New off‐centre system. Jahn‐Teller mercury‐like impurities in alkali halides

Abstract

Two slow decay components are found in the low-temperature decay kinetics of the A, emission of Ga+ and In+ centres in alkali halides. The emission, excitation, and polarization spectra of these components as well as the temperature dependences of their decay times and light sums and the influence of the electric and magnetic fields on the decay kinetics of the polarized A T emission are carefully investigated. It is shown that the observed effects are caused by an off-centre position of Ga+ and In+ ions in the triplet relaxed excited state (RES). The off-centre displacement occurs most probably in one of the (100) directions perpendicular to the tetragonal Jahn-Teller distortion axis. Rapid tunnelling transitions between various energetically equivalent off-centre positions of the impurity ion lead to the tunnel splitting of each metastable tetragonal (T) minimum of the triplet RES. Both the observed slow decay components of the AT emission are connected with the radiative decay of the two lower states. The orthorhombic structure of the A T RES, found by the technique of optical detection of electron spin resonance (OD ESR) several years ago for Gat centres in some alkali halides, is explained here as another evidence of the same phenomenon.

Investigation of Cu-doped Li2B4O7single crystals by electron paramagnetic resonance and time-resolved optical spectroscopy

“…The absence of the non-equilibrium component means that there is no direct initial population of the emitting doublet during the relaxation from the non-relaxed 3d 9 4s term. A complete absence of the non-equilibrium decay component has been observed, for instance, also for In + centres in alkali halides [22] and for the triplet emission of self-trapped excitons in tungstate crystals [23].…”

“…Note that the degree of polarization of the emission coming from the lowest tunnel sublevel is the same as that of the whole triplet system in thermal equilibrium. Such behaviour is characteristic of tunnel sublevels, where the lowest one tends to display the symmetry properties possessed by the centre-symmetric state (see [22]). The polarization degree of the sum of both components ( figure 7(b), curve 3 ) is the same as that of the 3.35 eV emission in the region 6-12 K, where the splitting is not yet observable.…”

“…It can be assumed that this may also happen in its excited state, as required in our case. The mechanism of the splitting of a metastable level by tunnelling transitions is described in detail in [22].…”

Time‐Resolved Polarization Spectroscopy and Off‐Centre Position of Excited Ga+ Ions in NaCl:Ga Crystal

“…Consequently, the rate of the tunnelling transitions increases, and the tunnel splitting energy ( A ) also grows. Really, a comparison of the results obtained for NaCl : Ga in the present work and those obtained for KCl : Ga in [7,8] shows that in the NaCl : Ga crystal (RNa+ = 0.098 nm) the manifestations of off-centre effects in the triplet emission decay kinetics appear at a higher temperture ( T 5 1.45 K) than those in KCl: Ga crystal ( T 5 1.15 K) (RK+ = 0.133 nm) just because of the larger energy A . One may expect this temperature to be still lower in RbCl : Ga.…”

“…Therefore, the off-centre displacement of Ga' ions in alkali halides (RGa+ % 0.1 nm) must continue existing up to temperatures higher than that of In' ions (Rr,+ x 0.13 nm). Since at T < 3 to 4 K Inf ions in the RES occupy the off-centre position3) in all alkali halides investigated [8 to 12, 181, the excited Ga' ions must undoubtedly be ,,off-centre" at these temperatures in the same crystals including NaCl : Ga.…”