Centers In KCl Research Articles

Hot luminescence from F centers in KCl studied by time-resolved polarization spectroscopy

A dynamical de-excitation process for optically excited electrons in F centers in KCl is studied by using time-resolved polarization spectroscopy. The ordinary luminescence (OL) and the hot luminescence (HL) in the OL region are separated from a resonant secondary radiation, and the HL is decomposed into two components with different behaviors. They are the HL in a short time after excitation and an emission from 2p-like states after a long time from excitation. The dynamics of the optical excitation is interpreted phenomenologically in terms of a configuration coordinate model.

Jahn–Teller effect of Au − centers in KCl xBr 1− x mixed crystals

We have investigated the A absorption band of the Au − center in KCl x Br 1− x mixed crystals. In Au −-doped KCl and KBr crystals, the obvious zero-phonon lines and vibronic structures appear on the absorption and luminescence spectra reflecting the fairly weak electron–lattice interaction. With increasing concentration of the hetero-halogen ions, the zero-phonon lines and the vibronic structures on the spectra disappear abruptly, the absorption and luminescence bands become broader and their Stokes–shifts increase. These facts evidently indicate the increase in the electron–lattice interaction strength in the mixed crystal system. The A absorption band at various temperatures can be deconvoluted into two Gaussian components due to the Jahn–Teller splitting. The temperature dependence of the splitting energy reveals the presence of a static perturbation due to the lowering of the site symmetry of impurity in addition to the dynamical Jahn–Teller effect.

Theoretical study of optical transitions of H centers in potassium halides

The H center in KCl, KBr and KI crystals gives rise to two distinct optical absorption bands. On the basis of the (halogen) 4 3− model, the peak energies and intensities of these two bands are theoretically studied by using the semi-empirical method, in which the spin–orbit coupling, the crystal field and the configuration interaction are included. The calculated results are compatible with the experimental values. The theoretical consideration is also presented on the intensity of transitions by the π-polarized light excitation.

Electronic States and Optical Absorption Spectra ofHCenters in Potassium Halide Crystals

Irradiation of undoped KI single crystals with a KrCl excimer laser ( h ν=5.58 eV) near liquid helium temperature results in the production of α, β, F , F ′ and H centers. The H centers, i.e., I 4 3- molecular ions in KI, give rise to two prominent optical absorption bands peaking at 2.23 and 2.77 eV. The theoretical results for the (halogen) 4 3- molecular ion, which are obtained by using the semiempirical self-consistent field method, show that the 2.23- and 2.77-eV bands are ascribed to the transitions of the ground 2 Σ u 1/2 + state to the excited 2 Π g 1/2 and 2 Σ g 1/2 + states, respectively. The same assignments are made for two distinct absorption bands arising from H centers in KCl and KBr crystals. The relative intensities of the two distinct bands for KCl, KBr and KI crystals depend on the magnitude of the spin-orbit interaction in halogen atoms. The peak energies and intensities of absorption bands caused by H centers in the potassium halide crystals are discussed in comparison between experi...

Determination of the Optical Constants of the Photostimulable BaFCl: Eu2+ and KCl:Eu2+ Crystals

A new method to determine the concentration of the defect centers in photostimurable luminescent materials is demonstrated. The method is representatively applied to BaFCl:Eu2+ and KCl:Eu2+ single crystals, which exhibt the photostimulable luminescence. Thereby the values of the oscillator strength, f, are calculated utilizing the results of the defect center concentrations as well as the measured refractive indices and the corresponding absorption bands by using the Smakula's formula. The values obtained are f=0.80 for the F(Cl-) centers in BaFCl:Eu2+ and f=0.60 for the F centers in KCl:Eu2+.

Polarized wave coupling and selection rules violation in alkali-halide crystals with F/sub A/ centers

Self-diffraction of light has been investigated in KCl with F/sub A/ centers in order to elucidate the role of F/sub A/-center kinetics in holographic recording under resonant optical excitation. It is shown that diffraction efficiency for holographic grating recorded in the spectral range of the F/sub A1/ transition can be substantially increased by populating the orientations which immediately interact with the polarized light by means of lateral illumination of the crystal with an auxiliary beam. The optimum range of auxiliary beam intensities leading to improvement of grating quality is found. Such a range is directly defined by the parameter /spl chi/, characterizing the selection rules violation for F/sub A/ center in KCl. It has been found that the experimentally determined value /spl chi/=0.04 is consistent with the self-diffraction data obtained in this paper.

