Excitation mechanism of cathodoluminescence of oxisulfides

This paper presents the first investigation on the diffusion of electron-hole pairs (generated under cathode-ray excitation) as a function of activator concentration in the oxisulfides $L_2$ ${\mathrm{O}}_2$S:${\mathrm{Tb}}^{3+}$ with L=Gd, La, or Y. The diffusion was studied by measuring the energy efficiency of the ${\mathrm{Tb}}^{3+}$ luminescence as a function of the accelerating voltage of the primary electrons. For all three host lattices investigated the diffusion length of the electron-hole pairs is found to be essentially constant at low activator concentrations (dominated by the killer concentration); then the diffusion length decreases with increasing activator concentration (increasing transfer to activators); at still higher concentrations, where the energy efficiency starts to decrease (concentration quenching), the diffusion length increases again. As the quantum yields with 254-nm excitation show hardly any decrease, the increasing diffusion length indicates a new mechanism, leading to concentration quenching. From our experiments we determined the cross section for excitation of a ${\mathrm{Tb}}^{3+}$ ion by a thermalized electron-hole pair to be about 30×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$.