Absorption- and uniaxial stress experiments in the a band of Pb2+ centres in KBr,RbBr, KI and RbI

The absorption lineshape and the stress induced dichroism of the A band of Pb²⁺ centres in KBr, RbBr, KI and RbI have been measured as a function of temperature from 4 K to room temperature. The moments of the absorption bands and of the dichroic spectra have been determined, and the effective coupling constants of the linear electron-lattice interactions of the ¦A〉 state as well as the frequencies of the interaction modes have been evaluated. Above 200 K the temperature dependence of the moments gives strong evidence that nonlinear electron-lattice interactions are important, particularly in the case of KBr:Pb²⁺.     

Luminescence and relaxed excited state origin in CsI:Pb crystals

Emission and excitation spectra, luminescence polarization and decay kinetics have been studied for CsI:Pb crystals in the 0.36-300 K temperature range. The origin of the excited states responsible for the optical characteristics has been discussed. It has been concluded that the doublet ≈3.70 eV absorption (excitation) band is caused by the electronic transitions into the Pb²⁺ triplet state split due to the presence of a cation vacancy near a Pb²⁺ ion, while the higher-energy bands are of the charge-transfer origin. Like in CsI:Tl, four emission bands of CsI:Pb have been found to belong to the main luminescence centres. Two emission bands, peaking at 3.1 and 2.6 eV, are suggested to arise from the triplet relaxed excited state of a Pb²⁺ ion. Two visible emission bands, peaking at 2.58 and 2.23 eV, are interpreted as the luminescence of an exciton localized near the Pb²⁺ ion.     

Luminescence and energy levels of Sm2+ in alkali halides

Photoluminescence spectra and their lifetimes of Sm²⁺ diluted in NaCl, NaBr, NaI, KI, RbI and RbBr at temperatures 15–300°K were studied in order to extend previous results. It is confirmed that an excited level labelled E lies in the proximity of the f⁶ (⁵D₀) state and that the energy of E is strongly dependent upon the host. This level is shown to give rise to luminescence in all the crystals studied here, with the exception of RbBr, and is assigned to the 4f⁵ 5d configuration of Sm²⁺. The E level shifts its energy from above ⁵D₀ in KCl, KBr, RbCl and RbBr to below ⁵D₀ in the other crystals and as a consequence the luminescence spectra are essentially of two kinds with lifetimes either of the order of 10^-2 sec or 10^-6 sec, respectively. We have determined the energy of the E level in all crystals; an interpretation of the several emission lines is also suggested.     

Optical absorption of s2-configuration ions in alkali halide crystals—III: The A band in Sn2+-doped crystals

The line shape of the A band in KCl:Sn²⁺, KBr:Sn²⁺, KI:Sn²⁺, RbCl:Sn²⁺ and RbBr:Sn²⁺ has been measured as a function of temperature between about 15 K and room temperature. As the spectra for all these systems are similar detailed results are presented only for RbCl: Sn²⁺, RbBr:Sn²⁺ and KBr: Sn²⁺. The experimental data resemble those of a previous study on KBr:In⁺ and KG:In⁺ in that the A band consists of two broad, overlapping components designated A₁ (the low energy component) and A₂. The separation between A₁ and A₂ is more marked at low temperature for Sn²⁺ than for In⁺, and no inversion of the ratio of the peak heights of A₁ and A₂ occurs as the temperature is varied, as was so for In⁺. Detailed calculations of the line shape are presented for KBr:Sn²⁺. These calculations show that one cannot account for the line shape purely in terms of the dynamical Jahn-Teller effect and that there is in addition a static splitting of the A state caused by a lowering in the symmetry of the crystal field.     

Luminescence spectra of lead-doped NaCl,KCl and KBr

The luminescence spectra of lead-doped NaCl, KCl and KBr have been systematically investigated. Special attention has been paid to the effects of concentration and thermal history of the crystals. In the three systems, the emission spectra for A and C band excitation consists mainly of two well-defined emission bands whose energy separation is ～0.7 eV. It has been concluded that none of the bands can be attributed to a single type of lead center but are both typical of Pb²⁺ luminescence. In fact, their behavior can be correlated with that found for most monovalent ions and interpreted in a similar way. The excitation spectra for the two emissions have shown that the A-band is complex. One of the components appearing in very low doped and quenched samples is ascribed to dipoles, whereas additional side bands are attributed to complexes or small aggregates involving Pb²⁺ ions.     

Magnetic field effect on the luminescence of the paired Ag- ions in alkali halides (II)

The fine structure of the relaxed excited state of paired Ag^- ions in KBr and RbBr is studied by measuring the magnetic field effects on the decay time of the luminescence at about 2 K. The experimental results obtained are found to be well explained theoretically by assuming thermal equilibrium, with the exception of an anomalous change at about 43 kG(KBr) or at about 34 kG(RbBr). The anomalous change exhibits a level crossing and also shows that the Zeeman levels do not reach the thermal equilibrium at least in the neighborhood of the level crossing.     

