Absence of a charge-transfer instability for rare-gas atoms adsorbed on metals

Abstract

Recent optical-absorption experiments on rare-gas atoms bonded to metals dramatically segregate various rare-gas—metal systems into two classes. Cunningham, Greenlaw, and Flynn have hypothesized that these two classes are characterized by the presence or absence of charge transfer from the (excited) rare-gas atom to the metal, and that such charge transfer is controlled by the sign of the difference $\Phi -I^{*}$, where $\Phi$ is the metal work function and $I^{*}$ is the energy required to ionize the rare-gas atom in its lowest excited state. Flynn and Chen have, in addition, collected data describing the dipole moments of adsorbed Xe; these also suggest a dramatic dependence on the quantity $\Phi -I^{*}$. As a test of this hypothesis, we have measured the dipole moment of Xe adsorbed on a low-work-function substrate [Gd(0001), with $\Phi =3.3\mathrm{}\pm 0.1$ eV for the clean surface]. The central new result is that both the Gd measurement and a variety of existing experimental data are inconsistent with the interpretation emphasizing $\Phi -I^{*}$. New calculations using the atom-on-jellium model are also introduced to supplement the discussion. Our analysis suggests that the behavior seen in the optical-absorption measurements could represent a physical effect even more unusual than the charge-transfer instability proposed by Flynn et al.