Centers In KCl Research Articles

Electron-spin-resonance study of Sn+(5p1) centers of the laser-active-type structure in KCl:Sn2+ and analysis of the hyperfine structure.

It is shown through an analysis of the electron-spin-resonance spectra that the ${\mathrm{Sn}}^{2+}$(5${\mathrm{s}}^{2}$) impurities in KCl can also give rise, after x irradiation above 220 K, to the so-called ${\mathrm{Sn}}^{+}$(1) centers of the laser-active-type structure. The essential core of this center is a substitutional ${\mathrm{Sn}}^{+}$(5${\mathrm{p}}^{1}$) ion strongly perturbed by an adjoining anion vacancy along the 〈001〉 direction. The observed orthorhombic symmetry, with the three crystallographic axes as the main axes, is induced by either one or two weakly perturbing cation vacancies in the neighborhood. Their exact positions are hard to establish and several possible, subtly differing, defect models are proposed. An analysis of the hyperfine interaction of all the ${\mathrm{np}}^{1}$ (n=4,5,6) centers in KCl is presented, and it is established that these atoms and ions possess large negative unpaired electron-spin densities at their nuclei when they are free or in crystal fields possessing inversion symmetry. The strong odd field component induced by the anion vacancy invariably adds, through s mixing, a substantial positive contribution to this spin density.

The Study of Tl 〈110〉 Centres in NaCl and RbCl

AbstractThe presence of 〈110〉 oriented Tl centres is confirmed in the ESR spectra of heavily doped NaCl:Tl+ and RbCl:Tl+ crystals following X‐irradiation. This identification is supported by an analysis of the spin‐Hamiltonian parameters as well as by the formation and optical bleaching properties, which are similar to the previously reported optical‐absorption and ESR studies on Tl〈110〉 centres in KCl. An analysis of the spin‐Hamiltonian parameters in the frame of a molecular‐orbital model is presented.

Broadly tunable oscillator–amplifier system using lithium (F_2^+)A centers in KCl

An oscillator-amplifier system has been constructed using (F(2)(+))(A) centers in KCl:Li crystals and a 1.41-microm laser pump generated by Raman shifting a 1.06-microm Nd:YAG line in liquid nitrogen. Output energies of 2.3 mJ and peak powers exceeding 0.4 MW were achieved. The system was tuned continuously over the 2.0-2.4-microm spectral wavelength range.

Two-color superfluorescence of O 2- centers in KCl

O 2 - centers in KCl which are excited at low temperatures (T < 30K) with single pulses of a mode-locked Neodymium-YAG laser exhibit spontaneous emission of coherent light at two different wavelengths (592,77 nm and 629,14 nm, respectively). The properties of this radiation were investigated in particular the statistics of the pulse intensities and delay times.

Resonant Raman scattering of the laser active Tl°(1) defect in KCl

Resonant Raman measurements were performed on the A 1 vibrational mode of the Tl° atom in the Tl°(1) center in KCl:Tl + by excitation in the σ-polarized absorption band at 550 nm. The Tl°(1) center consists of a substitutional Tl° atom perturbed by a single nearest neighbor anion vacancy and it is known to be laser active with output in the infrared around 1.5 μm. A localized vibration at 30 cm -1 is proved to belong to the A 1 representation of a C 4v [001] symmetry by application of the recently developed Behavior Type method to the polarized Raman scattering data. Furthermore, there is evidence for a first order phonon spectrum induced by the Tl°(1) defect.

Pulsed luminescence from non-equilibrium distribution of F A(II) centers in KCl:Li +

Time dependence of luminescence under polarized optical pumping shows the effect of reorientation of the off-axis dipoles of F A(II) centers in KCl:Li +.

