Effect of Pressure onF-Center Absorption in Alkali Halides

Measurements are reported for the effect of hydrostatic pressure to 5000 atmospheres on the $F$-center absorption of seven alkali halides. An interpretation is made of the shift of the absorption band maximum with pressure, and of the difference between the shift with pressure and that with temperature. The shift with pressure depends almost entirely on the local interionic distance at the $F$ center. The shift with temperature is largely governed by this same factor, but it must be supplemented by a contribution from the interaction of the $F$-center electron with the optical vibrations of the lattice. Effects of the pressure-induced structure transformation in RbCl are described for both photochemically and additively colored crystals.