Line shapes of narrow optical bands: Infrared absorption by U centers and heavier impurities in alkali halides

The shape of the bands for photon absorption and emission by the local constituents of a solid is governed mainly by processes involving many low-energy acoustic phonons. This applies not only to wide bands, such as those exhibited by F centers, but also to narrow ones, as those observed for infrared absorption by local vibration modes of U centers and heavier impurities. The line shapes are theoretically studied on a general basis to show they provide a nice example to illustrate the power of field theory and methods to reproduce experimental facts. To this aim, the phonon induced broadenings of infrared absorption lines by U centers in KCl and KBr, and by substitutional Ag+ in KI, were calculated to compare theoretical predictions with experiment. The agreement obtained between both is remarkable.