ENDOR Investigation of H(Li) Centres in KCl

AbstractIn KCl doped with Li+ recently a new paramagnetic hydrogen centre has been observed. From the ESR spectrum it was described to be a (LiH)+ molecule occupying a cation site with a Li+–H0 separation of 1.44 Å. Below 70 K it is frozen‐in along [111], at higher temperatures it undergoes a rapid reorientational motion. Using ENDOR now the shf interactions of the hydrogen electron with three equivalent Cl and one Li neighbour nuclei are determined precisely at 25 and 41 K and the ESR analysis essentially is confirmed. A theoretical calculation of the shf interactions shows, however, that the centre is not a (LiH)+ molecule. It is rather a perturbed interstitial atomic hydrogen centre (H(Li) centre) with a Li+–H0 separation of 2.17 Å, where the hydrogen atom is displaced from the interstitial site by about \documentclass{article}\pagestyle{empty}\begin{document}$ (1/4)\sqrt 3 d_0 $\end{document} d0 (d0 lattice spacing) towards the cation site.

Optical properties of atomic gallium and indium centres in KCl

In radiation-damaged In- and Ga-doped KCl crystals many In- and Ga-related defects are formed. Their optical absorption bands strongly overlap, making it impossible to determine the optical transitions of a particular defect directly. With measurements of the magnetic circular dichroism tagged by electron spin resonance absorption bands of Ga0(1) and In0(1) centres, the analogues of the laser-active Tl0(1) centres, could be identified. Ga0(1) centres have absorption bands peaking at 930, 890 and 620 nm and In0(1) centres at 965, 860 and 610 nm. Up to 2.7 mu m no emission was found for Ga0(1) centres, whereas upon excitation at 965 nm In0(1) centres probably have a very weak emission around 2.2. mu m. In a model calculation the occurrence of radiative and non-radiative transitions upon excitation of the lowest excited state in Ga0(1), In0(1) and Tl0(1) centres is discussed.

Effect of Powerful Pulsed Electron Irradiation on Creation of Hole Colour Centres in KCl at 300 K

Effect of Powerful Pulsed Electron Irradiation on Creation of Hole Colour Centres in KCl at 300 K

Dynamics of the Optical Excitation—Deexcitation Cycle of O2− Centers in KCl

AbstractThe time dependence of the fluorescence of O−2 centers following picosecond‐pulse excitation is studied. The risetime of the spontaneous fluorescence yields an upper bound for the relaxation time in the excited state: τ*R &lt; 20 ps. The fluorescence decay time decreases from 92 ns at 4.2 K to 50 ns at 300 K. By comparing the results to superfluorescence experiments a lower bound for the relaxation time in the ground state is obtained: τR ⪆ 50 ps.

Optical properties of the In 0(1) center in KCl

We report both experimental and theoretical studies of the optical properties in KCl of a defect-associated In center whose sole previously identified optical transition was a strong absorption band peaking at 610 nm. With the aid of an optical tagging technique, we have been able to identify with that center two new but much weaker absorption bands peaking at 860 and 960 nm, respectively. We show that the energy splittings, relative strengths, and less perfectly, the polarizations of those three transitions make fit to the In 0(1) center model, i.e., the model, initially developed for the Tl 0(1) center, of a neutral atom perturbed by the field of an adjacent anion vacancy. However, in contrast to the Tl 0(1) center, the In 0(1) center lacks a significant emission, excluding its use as a laser gain medium.

Optical Pumping and Reorientation of the FA Centers in KCl:Li+

Optical Pumping and Reorientation of the F_A Centers in KCl:Li⁺

The FH‐Centre Energy Structure in KCl and KBr

AbstractThe energy parameters of the FH(Br−)‐, FH(O−) — centres in KCl and FH(O−) — centres in KBr are calculated within the framework of perturbation theory. From the comparison of calculated and experimental data the following conclusions are drawn: (1) the long‐wave shift of the F‐centre absorption band in KCl with increasing Br− concentration may be due to the formation of FH(Br−)‐centres whose components are a \documentclass{article}\pagestyle{empty}\begin{document}$ a\sqrt 2 $\end{document} apart (a is the shortest interionic distance); (2) the decay of impurity excitons in KCl‐Br leads to the creation (or the manifestation in the stimulation spectrum) of the FH(Br−)‐centres with components being 2a rather than a \documentclass{article}\pagestyle{empty}\begin{document}$ a\sqrt 2 $\end{document} apart; (3) the change in the optical characteristics of the FH(O−)‐centre as compared to those of the F‐centre in KBr is consistent with the experiment assuming that an O− off‐centre position in the lattice site exists. The possibility of F‐centre RES life‐time lowering is mentioned.