Investigation of Radiation-Induced Defects in NaBr:Sr2+

An EPR/ENDOR study of two radiation-induced defects in NaBr doped with Sr 2 + is presented. X-irradiation at room temperature produces a large amount of F centres compared to undoped NaBr and an impurity-related hole centre of fJrj-type. An ENDOR investigation of the F centre yielded the superhyperfine interaction constants for the first two shells of neighbours. The Br 2 - -type hole centre has properties very similar to the H D S D 100 centre in KCl:Sr 2+ . In contrast to the centres in KCl, which decay thermally at 255K, the H F D D is stable at room temperature.

And centres in KCl and KBr doped with either carbonates or sulphates

In the present paper, it is shown experimentally: (i) that laser-active and centres may be produced with significant concentrations in -doped KBr (as was already known for -doped KCl); and (ii) that, in contradistinction to the results reported previously, large optical densities of and centres can be obtained in sulphate-doped alkali halides (KCl or KBr).

Structural and optical properties of low nnergy electrons irradiated KCl:LiF multilayer films

The optical behaviour of point defects created by low energy electron beams is studied in multilayer KCl:LiF films, grown by thermal evaporation onto amorphous substrates at constant temperatures ranging from 30 to 350°C. The surface morphology of the samples was observed by the atomic force microscopy (AFM) technique. Large concentration of F and F-aggregate centres has been produced by 3–7 keV electron irradiation both in KCl and LiF layers which constitute the film. By pumping our films with appropriate Ar + laser lines, we measured, for the first time, the luminescence in the near infrared region due to F 2 and F 3 centres in KCl and the typical emission of F 2 and F 3 + centres in LiF in the visible region.

Diode-pumped colour-centre lasers tunable in the 1.5 �m range

Under pumping with a 990 nm, 1 W laser diode, continuous-wave (cw) tunable laser emission in the 1.5 µm wavelength range was obtained from two different colour centres: Tl0(1) in NaCl:Tl+ and (F 2 + )H in NaCl:OH−. The results are compared to those recently obtained with a similar apparatus and Tl0(1) centres in KCl:Tl+. The highest output power (30 mW) and the broadest tuning range (1.48–1.68 µm) were achieved with (F 2 + )H centres.

Superhyperfine structures of the relaxed excited state of the F centers in KCl observed by zero-frequency optical detection of electron-nuclear double resonance.

The superhyperfine (shf) spectrum of the relaxed excited state (RES) of the F centers in KCl has been observed by detecting decrement dips of the quantum efficiency ${\mathrm{\ensuremath{\eta}}}_{\mathit{F}}$ of the F luminescence in its dependence on the magnetic field B below 0.31 T at 4 K. This method is called zero-frequency optical detection of electron-nuclear double resonance (ZF-ODENDOR) described earlier by the Jaccard group. The shf spectrum depends on the angle \ensuremath{\Phi} between B and the crystalline axis. From curve-fitting analysis of its \ensuremath{\Phi} dependence, the isotropic and anisotropic shf interaction constants (a and b) of the RES are determined up to the 32nd shell as a function of \ensuremath{\rho} being the shell distance from an anion vacancy divided by a lattice constant. From the \ensuremath{\rho} dependence of the a values, the envelope wave function (EWF) of the RES is estimated as a \ensuremath{\Vert}2s〉-like EWF admixed with a small fraction (13%) of \ensuremath{\Vert}2p〉-like EWF (Bogan and Fitchen's model). The amplification factors ${\mathit{A}}_{+}$=819 and ${\mathit{A}}_{\mathrm{\ensuremath{-}}}$=950, and an orbital extension constant \ensuremath{\eta}=0.65 for the EWF are adopted. Thus, the advantage of the vibronic model of the RES, that is theoretical extension of Bogan and Fitchen's model, is supported. The ${\mathrm{\ensuremath{\rho}}}^{\mathrm{\ensuremath{-}}3}$ dependence of the ${\mathit{b}}_{1}$ values is confirmed in the range \ensuremath{\rho}\ensuremath{\gtrsim}3.5. \textcopyright{} 1996 The American Physical Society.

