Photoemission from Core States of Cs and Rb

Photoemission yields and electron energy distributions of Rb and Cs for photon energies of 12-22 eV are reported. Over most of this energy range, photoexcitation from $n{p}^5\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$ $\mathrm{}(n+1\mathrm{})s$ and $n{p}^5\left({}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ $\mathrm{}(n+1\mathrm{})s$ core levels is observed to be at least two orders of magnitude larger than excitation from the conduction band [$n=4\mathrm{} \left(\mathrm{Rb}\right)$, $n=5\mathrm{} \left(\mathrm{Cs}\right)$]. Two peaks in the energy distributions which move to higher energy with increasing photon energy result from electrons being directly excited from the shallow core levels. A third peak, which remains stationary in energy position for all photon energies, results from Auger processes which refill the core levels. From the positions and shapes of the peaks, we have obtained values of the spin-orbit splitting of the core levels, the position of the core levels relative to the Fermi (or vacuum) level, and an estimate of the widths of the conduction band. Measurements of spectral yield show a threshold at the core-level-to-Fermi-level spacing that is consistent with this interpretation of structure in the energy distributions. Additional structure in the distributions results when an electron initially excited into one of the above peaks loses energy in exciting a surface plasmon as it escapes through the surface.