Damping Effects in Two-colour Superfluorescence

A theoretical model for the superfluorescent emission from a three-level system with rapidly decaying lower states is discussed. In the framework of this model a qualitative description of some features of two-colour super-fluorescence by O m2 centres in KCl is presented.

Positive muon behavior in KCl with and without F centers

Positive muon behavior in KCl containing F centers has been studied. The muon spin depolarization rate showed a maximum near 120 K, which was not found in pure KCl. This is probably due to the fact that free positive muons are trapped by F centers in KCl. However, the binding energy between a positive muon and an F center is not large, so that muons detrap again above 150 K.

V3 and FA Centers in KCl:Na Crystals. A Thermoluminescence Study

AbstractV3 and FA centers in KCl:Na crystals are studied using thermoluminescence and TL spectra as tools together with the data on X‐ray luminescence spectra. The charge species responsible for two different glow peaks occurring around 55 and 105 °C are identified as well as the sequence of steps involved, the mechanism of recombination, and the recombination sites. The results obtained enable to establish the origin of the 435 nm emission occurring in both, the TL and X‐ray luminescence as that due to the radiative decay of the self‐trapped exciton at a cation—vacancy site.

Pressure Effects on the Absorption and Emission of FA Centers in KCl

Pressure Effects on the Absorption and Emission of F_A Centers in KCl

Coherent interaction of picosecond pulses in a fluorescent medium and the timing of pulse trains for two-wavelength excite and probe measurements

Coherent interaction between two cw, picosecond pulse trains of the same wavelength absorbed by a fluorescent material and the effect on the emission has been studied. As the path difference between the two beams is varied, the infra-red emission from colour centres in KCl: Na or KCl: Li fluctuates in phase with the transmitted intensity of the weaker of the two beams. The effect may be applied to the accurate relative timing of cw pulse trains of different wavelengths for use in two-wavelength, excite and probe measurements. A small fraction of the pump pulse train is superimposed on the probe beam and passes along precisely the same optical path. Coherent interaction between the two pulse trains at the pump frequency causes modulation of the transmitted probe beam intensity when pump and probe pulses are accurately synchronised.

Effect of Magnetic Field on the Decay Kinetics and Polarization of the AT Emission of KCl:Tl and KI:In Crystals at 0.4 to 0.7 K

AbstractThe structure of emission and excitation spectra as well as the decay kinetics and polarization of the AT luminescence are investigated for Tl+ and In+ centres in KCl:Tl and KI:In at 80 to 0.4 K. The effect of a magnetic field up to 2 T on the decay times, intensities, and polarization properties of the emission occurring from the metastable tetragonal minima of the triplet excited state is studied at 0.4 to 0.7 K. The parameters of the centres are determined. Good agreement between the experimental results and theory is obtained.

Magneto-optical studies of atomic thallium centers in KCl: Magnetic circular dichroism tagged by spin resonance

We report measurement of the magnetic circular dichroism (MCD) of radiation-damaged crystals of KCl doped with Tl. By using microwaves resonant with ground-state magnetic splittings, we have been able to tag such MCD spectra to specific species. In particular, we have shown that bands peaking at 1040, 725, 550, 405, 340, and 254 nm all belong to the ${\mathrm{Tl}}^{0}$(1) center, where that center, as determined by earlier spin-resonance studies, consists of a neutral Tl atom perturbed by the field of an adjacent anion vacancy. Our experiment constitutes the first direct evidence that a recently discovered, thallium-associated, laser-active center whose pump band peaks at 1040 nm is indeed the ${\mathrm{Tl}}^{0}$(1) center; furthermore, the MCD of the two lowest energy bands are in rough quantitative agreement with predictions of a simple electronic model of that center. Finally, our technique has allowed the discovery of two other centers having distinctly different spin resonances, yet whose optical spectra bear some striking similarities to that of the ${\mathrm{Tl}}^{0}$(1) center. Because their fundamental bands overlap those of the ${\mathrm{Tl}}^{0}$(1) center, the new centers may affect the operation of ${\mathrm{Tl}}^{0}$(1)-center lasers.