Studies of the Charge Transfer States of Certain Rare-Earth Activators in Yttrium and Lanthanum Oxysulfides

Abstract

The lowest charge transfer state(CTS)of Eu³⁺ and,to a lesser extent, of Yb³⁺ and Tm³⁺, in Y and La oxysulfides, can be placed in energy by fitting their absorption bands, their emission bands (if present), their feeding behavior into lower rare-earth free-ion-like states, and their role in the sequential quenchings of the activator line emissions with increasing temperature. The CTS is found to be a source, at relatively low temperatures, of free holes which can be trapped for months and then released thermally or with IR irradiation to give the line emissions of the activator. A study of the fraction of CTS excitations which do create free holes as a function of excitation temperature allows determining the bonding energy of the hole to the Eu²⁺left behind. It is found to be about 1000 cm^-1. The binding energy of Eu³⁺ for a free electron can also be determined. It is found to be near 8000 cm ^-1. The current knowledge of the CTS is examined and many unanswered questions are raised. Some approaches to exploring these questions are suggested.