VUV-spectroscopy of scattered electrons in insulators

The influence of electron-phonon scattering on the structure of the photoelectron energy distribution has been studied in CsI for electron energies lower than the band gap. Photoelectron energy distributions at hv < 11.8eV have been measured as a function of CsI layer thickness. In this energy range hv is lower than the double band gap (Eg = 6.2eV) and the energy of photoexcited electrons is not enough for electron-hole pair production. The photoelectron energy distribution can then be changed only by electron-phonon scattering. Back-side illumination of the CsI layer has been used to separate the structures of the spectrum due to excitation and scattering. It has been shown that the initial energy distribution of photoelectrons is significantly distorted by the transport processes and new structure features appear in the energy distribution. These features correlate with the density of states in the conduction band: a peak in the density of states corresponds to a dip in the energy distribution at the same energy. Transport of excited electrons in the CsI conduction band has been described by solving the diffusion equation obtained on the basis of a kinetic equation. Photoelectron spectra have been simulated and compared with experimentally measured ones. As a result maxima in the CsI density of unoccupied states have been obtained at the energies of 6.7, 7.5 and 8.9eV above the top of the valence band.