Optical study in a novel quaternary ionic crystal doped with europium

In this Letter, the novel quaternary alkali halide crystal KCl:KBr:RbCl:RbBr doped with europium, each component in a 25% molar fraction and 0.5% of Eu⁺⁺, is reported. The value of its refractive index was measured, the crystallographic properties are characterized by X-ray diffractometry, the optical properties were studied by optical absorption in UV-VIS region, and by photoluminescence under N₂ laser excitation at 12 K and at RT. The diffractometer diagram of a powder sample shows a single phase and a well defined value of the lattice constant. From the optical absorption spectra of europium, the value of 10 Dq parameter is obtained, the result is in good agreement with X-rays analysis. It was also observed that decay times of the blue emission is independent of temperature and only one decay component is present in the quenched samples. From obtained data, it is suggested that the symmetry of the Eu⁺⁺ site in the quaternary crystal KCl_0.25KBr_0.25RbCl_0.25RbBr_0.25 is similar to a quenched sample of a single component crystal.     

Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

Multiplication of anion and cation electronic excitations in luminescent wide-gap ionic crystals

The spectra of intrinsic luminescence excitation by synchrotron radiation (6–32 eV) at 8 K have been analyzed for NaCl, KCl, RbCl, KBr, RbBr, CsBr, MgO, CaO and YalO₃ crystals. In all crystalsv (except MgO and CaO) the process of multiplication of electronic excitations (MEE) causes a sharp increase of the intensity of self-trapped exciton emission, but leads (in KBr and NaCl) to the decrease of intra-band luminescence efficiency. The analysis of the intensity ratio spectra for two components of exciton emission allows us to separate the process of secondary exciton creation by hot photolectrons (NaCl, KBr, YAlO₃). The threshold energies of excitonic and electron-hole mechanisms of MEE are compared for a number of alkali halides.     

Photophysics of RbCl co-doped with Eu2+ and Eu3+

Excitation and luminescence spectra of RbCl co-doped with divalent and trivalent europium ions are reported. Spectral dips appearing in the blue emission from Eu²⁺ are resulted from the radiative energy transfer from Eu²⁺ to Eu³⁺ and consequently induces the luminescence from Eu³⁺ that is responsible for the ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4) transitions. The induced luminescence has been characterized as a function of temperature and a decay time. In addition, the polarized emission from RbCl doped with only Eu²⁺ is also reported.     

Optical properties and stable,broadly tunable cw laser operation of new FA-type centers in Tl+-doped alkali halides

A new group of complex color centers with F_A-type properties, involving simple center production and high thermal and optical stabilities, has been found in six Tl⁺-doped alkali halides: NaCl, KCl, RbCl, KBr, RbBr and RbI. In its first tested examples, KCl and KBr, broadly tunable cw laser operation over the 1.4 to 1.7 μm range has been obtained, with output powers in the 100 mW range. In contrast to already existing F⁺₂ and F⁺₂-like centers, operating in the same wavelength range, the new F_A(Tl⁺) lasers are optically stable and do not show any bleaching effects under laser operation.     

Luminescence studies of thallium co-doped KBr:Eu2+ powder phosphors

In this study, photoluminescence (PL) and photostimulated luminescence (PSL) properties in KBr:Eu²⁺, Tl⁺ powder phosphors are reported. PL emission spectra of these Tl⁺ co-doped KBr:Eu²⁺ phosphors show four overlapping bands around 310, 325, 360 and 375 nm in addition to the characteristic of Eu²⁺ ions at 420 nm. These additional short wavelength bands were attributed to centres involving Tl⁺ ions. The decrease in PSL intensity of γ-irradiated KBr:Eu²⁺, Tl⁺ powder phosphors with Tl⁺ concentration and the absence of thallium emission bands in PSL were attributed to the efficient electron trapping by Tl⁺ ions during irradiation.     

The photoelectron spectra of gaseous alkali halides

The He I photoelectron spectra of the gaseous forms of CsF, NaCl, KCl, RbCl, CsCl, LiBr, NaBr, KBr, RbBr, CsBr, NaI, KI, RbI, and CsI have been obtaine using a high-temperature spectrometer equipped with an internally-located, laser-beam heated sample oven. The spectral bands have been assigned molecul origins by use of MO calculations, spin—orbital splitting considerations, band intensity relations, polymer formation tendencies, and mass spectromet     

Photoluminescence from deep centers in GaAs

Sharp line structure attributable to phonon assisted radiative emission has been observed in the 6 K photoluminescence spectra from deep centers in bulk samples of chromium doped GaAs. Two luminescence bands at 0.56 and 0.8 eV have been observed and both bands exhibit evidence of phonon assisted radiative recombination. An exploration of these luminescence bands in terms of excited state to ground state transitions of Cr³⁺ and Cr²⁺ ions is proposed.     

