Effect of Ionizing Radiation of Impurity-Vacancy Dipoles in Lead-Doped NaCl and KCl

The radiolytic behavior of NaCl: Pb and KCl: Pb upon exposure to 1.5-MeV electrons at various temperatures has been investigated by means of optical absorption and ionic thermocurrent (ITC) methods. Irradiation decreases the ${\mathrm{Pb}}^{2+}$ $A$ band at 272 nm and introduces two narrow bands at 207 (210) and 253 nm in NaCl (KCl). Since the new bands decrease in amplitude when holes are released by warming the crystals into the range where they become mobile, and increase in amplitude at the expense of the $A$ band when electrons are released by $F$-band excitation, we conclude that they are associated with Pb states of lower positive charge, i. e., ${\mathrm{Pb}}^0$ and/or ${\mathrm{Pb}}^{+}$. Irradiation also decreases the height of the ${\mathrm{Pb}}^{2+}$-cation-vacancy ITC relaxation peak near 227 °K and introduces small ITC peaks at lower temperature, which are not produced by similar irradiation doses in pure KCl. One of these, at 175 °K in NaCl and near 200 °K in KCl, anneals at 300 °K but is enhanced by $F$-band bleaching at lower temperatures. It is proposed that this new ITC peak is associated with the relaxation of a pseudodipole composed of a cation vacancy bound by elastic forces to a ${\mathrm{Pb}}^{+}$ ion.