Band In KCl Research Articles

Electrolytic reduction of Tl+ ions in KCl crystals

Electrolytic coloration has been made for KCl crystals doped with Tl+ ions and doped with Tl+ and Ca2+ ions. Intensity decrease of the Tl+ absorption bands is observed in the two colored crystals, while several new absorption and emission bands are observed. They are attributed to Tl− ions which were converted from Tl+ ions by electron injection during electrolytic coloration. Different absorption and emission bands are observed for KCl:Tl and KCl:Tl+Ca crystals. Sharp absorption band appears at 285nm in the colored KCl:Tl and at 400 and 503nm in the colored KCl:Tl+Ca. Infrared emission bands are observed at 1470, 1600 and 2185nm in colored KCl:Tl, while at 1630 and 1900nm in colored KCl:Tl+Ca. It is suggested that the absorption and emission bands in KCl:Tl+Ca are due to Tl− ions perturbed by Ca2+ or vacancy at the nearest neighbor.

Study of the phototransferred thermoluminescence in KCl:Eu2+ phosphors.

The phototransferred thermoluminescence (PITL) processes play an important role in the optical stimulated luminescence (OSL) and thermoluminescence (TL) dosimetric properties in KCl:Eu2+ crystals. In the present work, experimental evidence is presented about the participation of F and Fz centres in the associated recombination luminescence mechanisms involved with all three phenomena. An analysis of the TL glow curve of KCl:Eu2+ exposed to X ray ionising radiation shows three main thermoluminescence peaks around 370, 390 and 470 K. The 470 K highest intensity peak, considered the dosimetric peak due to its low fading and linear dose behaviour, is strongly correlated to the F and Fz centres. Through optical absorption spectra measurements of an optically bleached specimen, it was found that a minimum occurs at 560 nm, coinciding with the F centre band in KCl:Eu2+. Moreover, the diminishing of the integrated TL 470 K peak intensity is also seen to have a minimum at 560 nm (F centres); it also happens simultaneously with an increase of the integrated TL peak associated to the Fz band. This supports the close relation of the F and Fz centres in the PTTL process in KCl:Eu2+.

Optical properties of Ti2+ centres in alkali halides: Theoretical investigation

The properties due to substitutional Tl2+ ions in KX (X = Cl, Br, I) lattices have been explored by means of density functional and MS-Xα calculations carried out on clusters involving up to 81 atoms. Density functional calculations strongly indicate the existence of a relevant inwards relaxation (∼ 12%) for KCl:Tl2+. The present results support that the lowest optical absorption bands in KCl:Tl2+, peaked at 3.40 and 4.22 eV, can be associated with the t1u(π) → a*1g and t1u(σ) → a*1g charge transfer (CT) transitions of the (TICl6)4- unit. Also the 130 meV red shift undergone by the first CT transition on passing from KCl:Tl2+ to RbCl:Tl2+ is shown to reflect an increase of the equilibrium Tl2+−Cl− distance of only about 2 pm.

Cathodoluminescence and thermoluminescence studies on the three STE pi emission bands in KCl:KBr mixed crystals

Three different kinds of (Br2-+e) emission band in KCl:KBr mixed crystals have been investigated based on cathodo- and thermoluminescence measurements. the green emission band is considered to be the normal (Br2-+e) pi emission; the violet and blue bands observed below 120 K are suggested to originate from the (Br2-+e) emission perturbed by a Cl- ion at two different sites. The relationships between the intensities of these three bands and the temperature are also reported.

Time-Resolved Luminescence Study of Relaxed Excitons in KBr:I and KCl:Br

Luminescence from relaxed excitons associated with guest halogen dimers was investigated for KBr:I and KCl:Br through time-resolved measurements of emission and excitation spectra using synchrotron radiation pulses from UVSOR under single-bunch operation. A weak slow decay component was found in addition to a main fast one in the 4.89 eV band in KCl:Br, in contrast to the 4.35 eV band of KBr:I which consists of a weak fast decay component and a main slow one. The fast and the slow decay components are assigned to transition from the singlet and the triplet relaxed excitons in the on-center configuration. It is suggested that the adiabatic potential energy surface for the triplet state, as well as that for the singlet state, must have a local minimum at the on-center configuration but that the triplet exciton in KCl:Br relaxes preferentially into the off-center configuration.

