The Urbach Tails and Reflection Spectra of NaCl Single Crystals

Abstract

In the tail part region of the first exciton band, the absorption constant (10⁻¹ to 10² 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=σ₀F, it was found that σ₀=0.757 and the temperature of zero-point phonon is ωh⁄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.