Self-Trapping of Excitons Beginning with Tunneling Nucleation

Abstract

The rate of self-trapping (ST) is calculated, considering that ST begins with quantum-mechanical tunneling of a free exciton to a state of nucleation less localized than the relaxed self-trapped one. Three cases appear depending on the site-diagonal and off-diagonal exciton-phonon interaction energies S 1 and S 2 relative to the half width B of the exciton band. When \(S_{2}/B \gtrsim 0.21\), ST through the nucleation state of two-center type predominates. When \(S_{2}/B \lesssim 0.21\) and \(S_{1}/B \lesssim 1.25\), only ST through that of one-center type occurs. When \(S_{2}/B \lesssim 0.21\) but \(S_{1}/B \gtrsim 1.25\), the latter channel predominates at low temperatures but it is over-come by the former one with increasing temperature. Alkali halides except iodides are classified into the first case, RbI into the second one, and KI into the third one. The so called E x luminescence observed only in alkali iodides should be emitted from a still unknown self-trapped state of one-center type.