Investigation of the dynamical behaviour of the FH(CN−) centre in KCl with temperature dependent endor spectroscopy

Abstract The temperature dependence of the electron nuclear double resonance (ENDOR) spectra of FH(CN−) centres in KCl was investigated in the temperature range between 10–220 K. At the lowest temperature of 10 K only one CN− orientation with respect to the F centre electron is present, in which the nitrogen is thought to be nearer to the F-electron than the carbon. With the very small thermal activation energy of 2.9 meV the opposite orientation is occupied. The superhyperfine interactions of those first shell K nuclei nearest to CN− and of the 13C interaction of the CN− molecule are strongly temperature dependent between 10 and 60 K, following an exponential law with a thermal activation energy of 4.2 meV. It is assumed that a soft local mode involving those two nearest K nuclei and the CN− is causing the strong temperature dependence. The shf interactions of 14N nuclei have not been seen, probably because of the dynamical effects.

Laser diode pumping of a colour centre laser with emission in the 1.5 μm wavelength domain

We describe a colour centre laser pumped by a laser diode. The pumping is at 0.96 μm in the absorption band of the T1 0 (1) centre in KCl :Tl + . The laser emission is tunable around 1.5 μm with an output power of 17 mW. We have also obtained with the same device, 15 mW with (F 2 + ) H centres in NaCl :OH - .

E-V energy transfer in the FH(cn) center in KCl and CSCl

Abstract The electronic-to-vibrational energy transfer (e-v transfer) between the F center and CN−-molecule in CsCl(CN−) and KCl(CN−) is discussed in terms of nonradiative transitions between the electronic excited and ground states of the system. The role of dipole-dipole coupling between the F center and CN- molecule in the e-v transfer is also considered.

Time-resolved picosecond spectroscopy of the resonant secondary radiation of F centers in KCl

Abstract The linear polarization (P HL) of hot luminescence (HL) composing of the resonant secondary radiation of the F centers has been measured using a time-resolved picosecond spectroscopy over the whole Stokes wavenumber Ω range. The P HL holds constant value of about 40% until the onset of ordinary luminescence (OL), from where it decreases to vanishingly small with decrease of Ω This implies that the optically excited F center relaxes down along the 2p-like adiabatic potential energy surface (APES) trough, and transits to the 2s-like APES trough to form the relaxed excited state (RES). The lattice relaxation time and the dynamical transition time are ultra fast estimated to be less than 15 psec.

Peculiarities of interstitials in a simple cubic CsCl crystal

Abstract The processes of the thermal annealing of VK centres (160–190 K) and anion interstitials (I and II centres) have been studied in a CsCl crystal by means of luminescent and EPR methods. The thermal annealing of I centres in a CsCl crystal takes place in a narrow region of 16–20 K, in contrast to the multistage annealing of I centres in KCl. The tunnel recharge of a primary F, H pair leads to the formation of an α centre and a localized (not crowdion-type) split I interstitial.

Absorption Spectrum Study on Colloidal Centers in KCl:NCO:K

Abstract The absorption spectrum study on metallic colloidal centers in KCl:NCO:K crystals have been performed as a function of annealing temperature in the temperature range of 500–750K. Conversion process between F-centers and colloidal centers was observed and it is found that the NCO ions play an important role in the formation and stability of aggregative centers in alkali halides.

Direct determination of site symmetry forF A centres in alkali halides

The evaluation of site symmetry distortion for an impurity ion in cubic crystals is a problem which obtained attention from a number of investigators during the last decade. So far, all approaches were based on multi-parameter fittings of the experimental results by some theoretical curves. A new technique, described in this paper, makes it possible to obtain the site symmetry distortion parameter in a straightforward fashion directly from the experimental data. The method is based on precise measurements of the light polarization azimuth for a beam transmitted by a crystal with reorienting centres. results are reported forF A (Li) centres in KCl and RbCl.

Polarized Positrons as Spin Detectors for Paramagnetic Centres in Solids

A technique for studying paramagnetic defects in insulators or semiconductors acting as positron traps is presented. The spin polarization of the paramagnetic electrons of such centres can be determined by investigating the annihilation of spin-polarized positrons, e.g. by Doppler broadening (DB) or positron lifetime measurements. The viability of this technique is demonstrated by measuring the temperature dependence of the polarization of the paramagnetic electrons in F centres in KCl in high magnetic fields (± 4.5 T)

Formation of Two F2+-Like Centers Coexisting in KCl:Li+, Na+, OH- Crystal

To enlargen tuning range in a single host material, we have simultaneously created (F2+)Li+,H-(F2+)Na+,H color centers in KCl:Li+, Na+, OH- crystal, whose emission wavelength covers from 1.45 to 2.45 μm. The experiments prove that two coexisting F2+-like centers in KCl have high concentration and good stability.