Z-Center Energy Levels

Abstract

One of the centers proposed to account for Z‐band absorption in alkali halide crystals consists of an electron trapped at the divalent cation. A calculation of the absorption energy has been made using the method of Simpson. This method, though not free of objections, has been surprisingly successful in predicting F‐band absorption maxima. For Ca⁺⁺ in KCl and NaCl, the absorption maxima were found to lie at 3.92 and 3.70 eV, respectively, for Sr⁺⁺ at 9.00 and 8.70 eV. These locations are far removed from the Z bands, which lie near 2.0 eV in both crystals. While the numerical accuracy of the calculations is not high, the possibility of a Z‐type center absorbing in the far ultraviolet is raised; this has a bearing on recent thermodynamic arguments concerning the nature of Z centers.