Absorption of the K band in KCl:KBr Mixed Crystals

The shift with concentration variation of the optical absorption of the K band in KCl:KBr mixed crystals is studied, by use of point-ion and ion-size correction models. Gaussian wave functions were used for the ground and excited states. The K band in KCl:KBr was also experimentally studied. It is found that the point ion model produced better agreement with the experimental results

The overlapping of the FA1 and FA2 bands in KCl:Li crystals determined by reorientation of FA(II) centers

La deviation des bandes d'absorption par rapport a la forme gaussienne n'est probablement pas assez importante pour expliquer les differences d'angles hors axes determines par diverses methodes. La methode presentee ici peut etre utilisee pour d'autres chevauchements de bandes provenant du meme centre et ayant des proprietes de reorientation

Optical Absorption of U Center in Mixed Alkali Halide Crystals

The shift with concentration variation on the optical-absorption of U band in KCl:KBr and KCl:NaCl mixed crystals was studied by use of point-ion and size-ion correction models. We found that using the Bartram et al . method there is not appreciable correction to the point-ion energies. Assuming, for the ground state, that the electrons are concentrated at the lattice site, the calculated energies are in good agreement with the experimental results for KCl, KBr and NaCl in pure state.

Relaxation and Aggregation Studies of Impurity-Vacancy Dipoles by Decomposition of Complex ITC Bands in KCl:Cd2+

Dipole relaxation and aggregation were investigated on Cd2+ ion doped into KCl crystal using an ionic thermocurrent (ITC) method in the temperature range from 180 to 280 K. Two ITC bands with the activation energies (ER) of 0.57 and 0.50 eV were observed, in which the polarization temperatures were varied at 229, 232 and 235 K. The first intense band was attributed to the Cd2+–OH- dipole, in which OH- ion was probably formed by the decomposition of a water molecule intermixed during the crystal growth. After cleaning the second band, two new bands with ER=0.64 and 0.52 eV appeared and were regarded as the Cd2+-vacancy dipole and its aggregate, respectively. Dipole relaxation and aggregation processes were discussed including the aggregate models.

Far infrared emission spectroscopy of KCl

Far infrared emission spectrum of Δ v=1 bands of KCl has been observed in the range from 230 to 295 cm −1 with a resolution of 0.1 cm −1. Vibration-rotation transitions of the 1-0, 2-1, 3-2 and 4-3 bands of K 35Cl were analyzed by a single least-squares fit to a set of seven Dunham coefficients Y ij . Y 10 and Y 20 for K 35Cl were determined to be 279.963 (22) cm −1 and −1.2306(83) cm −1, respectively, with the standard errors in parentheses. Characteristic features of the far infrared emission spectroscopy of high-temperature molecules have been discussed.

Lattice Relaxation of Self-Trapped Excitons in Binary Mixed Crystals of KCl and KBr

Change of emission bands in KCl 1- x Br x mixed crystals has been investigated at liquid helium temperature (LHeT) to make clear the nature of lattice relaxation of the self-trapped exciton (STE). When x <0.03, two emission bands appear at 3.60 eV and 4.88 eV. These emission bands have been confirmed to originate from a triplet state and a singlet state of localized excitons created at Br - dimer centers, respectively. With increase of x , the 4.88 eV band shifts gradually toward the low energy side to become finally the σ emission band in KBr ( x =1.0), but the 3.60 eV band diminishes on the way. Instead, a new emission band, which appears in the lower energy side, is connected to the π emission band in KBr. The discontinuity between the 3.60 eV band and the π band has been attributed to an additional lattice distortion characteristic to the initial state of the π emission. An off-center displacement of the Br 2 - core of the STE is suggested as a candidate for the type of the additional distortion.

F and F A centers in mixed ionic crystals using gaussian functions

The shift with concentration variation of the optical-absorption F band in KCl:KBr and KCl:RbCl mixed crystals was studied using Gaussian wave functions and the Bartram et al. (Phys. Rev. 176, 1014, 1968) method. The optical absorption of KCl:KBr with Li impurities was also investigated as a function of concentration using Gaussian wave functions and the same pseudopotential method. The F bands are shifted but show the correct dependence as was founded by Asami and Ishiguro ( Phys. Rev. B34, 4199, 1986). The Gourary and Adrian type I functions produced better agreement with the experimental F A bands than the Gaussian functions.

Two types of lattice relaxation of self-trapped excitons in KClKBr mixed crystals3

Luminescent properties of excitons relaxed on Br − ions are studied in KCl crystal containing a small amount of bromine impurity (KCl:Br) and in KCl 1-xBr x mixed crystals. It is confirmed that both of 3.60 eV and 4.88 eV emission bands in KCl:Br originate from relaxed excitons localized at Br − ion pairs. No monomer band appears. Experimental results of KCl 1-xBr x mixed crystals suggest that the initial state of the π emission band in KBr should be accompanied with another type of lattice distortion in addition to the V K-type distortion. A candidate for this type of distortion will be off-center displacement of Br 2 − core of the self-trapped exciton.

Study of the F band in mixed ionic crystals using discrete lattice models.

The shift with concentration variation on the optical-absorption F band in KCl:RbCl and KCl:KBr mixed crystals is studied, by use of point-ion and size-ion correction models. Our calculations, using several types of variational wave functions for the F-center electron, are in agreement with the Asami and Ishiguro experimental observations of a nearly linear shift for KCl:KBr mixed crystals.

