Thermal Stress Origin of 207 and 272mμ Optical Absorption Bands in KCl Crystal

Abstract

The author has previously reported the appearance of remarkable absorption bands in KCI crystal at 207 and 272mμ, produced by the rapid cooling of the melt in a crucible. Several features of the absorption coefficient of these bands and their dependence on the cooling velocity of the crystal have been discussed. In the present paper it is assumed that the origin of these optical absorptions is due to a certain kind of lattice defects in the crystal, which are introduced by the thermal stress in the crystal when a crystal is cooled from its surface at the temperature directly below the melting point. This model can well explain the observed features of the absorption coefficient. It is suggested that the thickness of the naturally cleaved crystal fragments must be inversely proportional to the density of the stress-induced defects.