ESR studies of europium ions in alkali chlorides and bromides

ESR studies of europium ions in alkali chlorides and bromides were performed at room and liquid nitrogen temperatures. There were first reported spin Hamiltonian parameters for NaBr: Eu²⁺ and RbBr: Eu²⁺ crystals and also for coagulated europium ions in NaCl, KCl, KBr and RbBr crystals. The dependence of b₂⁰ and b₂² on the lattice constant, a, of host crystals was discussed; a single relation b₂⁰αa^?2.6 was obtained, while relations for b₂² were treated seperately for Na, K and Rb halide series, respectively. From the relative values of b₂² and b₂⁰, displacements of the nearest neighbor anions surrounding the Eu²⁺-vacancy pair were estimated to be 0.083a for NaCl and NaBr, 0.081a for KC1, 0.080a for KBr, 0.074a for RbCl and 0.078a for RbBr.     

Thermoluminescence in single crystals of RbBr:OH− and RbBr:Ca2+

Optical absorption, thermoluminescence glow and emission spectra of RbBr:Ca²⁺ and RbBr:OH⁻ have been studied and analysed. It is observed that both Ca²⁺ and OH⁻ ions enhance theF-centre concentration.F _Z1 band in RbBr:Ca²⁺ appears at 1.55 eV. TL glow peak corresponding toF _Z1 centre on analysis gives a trap depth of 0.84 eV. OH⁻ ions in the crystal seem to act as TL ‘killers’. Spectral distribution of emission under the glow peaks shows five bands around 1.5, 1.8, 2.1, 2.5 and 2.9 eV. Probable models of TL mechanism are suggested to explain the observed TL emission bands.     

On the colloidal absorption bands in rubidium chloride and rubidium bromide crystals coloured with sodium

Colloidal absorption bands in rubidium chloride and rubidium bromide crystals coloured with sodium have been investigated. After a certain heat treatment of the RbCl and RbBr crystals containing F-centers, colloidal bands were observed at 585 mμ for RbCl and at 600 mμ for RbBr. The measured positions of these bands were compared with those calculated by means of the theory ofDoyle. Measurements of the areas of the absorption curves were also carried out at different temperatures.     

Optical absorption of s2-configuration ions in alkali halide crystals—IV: Line shape of the C band in Sn2+-doped crystals

The line shape of the C band in the alkali halide phosphors KBr:Sn²⁺, RbBr:Sn²⁺, and RbCl:Sn²⁺ has been measured as a function of temperature between about 15 K and room temperature. In contrast to earlier measurements on In⁺-doped phosphors, the C band shows a well-marked triplet structure over the whole temperature range. This triplet structure cannot be accounted for solely in terms of the dynamical Jahn-Teller effect without assuming a very large temperature-dependence for the coupling constant to vibrational modes of trigonal symmetry. The temperature-dependence both of the second moment of the line shape and of the separation between the components of the C band, suggests that there is a contribution to the splitting of the C state from a lowering in the symmetry of the static crystal field and a model which includes such a static splitting in addition to the dynamical Jahn-Teller effect provides a good representation of the experimental line shapes.     

RAMAN STUDY ON (Pb) CENTERS IN SINGLE CRYSTALS OF ALKALI HALIDES DOPED WITH LEAD

Low frequency modes were observed in a series of alkali halides doped with lead after X-irradiation at 77K. The peak positions of the Raman signals are at 34cm^-1 for NaCl:Pb²⁺, 30cm^-1 for KCl:Pb²⁺,28cm^-1 for RbCl:Pb²⁺ and 22cm^-1 for KBr:Pb²⁺. They are assigned to the inelastic light scattering caused by the local vibration of X^-_i (Pb⁺) center, which consists of an interstitial halogen ion stabilized by a substitutional ion Pb⁺ . Polarized Raman measurements and Behavior Typer(BT) analysis show that the symmetry of the X^-_i (Pb⁺) center belongs to the point group C_3v A structure model of the X^-_i (Pb⁺) center is proposed based on the Raman data and the result of BT analysis.     

Spontaneous and stimulated red luminescence of Lu2O3: Eu nanocrystals

The spectra of spontaneous and stimulated luminescence of Lu₂O₃: Eu (7 at %) nanopowders at different optical pumping intensities have been investigated. The obtained results—changes in the shape of the red luminescence spectra and in the lifetime of the ⁵ D ₀ excited state of Eu³⁺ ions—indicate the onset of superluminescence with an increase in the excitation power. It has been found that an increase in the optical pumping intensity leads to a decrease in the luminescence decay time of the Lu₂O₃: Eu (7 at %) phosphor in the stimulated luminescence regime and to an increase in the quantum efficiency of red luminescence with a maximum at 611 nm.