Is the Barrier Height of Exciton Self-Trapping Evaluated So far Correct?–Quantum Mechanical Reevaluation–

Abstract

The barrier height E B for self-trapping (ST) has been evaluated so far by analyzing the data on thermal quenching of free-exciton luminescence with the use of the classical formula (∝ exp [- E B / k B T ]) of the ST rate even at temperature T considerably lower than the Debye one. The rate of multiphonon nonradiative transition such as the ST rate cannot be described by the classical formula at such low temperatures. Analysis of the data of RbI below 30 K with the quantum mechanical formula gives E B ≃38–39 meV much larger than 17–18 meV obtained so far. The analysis shows simultaneously that concerning n =1 free excitons ST into a state of one-center type determines the rate in RbI while in KI it is overcome by ST into a state of two-center type with increasing temperature. This enables us to predict that the so called E x luminescence should be emitted from a still unknown self-trapped state of one-center type.