Photoemission from CsI: Experiment

Photoemission spectra for $\hslash \omega \le 11.7$ eV were measured from thin CsI films at 77°K. The distribution of emitted electrons is interpreted in terms of optical excitation from a valence band comprising three branches of Bloch bands derived from the ${\mathrm{I}}^{-}$ $5p^6$ levels. A triplet in the energy distribution cannot be interpreted in terms of excitation from simple ionic states localized on the iodine ions. Measurements indicate a valence-band width of 2.5 ± 0.3 eV, somewhat larger than the 1.7-eV width calculated by Onodera. Flat regions in the conduction bands, about 0.8 and 2.1 eV above the band edge, cause a small enhancement of photoelectrons excited to these regions. Additional evidence for the flat conduction bands is seen in the distribution of scattered electrons and in the strong scattering of hot electrons photoinjected through CsI. Band parameters obtained by photoemission aid in an interpretation of optical-density measurements.