On the origin of the visible emission in KBr:Tl and KCl:Tl crystals

The intensity of the visible emission of KBr:Tl at 440 nm is shown to increase linearly with the absorption in the A₁ band at 4.64 eV (267 nm). The latter band increased with the square of the Tl⁺ ions concentration, which supports its assignment to thallium dimers. Similarities in behavior between the 440 nm emission of KBr:Tl and the 475 nm emission of KCl:Tl suggest that the latter is also related to dimers. This is supported by the superlinear dependence of this emission on the Tl⁺ ion concentration. Deviations ofthe superlinearity from the square dependence expected for dimers is explained by the overlap between the weak dimer absorption and the very strong A-band.The 440 nm emission of KBr:Tl was found to be excited only in the A₁ component of the dimer absorption and not in the stronger A₂ component. This indicates that the emission process may be compound.     

Photoluminescence, optical absorption and XRD studies on X-ray irradiated terbium doped KBr crystals

This paper reports the optical absorption, photoluminescence (PL), XRD, SEM studies made on KBr:Tb³⁺ crystals. The integrated light intensity is enhanced by about two orders of magnitude as compared to the undoped samples in spite of some OH^- present in the samples. PL of these crystals exhibits characteristic Tb³⁺ emissions due to transitions from the ⁵D₃ to ⁵D₄ levels to various levels of the ⁷F septet on F-bleaching X-ray irradiated crystals Z₃ centers are observed. The bands observed in the emission spectra of Tb³⁺ doped KBr are found to be shifted to higher wavelength. The stokes-shift of KBr:Tb³⁺ is determined and it is found to have a large value. The XRD studies have been made to determine the crystalline structure of KBr and KBr:Tb³⁺ and also the Miller indices. The SEM studies made reveal the presence of microcrystalline structure.     

Normal and wavelength-modulated absorption of thallium bromide solutions

New results on the absorption spectra of Tl ions in aqueous and alcoholic solutions of TlBr are reported and compared with spectra of Tl ions in single KBr crystals. We study the Tl absorption bands under different conditions and we show that the A-band can be attributed to Tl-Br complexes. The energy parameters of these complexes in alcoholic solutions are given.     

Electronic spectroscopy of Se2- in KBr

Rapid quenching of KBr crystals doped with Se^2- gives isolated Se^2- ions which are strongly luminescent. Two emission spectra were observed, one with a maximum intensity at about 14100 cm^-1 and the other with a maximum intensity at about 17200 cm^-1. Only the red emission is observed by excitation into the lowest energy absorption band at 24600 cm^-1, while between 10 K and about 150 K both red and yellow emissions are observed by excitation into the second absorption band at 26770 cm^-1. Both are assigned to the isolated Se^2- center in different distorted charge transfer states. The yellow emission is observed below about 100 K, reaches maximum intensity at about 35 K and is absent below 10 K. There are complementary changes in the intensity of the red emission so that the sum of the two emission intensities is constant. The absorption spectra of the isolated Se^2- and the (Se^2--vacancy) pair have been identified as well as the spectrum of a cluster of two or more (Se^2--vacancy) pairs.     

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.     

Photoluminescence and EPR studies on gamma irradiated Ce doped potassium halide single crystals

Electron Paramagnetic Resonance(EPR), Photoluminescence(PL), Thermoluminescence (TL) and other optical studies of γ-irradiated KBr, KCl:Ce³⁺ single crystals. Cerium when doped into the KBr, KCl is found to enter the host lattice in its trivalent state and act as electron trap during γ-irradiation, thereby partially converting itself to Ce²⁺. The Photoluminescence(PL) spectra of both KCl and KBr crystals doped with Ce exhibit the strong blue emissions of Ce corresponding to ⁵d(²D)→²F_5/2 and ⁵d(²D)→²F_7/2 transitions. The defect centers formed in the Ce³⁺ doped KBr and KCl. Crystals are studied using the technique of EPR. A dominant TL glow peak at 374, 422 K and KCl:Ce³⁺ at 466, 475 K is observed in the crystal. EPR studies indicate the presence at two centers at room temperature. Spectral distribution under the thermoluminescence emission(TLE) and optically stimulated emission(OSL) support the idea that defect annihilation process to be due to thermal release of F electron in KBr, KCl:Ce³⁺ crystals. Both Ce³⁺ and Ce²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

Optical absorption of triplet molecular excitons in alkali cyanides

The optical absorption spectra of alkali cyanides in the UV region present a set of weak absorption bands which are identified as triplet a'³Σ⁺ molecular excitons. The nature of the molecular exciton transitions in the ionic-molecular crystals is discussed and the existence of an admixture between molecular exciton and charge transfer exciton states is suggested.     

