Fluorescence of tin-sensitized manganese in single-crystalline NaCl.

Abstract

Energy transfer from ${\mathrm{Sn}}^{2+}$ to ${\mathrm{Mn}}^{2+}$ ions has been investigated in single crystals of NaCl slightly doped with tin and manganese ions. The data obtained indicate that the tin-sensitized manganese fluorescence which takes place in our doubly doped quenched crystals occurs between the Sn-Mn pairs which are preferentially formed in the crystalline matrix. The essential features of the kinetics of ${\mathrm{Sn}}^{2+}$\ensuremath{\rightarrow}${\mathrm{Mn}}^{2+}$ energy transfer are described considering an energy-level system in which both the sensitizer and activator ions are treated as two energy-level systems. From the solutions of the rate equations describing the time evolution of the excited-state populations for the isolated Sn and the Sn-Mn complex and our experimentally determined data, the rate of ${\mathrm{Sn}}^{2+}$\ensuremath{\rightarrow}${\mathrm{Mn}}^{2+}$ energy transfer was estimated to be greater than 7\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. This value was then compared with those calculated with use of Dexter's theory of energy transfer via either multipolar- or exchange-interaction mechanisms. To do this, several configurations for the Sn-Mn pair complex were employed. These calculations allowed us to get some insight into the possible nature of the tin-manganese complex and to establish that the experimental data can only be plausibly explained if a short-range interaction mechanism for energy transfer is active between these two impurity ions. Moreover, from the solution of the rate equations for the steady-state case, the ratio of the number of coupled ${\mathrm{Sn}}^{2+}$ ions which transfer energy to ${\mathrm{Mn}}^{2+}$ to the total number of tin ions in the samples was estimated to be approximately 5%. This result gives support to the ionic-radius criterion proposed by Rubio and co-workers to predict pairing between two doubly valent impurity ions in an alkali halide host between which energy transfer is desired.