Field-Emission Studies of Electronic Energy Levels of Adsorbed Atoms

Abstract

The relative changes in the total energy distribution of field-emitted electrons upon adsorption of single atoms have been measured for adsorption of the alkaline-earth atoms (Ba, Sr, and Ca) on several crystal planes of tungsten. The expected perturbations of the energy distribution due to the tunneling resonance through an atomic "virtual level" at various positions relative to the Fermi surface and of various half-widths $\ensuremath{\Gamma}$ is demonstrated by a simple one-dimensional calculation. The measured energy-dependent structure in the current-enhancement factor due to the adsorbate has been interpreted in a tunneling-resonance model to yield the positions and shapes of the atomic "virtual levels." The ground-state $^{1}S$ $6{s}^{2}$ level of Ba is broadened to a half-width ${\ensuremath{\Gamma}}_{S}=0.75$ eV and shifted upward by $\ensuremath{\Delta}{E}^{S}=0.95$ eV. This causes it to overlap and mix with the first two excited states: a triplet $^{3}D$ $6s5d$ and a singlet $^{1}D$ $6s5d$. The observed $^{3}D$ and $^{1}D$ levels were not shifted and had a half-width ${\ensuremath{\Gamma}}_{D}=0.1$ eV. Similarly, the first excited state of Ca, a triplet $^{3}P$ $4s4p$, was shifted by $\ensuremath{\Delta}{E}^{3\mathrm{P}}\ensuremath{\approx}0.4$ eV with a width ${\ensuremath{\Gamma}}_{3\mathrm{P}}\ensuremath{\approx}0.3$ eV ($2\ensuremath{\Gamma}=\mathrm{FWHM}$). The effect of the band structure of the substrate on tunneling resonance is discussed.