Energy Distribution of External Photoelectrons fromF-Centers in RbI

Abstract

Energy distributions of the external photoelectrons from $F$-centers in RbI were determined by retarding-potential techniques. Precautions were taken to secure electrically uniform surfaces. The photoelectrons then emerged in two separable groups. The first, termed the $f$-group, was composed of relatively fast electrons that were attributed to direct photon ionization of $F$-centers. Photoelectron energies were distributed in a band, roughly gaussian in form, that was treated according to a theory given by Herring. The Franck-Condon principle was taken into account; scattering of excited electrons was neglected. The second group of photoelectrons, termed the $s$-group, was comparatively slow. The energy distribution had an unusual form and peaked at the surprisingly low value of 0.3 or 0.4 ev, independent of $h\ensuremath{\nu}$. Exciton-enhanced emission was almost entirely of this type. Three factors that may influence the $s$-type distribution are mentioned: Hebb's calculations indicate that lattice scattering of exciton-induced emission may be important because of the relatively large depth of origin of these photoelectrons. As suggested by Seitz, degradation of exciton energy prior to $F$-center stimulation may be involved. Finally, some of the photoelectrons in the $s$-group may arise in other types of centers.