Direct excitation of Tl+ impurity ions by hot photoelectrons in wide-gap crystals

The reflection spectra of KCl and RbCl crystals and excitation spectra of A and B luminescence and recombination phosphorescence of Tl+ centers for RbCl:TI and KCI:Tl crystals have been measured in a spectral region from 5 to 30 eV. The energy distribution curves of emitted electrons have been measured on the excitation of RbCl thin films by 13-30 eV photons. The comparative analysis of optical and photoelectric characteristics allowed us to reveal the effect of direct excitation of Tl+ impurity ions up to A(P-3(1)), B(P-3(2)), and C(P-1(1)) states by hot conduction electrons. The threshold photon energies of this effect are determined. The estimated energies of conduction electrons, formed by such photons, are close to the energies of direct optical transitions S-1(0)-->P-3(1) (5 eV) and S-1(0)-->P-3(2) (6 eV) of Tl+ centers in KCI:TI and RbCl:TI crystals. Similar to the case of ns(2) free atoms, the excitation functions for triplet states of Tl+ centers have a sharp maximum at the energy just above the threshold in RbCl:TI and KCl:Tl crystals. The peculiarities of electron impact spectroscopy of impurity ions in wide-gap crystals are discussed.