Luminescence ofFCenters in Alkali Halides under Hydrostatic Pressure

F-center luminescence spectra have been measured at 78^∘{}K in NaCl, KCl, KBr, KI, RbCl, and CsBr under hydrostatic pressure to 3.5 kbar. From a comparison between the shift of the emission peak with pressure and the shift with temperature, it is concluded that most of the latter arises from the electron-lattice interaction and not from thermal expansion. This is the opposite of the conclusion for F-center absorption, for which thermal expansion dominates the temperature coefficient of the peak energy. The halfwidth of the function $W(E)$, the probability per unit time of emitting a photon with energy between E and $E+dE$, does not vary with pressure in NaCl, KCl, and RbCl, but the half-width of the "shape function," E^-3W(E), changes with pressure. This is interpreted as meaning that for F-center emission, it is more suitable to make the Condon approximation on the momentum operator than on the position operator. Lifetime measurements in KCl under pressure at 15 and 78^∘{}K give the pressure derivative of the thermal ionozation energy of the relaxed excited state of the F center in KCl, + 19×{}10^-6 eV bar^-1. The pressure coefficients of a few K- and L-band peaks have been determined. These provide additional evidence that the final states in the L-band transitions are not similar to those for F- and K-band transitions. The K band in RbCl changes shape with pressure. The FA-band splitting has been found to increase with pressure in KCl:Na. The F-center emission in KBr under pressure has been found to have two components, and the excitation spectrum has two peaks, the F band and a peak very close to the K^' band previously found in absorption at much higher pressures. Emission data at 12^∘{}K on diverse samples of additively colored KBr strongly suggest that there are two components to the normal F-center luminescence in KBr with no applied pressure.