Franck–Hertz effect in cathodo- and photoluminescence of wide-gap materials

Abstract

a b s t r a c tA brief overview of previously obtained and novel data on the manifestations of an analogue of FranckHertz effect in photo-and cathodoluminescence of wide-gap inorganic materials is presented. On the example of NaCl:Tl þ and MgO:Cr 3þ single crystals, the excitation processes of the luminescence of 6s 2 Tl þ ions and 3d 3 Cr 3þ ions by 5-15 keV electrons or 5-20 eV photons at 6-420 K have been studied. The rapid processes of the direct energy transfer to Tl þ by hot conduction electrons or to Cr 3þ centers by hot electrons and/or hot valence holes have been separated from rapid excitonic and more inertial electronhole processes.

Low-temperature creation of Frenkel defects via hot electron-hole recombination in highly pure NaCl single crystals

“…A further F stimulation leads to the appearance of the triplet luminescence of self-trapped excitons (STEs, peaked at $3.4 eV) and a similar recovery of a regular lattice. In addition, there are some experimental manifestations that NaCl is a unique system, in which the energy of a close F-H pair exceeds the level of a relaxed triplet exciton, 29 and an H center becoming mobile (for instance, with a rise of temperature) recombines after several jumps with a complementary F centre via the STE triplet state. Nevertheless, in all cases the light sum of PSL is proportional to the number of radiation-induced F centres.…”

“…So, the FranckHertz effect was considered as some kind of "luminescent protection" against the radiation damage induced by hot e-h recombination. 12,14,15,36,37 However, several limitations do not allow to use this approach for significant increase of material radiation resistance. 15 In NaCl at room temperature, the value of E FD is close to E g .…”

“…This fact was confirmed once again by measuring the creation spectrum of F centres by SR in NaCl:Tl at 295 K using a similar luminescent method. 29 At room temperature, H centres are highly mobile and do not exist as single centres, therefore, the light sum of impurity luminescence stimulated in the F-absorption band was taken as a measure of radiation-induced F centres (see, e.g., Refs. 33 for details).…”

“…2,16,17 The excitonic and e-h processes as well as the creation of FDs in pure and doped NaCl crystals have been studied for a long time. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Because of the great difference in radii of sodium and chlorine ions, the interstices in the closely packed NaCl lattice are small that impedes the displacement of anions from regular sites with the formation of interstitials. Furthermore, in contrast to the majority of alkali halides, H centres (a dihalide X À 2 molecule located at one anion site) in NaCl are oriented along h111i directions.…”

Influence of complex impurity centres on radiation damage in wide-gap metal oxides

“…A photon with the energy of ~2Eg creates a hot conduction electron (an energy excess of Eg) that already participates in the so-called multiplication process forming a secondary e-h pair (see, e.g., [43,45]), while both existing conduction electrons become "cold" for the direct excitation of an impurity centre. [56].…”