High-pressure optical studies of doped alkali halides. I. Peak shifts and peak shape changes

An apparatus has been developed which permits optical emission measurements to 150 kbar using a quasihydrostatic medium or to 12 kbar using a hydrostatic medium. Using this apparatus and the absorpion rig already available, a series of high-pressure studies have been made on alkali-halide crystals doped with ${\mathrm{T}1\mathrm{}}^{+}$ or ${\mathrm{In}}^{+}$. In this paper we present data on the peak shifts and changes of shape and bandwidth. The following papers discuss Jahn-Teller effects, photothermally induced electron transfer, and mixed crystals. We discuss the shifts and shape changes in terms of a simple configuration coordinate model showing its possibilities and limitations. In spite of the fact that these systems are clearly too complex for such a simple description, one observes certain regularities. For several systems the decrease in volume upon electronic excitation is ∼ 10% of the molecular volume. The ratio of force constants $R$ of the excited state compared with the ground state is less than one. We demonstrate that several different methods for calculating $R$ give consistent results.