We measured the rotationally inelastic diffractive scattering probabilities for an HD beam colliding with a smooth Pt(111) surface. These large T yield R inelastic probabilities were measured as a function of incidence angle for a 110 meV beam energy. The results are in excellent agreement with a simple physical model of an eccentrically weighted sphere colliding with a hard wall with an attractive well depth of 55 + or - 10 meV. The numerical GR method of Garcia and co-workers was found superior to an eikonal method in solving this rotational quantum boundary value problem.

HD scattering from Pt(111): rotational excitation probabilities. Technical report

In a flowing Knudsen gas, molecular angular momentum polarization can be produced by tangential forces resulting from nonspherical molecule–surface interaction. This is investigated by studying the change in the flow due to molecular precession around an external magnetic field. Various types of polarization are observed in CH4, N2, and HD flowing through a Au‐plated channel. The results suggest a strong correlation between type of polarization produced and molecular structure. These data call for model calculations which include surface corrugation.

Angular momentum polarization produced by molecule–surface collisions in a Knudsen flow

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.

Experiments on surface light-induced drift for various systems

In a Knudsen flow of polyatomic molecules, polarization of the angular momenta can occur due to non-spherical molecule-surface interactions. The polarization has been studied by observing a change in the particle flux caused by the precession of the molecules in an external magnetic field. It is show how the experimental results as a function of field orientation and field strength allow a determination of the type and magnitude of the polarization. The experimental technique is described and results are given for N2 flowing through a Au-coated channel at 300 K. It is found that the dominant types of polarization are Jy and JxJz, where x is the flow direction and z is the normal to the surface.

Molecular Angular momentum polarization produced in a Knudsen flow

Role of corrugation in molecule-surface interaction : flow experiments using the liF (001) surface in two orientations

In a Knudsen flow of polyatomic molecules, non-spherical molecule-surface interaction can give rise to polarization of the angular momenta. This phenomenon is studied for various gases flowing through a Au-coated channel, by observing the magnetic field effect on the flow. Different types of polarization are found to be produced in different types of molecules. This is qualitatively explained with simple models for a molecule-wall collision. The effects in strongly polar gases are found to be very small. This is attributed to non-instantaneous scattering.

Angular momentum polarization in a Knudsen flow; Results for various gases on a Au-surface

Experimental determination of angular momentum polarizations produced in a Knudsen gas flow

Polarization of molecular angular momentum in a Knudsen flow, which is caused by non-spherical molecule-surface interaction, is investigated by means of the magnetic field effect on a particle flux. Experiments are reported on a number of gases flowing through channels coated with Au, Pt or mica. The results show that the characteristic features of the effect do not vary significantly with the surface, which indicates that the gas-surface scattering is dominated by adsorbates. The differences in magnitude of the effects in the three channels can be explained with a gas-independent scaling factor reflecting differences in macroscopic surface roughness.

Angular momentum polarization in a Knudsen flow; results for different surfaces

Magnetically induced changes in the flow rate of CO and CH 4 are investigated experimentally in the intermediate pressure regime (Knudsen numbers between 0.1 and 10) for the three main field orientations. From the results a qualitative understanding of the role of the various angular momentum polarizations involved can be obtained.

J.J.G.M. Van Der Tol

Papers

Molecular Angular momentum polarization produced in a Knudsen flow

Angular momentum polarization in a Knudsen flow; Results for various gases on a Au-surface

Experimental determination of angular momentum polarizations produced in a Knudsen gas flow

Angular momentum polarization in a Knudsen flow; results for different surfaces

Magnetic field effects on a molecular flow at Knudsen numbers of order unity for CO and CH4