Ultraviolet photoelectron spectra of cesium halides

Energy distributions of photoelectrons from evaporated films of cesium halides were measured at photon energies of 11-23 eV. The photoelectron spectra show structures that can be identified as relatively high densities of states in the ${\mathrm{Cs}}^{+}$ $5p$ core as well as in the valence and conduction bands. Doublet core states due to spin-orbit splitting are observed for all the cesium halides. The bandwidths of these states are 3.1 \ifmmode\pm\else\textpm\fi{} 0.3 eV for CsCl, CsBr, CsI, and 4.0 \ifmmode\pm\else\textpm\fi{} 0.3 eV for CsF. The valence-band widths for CsCl, CsBr, CsI, and CsF are found to be 1.8 \ifmmode\pm\else\textpm\fi{} 0.3, 2.1 \ifmmode\pm\else\textpm\fi{} 0.3, 2.4 \ifmmode\pm\else\textpm\fi{} 0.3, and 1.7 \ifmmode\pm\else\textpm\fi{} 0.3 eV, respectively. The energy distribution curves of CsBr indicate a valence-band splitting of 0.5 \ifmmode\pm\else\textpm\fi{} 0.2 eV, which is consistent with the spin-orbit splitting of the Br ($4p$) atomic state. The photoelectron spectra also show the triplet structure associated with the excitation from the ${\mathrm{I}}^{\ensuremath{-}}$ $5p$ band of CsI as reported recently by DiStefano and Spicer. Some evidence of core-exciton decay can be seen in the spectra of CsCl. The photoemission results are used to estimate the binding energies of the $\ensuremath{\Gamma}$ core excitons of cesium halides.