Discrete Phonon and Zero-Phonon Optical Absorption Spectra in Alkali Halides

In addition to 0-phonon lines associated with the $R_2$ and $N$ bands of a number of the alkali halides at low temperature, other subsidiary peaks often occur on the high-energy side of the 0-phonon line. We have observed such peaks on the $R_2$ and $N$ bands of LiF, NaCl, KCl, and KBr, and on the $N$ bands only of NaF and KI. It is found with very few exceptions that energy separations between the 0-phonon lines and associated subsidiary peaks agree very well with theoretical normal lattice phonon energies near the zone boundary as determined by Karo and Hardy. It is felt that this is reasonable, as these are the lattice vibrations of highest probability density and of wavelength comparable to small lattice defects. The most common lattice coupling to the defects appears to be through transverse acoustical phonons propagating in $〈100〉$ directions, although the results show that in most of the materials the defects are coupled with comparable strength to several phonon modes. Indeed, possible correlations with all normal modes are indicated in the data. The exceptions mentioned (on the $N$ bands of NaCl, KCl, and KBr) are all considerably smaller in energy than any of the lattice phonons at the zone boundary. These may be associated with pseudolocalized modes due to resonance scattering. In addition to the structure listed above, 0-phonon lines alone have been observed on the $R_2$ bands of NaF, KF, and KI.