Excited-state atomic-beam studies of submonolayer adsorption on a fine mesh

Abstract

A surface dipole layer due to adsorbates on a fine mesh can produce localized electric fields capable of ionizing highly excited atoms. This effect is observed when an atomic beam containing high-Rydberg states of Li is incident on a degassed Au mesh having square openings 6 \ensuremath{\mu}m across. During the formation of an adlayer on the surface, the adsorbate field is analyzed through measurements of the excited-state atomic-beam flux transmitted through the mesh. The beam-transmission experiments confirm that the excited-state survival probability is surface-layer dependent. Two distinct dips are observed in the transmission as a function of time, during submonolayer adsorption of ${\mathrm{H}}_{2}$O and Li on the mesh surface. A cusp between the dips is interpreted as evidence for a change in the sign of the net surface dipole-moment density. These results are compatible with a Langmuir-type model for the survival probability of excited atoms subject to electric-field ionization as they pass through the mesh. Demonstrating that high-Rydberg atoms may be useful as probes for localized electric fields, the experiments serve as feasibility studies for a method of analysis which can readily determine surface dipole orientations and which may have the ability to detect changes in adsorbate coverage at the level of ${10}^{\mathrm{\ensuremath{-}}4}$--${10}^{\mathrm{\ensuremath{-}}5}$ monolayer.