The Urbach Tails and Reflection Spectra of NaCl Single Crystals

In the tail part region of the first exciton band, the absorption constant (10 -1 to 10 2 cm -1 ) was measured on zone-refined and Harshaw NaCl single crystals from 10°K to 573°K. Fundamental absorption tails of pure NaCl single crystals are represented by the modified Urbach rule, which describes the influence of zero-point lattice vibrations at low temperature. Analysing a temperature dependence of Urbach constant σ=σ 0 F , it was found that σ 0 =0.757 and the temperature of zero-point phonon is \(ω/2{=}58.2\)°K. Reflection spectra of single crystals were measured from 2112Å to 1060Å in the same temperature range. The peak shift of the first reflection band is investigated as a function of temperature to clarify the mode of phonons interacting with the exciton in the main part of the band.