The Use of a High-Rydberg Lithium Beam and a Fine Mesh Target to Probe Atom-Surface Interactions.

A new experimental technique for the study of atom-surface interactions is obtained by passing an atomic beam containing high-Rydberg states through a fine mesh target. The technique is demonstrated to be a sensitive probe of the electric fields of adsorbed dipole layers in transmission experiments in which a time-dependent beam reduction is attributable to field-ionization of the high -Rydberg atoms. A thermal-energy Li beam, excited by electron impact to states with principal quantum numbers near n = 35, is passed through a 2.1-μm -thick gold mesh with 6.45-μm square holes. The fraction of excited atoms surviving passage through the target shows a decrease to a minimum as a function of time, followed by a rise to a final constant value. The rate of these changes is unaffected by changes in the Li flux, is decreased by an increase of the mesh temperature, and seems to be increased with a higher residual gas pressure. A model of the time dependence of the transmission based on field-ionization of the high-Rydberg atoms in the presence of dipole layers of adsorbed H_ {2}O and LiOH (formed in the reaction of H_{2}O and Li) agrees with the experimental observations. A calculation of the rate of diffusion of Li into Au shows that diffusion proceeds too rapidly to allow observation of effects due to adsorbed Li. A field-ionization of a beam of high-Rydberg atoms is shown to be a more sensitive probe of the field of a surface-dipole layer than the deflection of an ion beam in the same experimental geometry. Surface-dipole-moment density measurements with a sensitivity of 6 times 10^{-5} ea _{rm o} (1.5 times 10^{-4} debye) per adsorption site are within reach by extending the experiments to principal quantum number n = 80. The new technique, with its insensitivity to surface roughness, promises to advance understanding of surface ionization and van der Waals forces as well as provide measurements of adsorption rates, surface densities, and dipole moments at sub-monolayer coverages.