Absorption and emission spectra of yttria-stabilized zirconia and magnesium oxide

We have investigated the luminescence and absorption spectra of doped and undoped ZrO₂-Y₂O₃ and MgO crystals at room- and low temperatures. The crystals used are partly doped with the transition metals Ni, Co, Cr and the rare earth Pr. The emission spectra were obtained under laser excitation at different wavelengths. The observed optical emission and absorption bands of the MgO crystals doped with Ni, Co and Cr correspond to transitions between spin-orbit split crystal field levels of the transition metals. Luminescence and absorption bands of undoped yttria-stabilized zirconia (YSZ) crystals are due to color centers, absorption bands of the doped YSZ correspond to the well known transitions of the Ni²⁺, Co²⁺ and Pr³⁺ ions, respectively. The emission spectra of the doped YSZ obtained under various laser excitations can be explained by an energy transfer process between the color center and the doping materials. The influence of annealing on the absorption and emission of Pr³⁺/Pr⁴⁺ is investigated.     

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.     

Thermomodulated A-band absorption of KBr: Tl

A marked oscillator-strength decrease of the A band in KBr: Tl has been pointed out by means of thermally-modulated optical measurements throughout the temperature range 88–380°K. The experimental results suggest the existence of a clear correlation between the states responsible of the absorption bands and are discussed in order to clarify the role of the thermal perturbation upon Tl⁺ energy levels.     

Elektronenspin-Resonanz und Photochemie des U2-Zentrums in Alkalihalogenid-Kristallen

The photochemical reactions involved in the production and in the bleaching of U₂ centers (interstitial hydrogen atoms) in KCl, KBr and NaCl crystals are investigated by measuring the optical absorption spectra of the crystals at low temperatures. The U₂ centers are prepared by UV irradiation of hydroxide- and hydride-doped crystals. The electron spin resonance of KCl, KBr and NaCl crystals with U₂ centers at low temperatures reveals a well-resolved hyperfine splitting due to interactions of the hydrogen atom with four neighbouring halogen ions. The U₂ center can be described as a tetrahedal [HX₄]^4?-molecule ion, in which one hole, bound mainly at the hydrogen atom and partially at the surrounding halogen ions, gives rise to the spin resonance.     

Emission and decay time studies on Pb2+ centers in KBr,RbBr, and RbCl

The emission spectra and the luminescence decay times of KBr, RbBr, and RbCl crystals doped with Pb²⁺ and excited in the A-absorption band have been studied in the temperature range 5–300 K. The emission-lineshape spectra have been analysed in terms of skew-Gaussian bands. New bands have been observed in RbCl and RbBr at very low temperatures. While the luminescence decay of KBr:Pb²⁺ and RbBr:Pb²⁺ show only a single component with a decay time τ ～ 20 ns, RbCl:Pb²⁺ shows a short and a long component. The reason for the missing long component in KBr:Pb²⁺ and RbBr:Pb²⁺ is tentatively attributed to an anomaly in the structure of the adiabatic potential energy surface (APES) of the excited states.     

Absorptions- und Fluoreszenzspektren des Systems KCl:Sm2+ bei tiefen Temperaturen

Sharp emission line spectra of Sm²⁺ in KCl single crystals are observed up to 40 °K. From these it is possible to verify the idea of a substitution of a regular cation by an impurity ion and of a simultaneous association of a cation vacancy in the nearest neighbourhood [1/2 (110)]. This means that the crystal field at the place of the bivalent Samarium has the site symmetryC _2v . A cubic symmetry does not explain the spectra. It is possible to identify the line groups with the transitions, which are permitted by reason of the selection rules, and to set up the energy level diagram of the Sm²⁺ ion in KCl. The separation of the single impurity-vacancy complexes was reached by quenching the crystals from 650 °C to room temperature. The assumption of excitation exchange between neighbouring Samarium ions explains the spectra of the non-quenched crystals. The number of emission lines is determined for two different orientations of two dipole complexes towards one another. An interpretation of the absorption spectra of Sm²⁺ in KCl is proposed.     

K+3P-core excitons in potassium halides studied at low temperatures with high resolution

Reflection spectra of KF, KCl, KBr and KI single crystals have been measured at photon energies of approximately 20 eV with a bandwidth of 10 meV using synchrotron radiation in order to study fine structure and temperature dependence of the excitonic transitions associated with the K⁺3p core level. The crystals were cleaved under ultrahigh vacuum and cooled down to 20 K. Information on energy positions, halfwidths and line shapes for the K⁺3p-core excitons and their temperature dependence has been obtained. A new exciton predicted by the ligand field model was observed. For KI we evaluate an electron-hole exchange energy of only 30 (± 7) meV.     

