Ns2 Ions Research Articles

Luminescence of CsCl: Tl Single Crystals

AbstractSpectral, polarization, and kinetic characteristics of CsCl:Tl luminescence are studied between 4.2 and 350 K. The Ax band at 3.88 eV and the AT band at 3.65 eV are observed in the triplet emission spectrum of Tl+ centres. It is shown that in the triplet relaxed excited state (RES) the trigonal (X) minima are of higher energy than the tetragonal (T) ones. The symmetry of both the emitting and the metastable states in each (X or T) minimum is the same, and in many respects their structure is similar to that of other ns2 ions in alkali halides. However, some peculiarities are observed in the luminescence characteristics of CsCl:Tl crystals (e.g. very small quantum efficiency of triplet emission at excitation in high‐energy absorption bands; population of the lower (T) minima mainly in the process of vibronic relaxation in the triplet excited state; too large polarization degree of the slow decay component of the Ax emission, exceeding 1.5 times its theoretical limit; large difference in the spin‐orbit splitting energies δ of the triplet RES in the × and the T minima as well as in radiative decay probabilities y1 of their metastable states; the absence of an off‐centre displacement of the excited Tl+ ion from the crystal lattice site, etc.). Possible reasons for these features are discussed.

Polarized Singlet and Triplet Luminescence and Off‐Centre Position of Excited In+ Ions in a CsBr :In Crystal

AbstractSpectral and polarization characteristics as well as the decay kinetics of the CsBr : In luminescence are investigated in the temperature range from 1.75 to 300 K. Four emission bands (at 2.44, 2.75, 3.27 and 3.32 eV) are observed at low temperatures and interpreted correspondingly as AT, Ax, B, and C emission of In+ centres. The symmetry of the Jahn‐Teller minima of the triplet relaxed excited state (RES) responsible for the Ax and the B emission is established to be trigonal and rhombic, respectively. The AT and the C emission is caused by the radiative decay of the tetragonal Jahn‐Teller minima of the triplet (A) and the singlet (C) excited state. Two slow components are found in the decay kinetics of both, the AT and the Ax emission at T ⪅ 3 K. It points to the splitting of the metastable level both, in the tetragonal (T) minima and in the trigonal (X) minima of the triplet RES. It is shown that the effects observed are caused by an off‐centre displacement of the In+ ion in the triplet RES from a crystal lattice site. For excited ns2 ions in CsCl‐type hosts these effects are not observed before. This is the first observation of the off‐centre effects in the trigonal minima of the triplet RES of ns2 ions.

Triplet relaxed excited state structure and luminescence of thallium‐doped alkali halides

AbstractFor the first time a slow component is observed and studied in the decay kinetics of AT emission of TI+ centres in KBr, KI, RbCl, and RbI crystals. The existence is established of a singlet metastable level below the doublet emitting one in each Jahn‐Teller minimum of the triplet excited state of the TI+ centres. It is concluded that for all ns2 ions in alkali halides the structure of the relaxed excited state is qualitatively the same, and it may adequately be described in terms of a theory that considers the Jahn‐Teller effect as a main interaction in the relaxed excited states of the luminescence centres. The nature is discussed of the weak emission bands appearing on excitation in the A absorption band of TI+ centres besides the main AT and AX emission bands, earlier interpreted as hot luminescence of Tl+ centres.

ODMR in Triplet Excited States of ns2 Ions in Alkali Halide Crystals

AbstractOptically detected magnetic resonance (ODMR) is applied for the first time to the investigation of triplet relaxed excited states (RES) of divalent ns2 ions in alkali halide crystals. Four types of ODMR spectra related to the RES of Ge2+ are found. Spectral and angular dependences of ODMR are studied. ODMR data on the RES of monovalent (Ga+) and divalent (Ge2+) ns2 ions are discussed. These results provide the first experimental evidence of the effect of nuclear charge on the wavefunction density distribution for excited states of impurity ions in crystals.