Knowledge Bank

The temperature dependence of the rate of photo-orientation of $PO2-$ anion impurity centers in potassium chloride single crystals has been determined over the temperature range 2-50 K. The temperature dependence of the photo-orientation rate constant has been interpreted in terms of a kinetic model for thermal and photo-physical reorientation of the molecular axes in a six-fold orientationally degenerate orthorhombic site. Analysis of the experimental data yields values for the potential barrier to reorientation in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1$ electronic ground state of $783cm-1$, and in the first electronically excited $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$ state of $350cm-1$. The experimental results for the potential barriers to reorientation are compared with theoretical calculations using a short-image repulsive potential. Several mechanisms are discussed for the photo-orientation process.