Photoemission and optical processes in multialkali photocathodes

Abstract

Multialkali (${\mathrm{Na}}_2$KSb)Cs photocathodes have been studied in the range of photon energy of 1 to 6 eV by means of photoelectron spectroscopy, measurement of optical properties, and photoemissive quantum efficiency. The optical properties of these photocathodes are similar to the other alkali antimonides. The coincidence of the position of peaks and other similarities with the optical spectra of ${\mathrm{Na}}_2$KSb supports the surface heterojunction hypothesis proposed by us earlier. The pseudopotential band-structure calculation by Klimin et al. on these materials has been used to identify the possible transitions taking place in the Brillouin zone corresponding to the observed structures in the optical spectra. From photoelectron spectroscopy the optical transitions appear to be of direct type, which puts this material in sharp contrast with other alkali antimonides, all of which showed the features of nondirect transitions. The threshold energy of electron-electron scattering for the photoexcited electrons have been found to be 2.3 eV which is more than twice the band gap above the conduction-band edge. The mean free path for this scattering has been found to be decreasing with increase of energy of the photogenerated electrons and, at an excitation energy of 6 eV, it is 43 Å. Apparently, the scattered electrons fall into a valley located 0.95 eV above the bottom of the conduction band. The increase of quantum efficiency with decrease of absorption after the onset of electron-electron scattering probably takes place due to generation of secondary electrons from the valence band due to scattering. Some of the peaks in the photoelectron spectra have been identified with the transitions predicted through the band-structure calculations.