Electron spectroscopy of surfaces by de-excitation of metastable noble gas atoms

Abstract

Electronically excited, metastable noble gas atoms A* (for example H e*21S, excitation energy E* — 20.6 eV) with thermal kinetic energy are very efficiently quenched upon collision with a surface S, i.e. A* + S -> A + S+ + e~. De-excitation proceeds through two competing mechanisms: (i) Auger de-excitation (equivalent to Penning ionization), or (ii) resonance ionization followed by Auger neutralization. The energy distribution of the emitted electrons is governed by the overlap between the wavefunctions of the target and the unoccupied (ground-state) level of the impinging atoms. As a consequence, this technique is extremely sensitive to the density of valence electronic states of the outermost atomic layer. Results for clean and adsorbate-covered surfaces are presented in comparison with data recorded by ultraviolet photoelectron spectroscopy, to demonstrate the capabilities of this method.