Influence of a vVacancy on Luminescence of Pb+ Centres in Alkali Halides

AbstractLuminescence of Pb+ centres in X‐irradiated Pb2+ doped KCl, KBr, and KI crystals is first observed at 1.29, 1.21, and 1.14 eV, respectively. It is excited in the bands of KCl:Pb2+ at 4.93, 4.45, 3.70, 2.03 eV, of KBr:Pb2+ at 5.20, 4.55, 4.20, 3.20, and at 5.10, 4.30, 3.91, 3.52, 3.02, eV for KI:Pb2+. The effect is studied of the presence of the cation vacancy on both, the excitation and emission spectra of the Pb+ centre. Temperature, kinetic, and polarization characteristics of Pb+ and Pb+vc−‐centres in KCl:Pb2+ are investigated.

Hydrostatic Pressure Effects on the Moments of the A-Absorption and AT-Emission Bands in KCl:Tl+

The pressure dependences of the first, second and third moments of the Jahn-Teller-induced A -absorption and A T -emission bands in KCl:Tl + have been observed at room temperature. The changes in the first and third moments with application of a hydrostatic pressure P are proportional to P in both the A -absorption and A T -emission bands. Moreover, the observed change in the third moment of the A -absorption band is equal to 3 and that of the A T -emission band is nearly equal to 3 . Here and are the change in the first moment and the second moment of the corresponding band, respectively.

X‐Ray Induced Relaxation Band Due to Higher Aggregates in KCl:Pb

AbstractAlthough aggregation of impurity centres in doped alkali halides upon X‐irradiation is proposed earlier, direct evidence of such centres having dipole moment is reported here for the first time by the method of ionic thermocurrents (ITC). The norma Idipolar relaxation band in KCl: Pb is located around 224 K, while sometimes a small band at about 160 K is found in the ITC spectrum of heavily doped systems. Exposure to X‐rays increases this band around 160 K at the cost of the 224 K dipolar band. Optical absorption in the A‐band of KCl:Pb, along with ITC measurements indicates that higher aggregates of I–V dipoles are easily formed in heavily doped systems. Formation and growth of the 160 K ITC band upon X‐ray irradiation are explained from the fact that the effect of X‐irradiation is to increase the rate of aggregation of I–V dipoles, through temporary electron trapping by lead ions.

Line Shape Functions and Moments of the Jahn-Teller-InducedA-Absorption andA-Emission Bands in KCl:Tl+-Type Phosphors

The line shape functions and the first, second and third moments of the A -absorption and A -emission bands in KCl:Tl + -type phosphors have been investigated in consideration of the spin-orbit interaction and the electron-lattice interaction with three vibrational modes α 1 g ( Q 1 ), e g ( Q 2 , Q 3 ) and τ 2 g ( Q 4 , Q 5 , Q 6 ) within the Condon and Franck-Condon approximations. The linear and quadratic interactions are considered with the anharmonic potential terms in the electron-lattice interaction. In the approximation that the adiabatic potential energy surfaces are described as the quadratic terms of the interaction modes \(\bm{Q}\) around their minimum points, the second and third moments of the A -emission band for the tetragonal distortion are expressed by using the coordinates of the minimum point and the coefficients of the quadratic terms of \(\bm{Q}\).

Luminescence of Localized Excitons in KCl:I at Low Temperatures

Temperature dependence of the intensities of 5.88 eV ( Near-Edge ) and 2.64 eV ( Blue-Green ) emission bands in KCl:I has been investigated in crystals containing various amount of iodine impurities under excitation into each of three absorption bands due to isolated I - ions. Localized excitons created on the I - monomer sites have two relaxed states, NE-state ( one-center type ) and BG-state ( two-center type ). It is confirmed that there is a thermal activation process from the NE-state to the BG-state. Fraction of the relaxation rate into the BG-states, γ BG , is found to increase with increasing iodine contents. This means that the relaxation mechanism of the localized exciton is influenced markedly if another iodine ion is present in the vicinity of the localized exciton.

Bleaching of F-type centres in alkali halides—II: Laser-induced F centre bleaching in KC1

Bleaching of the F band in KCl occurs on pulsing with 532 nm laser light. The bleaching is accompanied by the appearance of an absorption on the red side of the F band. Both bleaching and absorption are transient phenomena, and no permanent bleaching or absorption is observed. The same exponential lifetime of about 5μ S was obtained for both bleaching and absorption, and all the features were found to be independent of both temperature (between 200 and 300 K) and the intensity of the analysing light. This eliminutes the possibility of a trapped species, such as F -, being responsible. The phenomena are qualitatively different to the bleaching observed at low light intensity, where bleaching of the F band is accompanied by formation of F -, which is thermally stable below 250K. The most likely explanation of the results is that the high concentration of conduction band electrons following the laser pulse allows formation of free or quasi-free electron pairs, e = 2, which decay back into F centres on a time scale which is long compared with that for individual electrons.