High-pressure optical studies of doped alkali halides. III. Rates of electron-transfer processes

Abstract

High-pressure studies have been made on the rates of photoinduced electron-transfer processes in NaCl:In. Continuous irradiation in the $C$ band ($^{1}S_{0}\ensuremath{\rightarrow}^{1}P_{1}$) of the impurity gives a luminescence which changes (generally decreases) with time, and ultimately reaches a steady state. At the same time absorptions appear which are assigned to ${\mathrm{In}}^{++}$ and to an $F$ center (also, under some conditions a peak assigned to ${\mathrm{In}}^{0}$ may appear). The process involves competition between, on the one hand, ionization of the $^{1}P_{1}$ electron (initial trap depth \ensuremath{\sim} 0.1 eV) and trapping at the $F$ center, and, on the other hand, the release of an electron from the $F$ center by absorbing ${\mathrm{In}}^{+}$ emission. The processes are characterized by the rate coefficients ${k}_{i}$ and ${k}_{r}$. ${k}_{i}$ decreases with pressure---probably owing to an increase in the depth of the $^{1}P_{1}$ state below the conduction band. ${k}_{r}$ exhibits a maximum at \ensuremath{\sim} 25-30 kbar. This is associated with the overlap of the ${\mathrm{In}}^{+}$ emission and $F$-center absorption which at first increases and then decreases with pressure, as well as with the increased trapping ability of the $^{1}P_{1}$ state of the indium ion. Irradiating the sample in the ${\mathrm{In}}^{++}$ absorption band gives a luminescence which decreases rapidly in intensity with time, since this luminescence overlaps the $F$-band absorption in energy and bleaches it. The rate of bleaching decreases rapidly with increasing pressure since the overlap decreases.