Influence of Ca and Pb impurities on the bulk optical strength of ultrapure NaCl and KCl crystals

A significant increase (by a factor of about 3) of the bulk damage threshold in the case of interaction of CO₂ laser radiation pulses with ultrapure NaCl and KCl crystals grown in a reactive atmosphere was observed on introduction of divalent metal ions Ca and Pb in concentrations of 10^–5–10^–6 mol/mol. Impurities were introduced in concentrations of 10^–8–10^–3 and 2×10^–7–10^–4 mol/mol into the melts of KCl and NaCl, respectively. The concentration of other impurities (including OH) did not exceed ~10^–6 mol/mol. A physical model was developed to account for the observed dependence on the basis of an analogy between a system of colloidal particles and F centers in a crystal and a liquid–vapor system.