On the temperature dependence of the visible emission in thallium doped KCl and KBr crystals

The visible emission of KBr : Tl and KCl : Tl crystals was measured in the temperature range 10–300 K, on excitation in the low-energy tails of the corresponding A-bands. The peculiar temperature dependence of the visible emission is explained in terms of absorption changes due to thermal broadening of the A₁-side band itself and due to the growing overlap of A₁-band with the A-band on warming. A quantitative analysis is given which enables the computation of some parameters of the A₁-side band.     

Farbzentrenassoziate mit (100)-Symmetrie (A und B-Zentren)

By light absorption in additively colored KCl A-centers can be developped under certain conditions. Irradiation with (100) polarized F-light produces a reorientation of these centers, which is observed by a characteristic dichroic absorption. From these measurements the symmetry and the two optical transitions A₁ und A₂of the A-center are derived. Experiments with Na-doped crystals suggest as a model for the A-centeran F-center assoziated to a single Na ⁺ ion in the KCl lattice. From A-centers a new center can optically be developped, which has again (100) symmetry and two optical transitions. This ?B-center“ is interpreted as the assoziation oftwo F-Centers to a Na ⁺ion in KCl.     

Excited-state absorption spectroscopy from the 3d9 4s relaxed excited state of Cu+ in KBr and NaBr

Data on the low temperature optical absorption from the optically-pumped relaxed excited state 3d⁹ 4s of Cu⁺, substitutional in KBr, are here reported for the first time. These results are discussed and it is suggested that the absorption peaks at higher energies are due to transitions from the 3d⁹ 4s to states of the conduction band continuum.     

Radiation induced defects in KBr:Tl+ crystals

The defects produced in KBr:Tl⁺ crystals during x-irradiation at 77 K were studied using thermoluminescence (TL), thermally stimulated currents (TSC), and absorption and emission spectra. Three main glow peaks at 165, 193 and 258 K were observed both in the TL and in the TSC curves. A variety of irradiation induced absorption bands were observed in the UV, visible and infrared up to about 2 microns. The 165 K TL peak was found to emit only the 440 nm band assigned to thallium dimers, while the peaks at 193 and 258 K exhibited the UV bands at 310 and 365 nm as well as the 440 nm band.The defects produced during the irradiation were the V_k hole center, the Tl° and the Tl⁺₂ electron centers. Smaller concentrations of Tl²⁺ and (Tl⁺)⁺₂ centers were also produced.An analysis of the results including measurements on lightly and heavily doped crystals enabled to draw conclusions on the nature of the defects and on the recombination processes involved. A close correlation has been found between the temperatures at which changes in the various absorption bands take place and the temperatures of the TL peaks. The analysis enabled also a full classification of the absorption bands.     

Color centers in heavily irradiated CsI(Tl) crystals

The absorption and luminescence properties of CsI(Tl) crystals colored by irradiation are studied by the method of the time-resolved spectroscopy. The scheme of the electron transitions in CsI(Tl) crystal is suggested to explain the appearance of the color centers under exposure to the near-UV light. It is established that either of the two types activator color centers holds the charge carrier with opposite sign. The model of the hole Tl²⁺v_c⁻ activator color center is suggested. According to the model the positive charge of Tl²⁺ ion is compensated by the negative charge of a close cation vacancy v_c⁻. The color center emission reveals in the cathode-luminescence spectrum of the colored CsI(Tl) crystal. The high-dose irradiation of CsI(Tl) crystal results in the reduction of the decay time of the near-thallium self-trapped excitons (STE) emission. The decay kinetics of Tl²⁺v_c⁻ emission contains the time components typical for the decay kinetics of near-thallium STE emission. The reason of the observed effects is the energy transfer from the near-thallium STE excitons to the color centers via the inductive-resonant mechanism.     

The valence states of NO+

Spectroscopic constants for the eight lowest electronic states of the NO⁺ ion are tabulated. These constants result from reanalysis of previously reported optical and photoelectron spectra and interpolation from corresponding states of the isoelectronic molecules CO and N₂. Similar spin-orbit perturbations of the A¹Π states of NO⁺ and CO are compared. An interpretation is given of previously reported emission from a beam of long-lived states of NO⁺. The intensities of ionizing transitions from NO X²Π (v = 0) observed in photoelectron spectra are compared with calculated Franck-Condon factors.