Effect of applied fields on the optical properties of color centers

Abstract

A theoretical analysis is made of the effect of an external perturbation, such as an electric field or magnetic field, on the optical properties of color centers. The analysis is restricted to centers with orbital singlet ground states. It is especially applicable to centers with orbitally degenerate excited states which are interacting strongly with the lattice, such as the F center in the alkali halides. The changes in the optical properties are described by computing the changes that occur in the moments of the optical absorption line shape when the external perturbation is applied. The changes in the zeroth, first, second, and third moments may be computed quite simply. Although this does not give the detailed change in the line shape, it provides a precise method for evaluating parameters of the center, such as the spin-orbit coupling constant of the excited state of the F center. The method of moments is used to analyze experiments on the circular dichroism, Faraday rotation, Stark effect, and Voigt effect of the F center, and the circular dichroism of the A center. The analysis of circular dichroism data yields for the first time a method for determining the separate contributions to the broadening of the F band due to the cubic vibrational modes, the non-cubic vibrational modes, and the spin-orbit interaction. The data of Margerie and Romestain on the circular dichroism of the F band in CsBr and CsCl are analyzed. The analysis resolves the discrepancy between their two methods of determining the spin-orbit splitting (measurement of the peak separation of the resolved structure and measurement of the circular dichroism), and shows that the structure arises from a combination of a large spin-orbit coupling and a large non-cubic lattice vibrational interaction.