Luminescence Due to Radiative Transitions between Valence Band and Upper Core Band in Ionic Crystals (Crossluminescence)

Abstract

I n some wide band gap halides of heavy alkali and alkaline-earth metals (BaF2, CsCI, R b F etc.) the energy separation between the valence band involving mainly the halogen electronic states and the upper cation core band is smaller than the band gap. Under the excitation generating holes in the core band radiative electronic transitions from the valence band to the core band take place giving rise to a specific kind of intrinsic emission which is proposed to call crossluminescence. I t s decay time is about 1 ns and the thermal quenching starts a t temperatures well above room temperature. The width of the emission spectrum practically coincides with the width of the valence band deduced from ultraviolet photoelectron spectroscopy and the shape of the spectrum depicts the electronic structure of the valence band.I n einigen Alkali-und Erd-Alkalihalogeniden (BaF,, CsCI, R b P u. a.) ist die Energiedifferenz zwischen dem Valenzband und dem hochsten Atomrumpfband kleiner als die Energielucke. Unter hochenergetischer Strahlungsanregung, die Locher im Atomrumpfband generiert, treten stra hlende elektronische UbergLnge von Valenzband zum -4tomrumpfband auf, eine spezifische Eigenlumineszenz, die ,,Crosslumineszenz" genannt wird. Die Abklingzeit dieser Lumineszenz ist etwa 1 ns und die thermische Tilgung beginnt iiber Zimmertemperatur. Die Spektrumsbreite korreliert mit der Valenzbandbreite, wie durch Photoelektronenspektroskopie festgestellt wird, und die Struktur des Spektrums charakterisiert die Elektronenstruktur dieses Valenzbandes.

Effect of local environment on crossluminescence kinetics in SrF2:Ba and CaF2:Ba solid solutions

“…In BaF 2 the excitation of electrons from the Ba 2+ (5p) core band to the conduction band starts at 18.1 eV, in agreement with previous data. [1][2][3][4] The thresholds of the impurity CL excitation spectra in SrF 2 :Ba and CaF 2 :Ba, when electrons are excited from the Ba (5p) level of the impurity ion, are shifted to higher energy by ∼0.5-1 eV. These data are in reasonable agreement with the above results of first-principles calculations for the energy separation between the top of the Ba 2+ (5p) band and the conduction band edges in BaF 2 , SrF 2 :Ba, and CaF 2 :Ba.…”

A Quasi-Molecular Model of Cross-Luminescence of the CsCl Crystal

“…In [2] cross-luminescence spectra of CsCl and CsBr crystals were interpreted based on the results of band calculations. However, impurity-induced cross-luminescence at electron transitions from the valence band to the core impurity level, for example, in the KCl: Cs crystal [5,7] suggests that this physical process has a local character and hence can be described within the framework of local approaches.…”

“…The characteristic features of cross-luminescence are extremely short (subnanosecond) de-excitation time, highly stable luminescence intensity for a wide temperature interval, and weak temperature dependence of other radiation characteristics (including the band position and width and the decay time) [1][2][3][4].…”

Luminescence of CsCl : Tl Crystal under Synchrotron Excitation

“…However, as this emission is very fast (1.2 ns at 295 K [5]) and appears only under excitation at E exc > 14 eV, it has been interpreted as the radiative recombination of valence electrons with outermost 5pCs þ core holes (see, e.g., Refs. [5][6][7][8][9] and references therein).…”

“…Time-resolved emission spectra of a CsCl : Tl 10 --2 mol% crystal measured under excitation in the A absorption band of Tl þ centres, E exc ¼ 5.17 eV Similar bands have been observed in the emission spectra of undoped CsCl crystals under excitation at E exc > 14 eV as a result of the radiative recombination of valence electrons with holes of upper 5pCs þ core band[5][6][7][8][9].It should be noted that the intensity ratio of the two localized exciton emission bands (%3.0 and 2.55 eV) depends on the excitation energy E exc . It explains the shift of the visible emission band maximum (E em ) with the change of E exc (see, e.g., Figs.…”