Far-Infrared Absorption Induced by Isotopes in NaCl and LiF

We have made measurements of the far-infrared absorption at 7°K in NaCl. The natural ${\mathrm{Cl}}^{35}$ and ${\mathrm{Cl}}^{37}$ isotopes acting as simple mass defects result in absorption in the acoustic band and yield information about phonon frequencies at critical points. The theory of this effect is derived and applied to NaCl. Similar measurements on LiF are also reported. We believe this to be the first observation of isotope-induced band-mode absorption in alkali halides.