Far-infrared difference-band absorption in potassium iodide

A far-infrared measurement of the temperature-dependent absorption in KI between 4.2 and 82 K shows a number of absorption edges which correlate very closely with calculations of the two-phonon difference-band effect in the limit of energy-conserving processes. A temperature-dependent peak in the absorption coefficient at 11.3 cm^-1 corresponds to a weak difference-band contribution stemming from vertical transitions between the Σ₁(LA)-Σ₄(TA) symmetry branches in the region where the slopes are nearly parallel. This result demonstrates that at finite temperature a true gap does not exist in the energy-conserving two-phonon difference-band absorption spectrum for a rock-salt crystal which has a well-defined gap between the acoustic and optic branches.