results are different from those of previous work. Emphasis is placed on considerations of comparison with experiment. This is part II of a series of papers, part I having studied LID with flat-plate geometry [F. O. Goodman, preceding paper, Phys. Rev. A 65, 063409 (2002)].

Theory of light-induced drift. I. Flat-plate geometry

Light-induced drift (LID) of a rarefied gas in a cell with flat-plate geometry is studied in the limits of large length and width of the cell, and exact solutions to the model rate equations are obtained, with exact analytical solutions for the case of surface LID (SLID); the special case of the limit of low radiation absorption by the gas in SLID is given particular attention. Many results are different from those of previous work. Emphasis is placed on considerations of comparison with experiment.

On the problem of rotational-state-dependent molecule–surface interaction: Relaxation, accommodation and trapping at low J

In the framework of a unified kinetic theory of particle–surface interactions, dependences of the relaxation, accommodation, and trapping of molecules on their angular momentum J are discussed. One of the basic parameters of the theory, the free flight time through the interaction region, is calculated using a simple model potential for which the classical equations of motion can be integrated analytically. With increasing J, an increase in molecular relaxation and accommodation is predicted at small J, and a decrease at larger J. These results constitute a microscopically founded interpretation of recent experiments on surface light-induced drift.

Surface light-induced drift resulting from inelastic scattering

Surface light-induced drift (SLID) is known to give rise to an extremely sensitive method of studying the state-specificity of gas-surface interactions. SLID occurs under velocity-selective laser excitation of a resonant gas whenever the scattering, trapping, or chemisorption by the cell walls are state dependent. In this brief report we derive and solve kinetic equations to analyze SLID arising in the case of inelastic scattering. We analyze both (a) the case when the coefficients describing the inelasticity of gas-surface quasi-specular scattering are state dependent, but are not dependent upon the velocity of the impinging particle, and (b) the case of velocity-dependent coefficients. Finally, we briefly outline the way to use this approach to interpret the results of possible experiments on SLID for atoms interacting with simple surfaces. PACS Nos.: 34.50Dy, 78.66-w

Surface light-induced drift resulting from inelastic scattering

Surface light-induced drift (SLID) is known to give rise to an extremely sensitive method of studying the state-specificity of gas-surface interactions. SLID occurs under velocity-selective laser excitation of a resonant gas whenever the scattering, trapping, or chemisorption by the cell walls are state dependent. In this brief report we derive and solve kinetic equations to analyze SLID arising in the case of inelastic scattering. We analyze both (a) the case when the coefficients describing the inelasticity of gas-surface quasi-specular scattering are state dependent, but are not dependent upon the velocity of the impinging particle, and (b) the case of velocity-dependent coefficients. Finally, we briefly outline the way to use this approach to interpret the results of possible experiments on SLID for atoms interacting with simple surfaces. PACS Nos.: 34.50Dy, 78.66-w

Surface light-induced drift due to anisotropic gas-surface scattering in models with and without boundary conditions

Surface light-induced drift (SLID), which occurs under velocity-selective excitation due to the state-dependence of the probability of trapping or chemisorption, has demonstrated its extreme sensitivity in studying the state-specificity of gas-surface interactions. Contrary to an intuitive perception, SLID can be also used to obtain the information on the angular dependence of the gas-surface scattering. In this paper we describe this effect qualitatively and then analyze it in two different models, with and without boundary conditions. We also discuss the implications of the results for studying the scaling laws of the angular dependence of surface scattering by the SLID technique. PACS No. 34.50.Rk

Surface light-induced drift due to anisotropic gas-surface scattering in models with and without boundary conditions

Surface light-induced drift due to anisotropic gas-surface scattering in models with and without boundary conditions

Rotational- and vibrational-state resolved HF-surface interactions investigated by surface light-induced drift

Experiments using surface light-induced drift are performed to yield information on the rotational (J) and vibrational (v) state dependence of molecule–surface interactions. Data are presented for the change in accommodation coefficient for tangential momentum transfer α upon excitation of HF interacting with a polycrystalline LiF surface (on a Cu substrate) and a hydrophobic stearic-acid monolayer (on a stainless-steel substrate). We employed both P- and R-branch excitation of HF in the fundamental vibrational band (v=0→1) with J=0–4, using a continuously tunable color-center laser (λ≈2.5 μm). By combining the results for the P- and R-branch, we find that the influences of J and v upon the molecule–surface interaction can be considered independent to a good approximation. It is found that α decreases upon vibrational excitation v=0→1, whereas it increases with increasing J. The J and v dependences of α are discussed in the framework of a unified kinetic theory of molecule-surface interaction.

The study of anisotropy of gas-surface scattering using surface light-induced drift

Surface light-induced drift (SLID) which occurs under velocity-selective excitation due to the state-dependence of the probability of trapping or chemisorption has demonstrated its extreme sensitivity in studying the state-specificity of gas-surface interactions. In this paper we show that, contrary to an intuitive perception, SLID can be used to obtain the information on the angular dependence of the gas-surface scattering. We present the initial results and discuss the ways to study the scaling laws for the angular dependence of surface scattering by the SLID technique.

Surface light-induced flux magnitude at low radiation intensities

Surface light-induced drift (SLID) of a rarefied gas in rectangular geometry is analyzed. In the case of low radiation intensities, the expressions for SLID flux and the relative pressure drop along the cell are found to contain a factor diverging logarithmically with the rise of the parameter L/R (L is the length of the cell, R is the distance between the walls). This divergence is attributed to the contribution to the SLID flux of the molecules moving to the wall at grazing angles.

Surface light-induced drift in porous media.

Diffusion and flow of gases in porous media have attracted considerable attention for many decades [1]. Interest in such research has been motivated by its many fields of application, such as surface catalysis, gas separations, soil physics, etc. Gas flows often arise here due to the difference in gas pressure or concentration between the opposing surfaces of the porous medium. In the following report, gas flows in porous media, caused not by the concentration or pressure gradients but by Surface Light-Induced Drift (SLID) [2–5], are discussed. Unlike the former case, the flow of a gas now becomes selective. Namely, one can force only the molecules of the resonant gas through the porous membrane, while leaving the other components untouched. It implies possible benefits in using the SLID for gas separations. Also, due to a high surface/volume ratio, porous media can be especially favorable for studying the state-specificity of surface chemical reactions [4] and catalysis.

Surface light-induced drift as a tool for studying gas-surface interactions

Surface light-induced drift (SLID) has been demonstrated to be a highly sensitive tool for studying state-specificity of gas-surface interactions. We compare here the sensitivity of SLID with that of molecular beams. Due to backscattering from roughness facets, SLID can occur even when scattering is completely locally diffusive, providing a significant expansion of the scope of situations in which SLID can show up, compared to the case of a flat surface. Rough surfaces are also a site of enhanced chemical activity, which can affect SLID. We propose a T-shaped cell as a sensitive tool for studying state-specificity of scattering and the chemical reaction process. We discuss prospective directions for further research on SLID.

Surface light-induced drift as a tool for studying gas-surface interactions

Surface light-induced drift as a tool for studying gas-surface interactions