Ultraviolet A-band absorption in NaCl:Pb2+and clustering of lattice defects

Abstract

Changes in the A-band absorption spectrum of NaCl:Pb²⁺, resulting from isothermal aging, have been analysed using information obtained from dielectric relaxation measurements about the state of aggregation of the Pb²⁺ ions and their associated vacancies. Optical absorption has been measured at room and liquid nitrogen temperatures. It is shown that the A band, centred at 4.54 eV for (Pb²⁺-vacancy) pairs, splits into two bands, when clusters of three (Pb²⁺-vacancy) units form during the first stage of aggregation. One band is at the same energy (4.54 eV) and the other at 4.34 eV. In the second stage of aggregation these trimer bands persist with no change in intensity, and two new bands appear at 4.66 eV and 4.34 eV. The significance of these measurements for models of the aggregation process is discussed.