Electron emission spectra of thermal collisions of He metastable atoms with Au(111) and Pt(111) surfaces: Evidence for Penning ionization

Abstract

Electron emission spectra obtained by thermal collisions of ${\text{He}}^{\ensuremath{\ast}}(2^{1}S$ and $2^{3}S$) atoms with Au(111) and Pt(111) surfaces were measured to clarify the electronically excited atom-metal interactions. It has been recognized that the metastable atoms de-excite on ordinary noble- and transition-metal surfaces via resonance ionization (RI) followed by Auger neutralization (AN) without no indication of Penning ionization (PI). Our data show that this traditional criterion partially breaks down in the ${\text{He}}^{\ensuremath{\ast}}\text{-Au}(111)$ collision system. The local electronic states near the surface were examined by first-principles calculations using density functional theory. It reveals that the itinerant $sp$ states are significantly spilled out toward the vacuum compared to the localized $5d$ states, and their asymptotic features play a crucial role in determining the branching ratio between PI and $\text{RI}+\text{AN}$.