Centers In Alkali Halides Research Articles

Optical properties of cubic and tetragonal Cu- centers in alkali halides

Optical properties of cubic and tetragonal Cu^- centers in alkali halides

COLOUR CENTRES IN ALKALI HALIDES (2).Defects induced in NaMgF3 crystals by X- and VUV-irradiation

COLOUR CENTRES IN ALKALI HALIDES (2).Defects induced in NaMgF₃ crystals by X- and VUV-irradiation

Creation and Destruction of X Centres in Alkali Halides by X‐Radiation

AbstractThe production (η ) and destruction efficiencies of X−3 centres in NaCl, KCl, KBr, and KI by X‐radiation versus the irradiation temperature (in the range of 80 to 320 K) are investigated by optical absorption and thermoluminescence methods. Both, the increase (80 to 150 K) and decrease (200 to 250 K) of η in KBr and KI at the adsorbed dose of 1019 to 1021 eV cm−3 are shown to be in correlation with the F‐centre creation efficiency (ηF). In chlorides ηF does not decrease with ηX−3 and these crystals are characterized by a high defect creation efficiency at approximately room temperatures. It is supposed that the creation of large halogen aggregates is much more efficient in chlorides than in bromides and iodides.

THEORY OF COLOUR CENTRES.Cluster-Bethe Lattice calculation for the electronic structure of hydrogen centres in alkali-halides

THEORY OF COLOUR CENTRES.Cluster-Bethe Lattice calculation for the electronic structure of hydrogen centres in alkali-halides

Vibrational modes of the Vkcentres in alkali halides

The frequencies are calculated for the local vibrational modes of Vk centres in alkali halides by considering a cluster of relaxed ions around the defect embedded in a non-primitive unit cell of the lattice, and using the shell model. The results give excellent agreement with values obtained from Raman spectra. The vibrational spectrum obtained can be used to calculate some thermodynamic properties of the defects and some illustrative calculations are reported.

A study of the extended-ion model for F centers in alkali halides

The extended-ion model in the calculation of the electronic structure of simple defects in ionic crystals is studied with the example of the F center in LiF. An efficient scheme of computation is set up in which orthogonalization of the defect wave function to the core orbitals and the exchange terms between the defect and the core electrons are treated exactly for a large enough number of shells around the defect. This is made practical by a systematic use of lattice harmonics and Gaussian-type orbitals. The effect of using different sets of core orbitals in the extended-ion model calculation is studied next. It is found that the results are quite sensitive to the choice of core orbitals. It is pointed out that as anions undergo contraction and cations expansion in an ionic lattice, this effect should be taken into account in an extended-ion model work by employing appropriate ionic orbitals.

On the luminescence quenching of F centres in alkali halides

Non-radiative transitions from the excited state of F center to the ground state occur very efficiently around and above, over a rather large interval of energy, the crossing point of the two adiabatic potential energy curves. Taking this effect into account the criterion for luminescence quenching originally proposed by Dexter et al. is re-examined. The luminescence quantum efficiency is estimated for those alkali halides in which the luminescence is weak, and the results are in general accord with experiment. The border-line case of NaCl is also explained.

Photochemistry and reactions of OH − defects and F centers in Alkali Halides

Additively colored KCl:OH − crystals show under combined UV and visible irradiation an irreversible destruction of F centers and visible absorption. This process, which is effective over the whole temperature range 4–300 K, was studied systematically in terms of the reaction efficiency and products using UV-visible and IR (local mode) spectroscopy. The UV photo-dissociation of OH − defects and photo-ionization of F centers produce a combined net effect, in which at higher temperatures all F centers are converted into U centers, thus completely bleaching the colored crystal. By these photo-reactions, high contrast visible images can be produced which are stable under visible light at RT, and are thermally stable up to 650°C. Besides the optical information-storage aspect, these photo-reactions can be used for controlled production of U A centers if the crystal contains alkali-ion impurities like Na +.

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B 3 u to the A 1 g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B 3 u near or above the crossing point of the A 1 g and B 3 u potential curves to the A 1 g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.

Spin-lattice relaxation andg-shift in insulators

The spin-lattice relaxation of a Kramer's doublet arising from as-like ground state in any crystal symmetry is discussed. It is shown that in the cubic case the relaxation rate for one-phonon processes can be expressed in terms of the staticg value and its rate of change under dilatation. A general formalism and its application to the case ofF centres in alkali halides are given.

Deformation-induced nonradiative decay of color centers in alkali halides

It is shown that the coloration of the microcrystalline powders of KCl, KBr, and KI is completely lost within 24 h when the colored powders are stored in dark, at room temperature, whereas the single crystals of these materials subjected to the same treatment can retain the coloration for at least several years. The decay of color centers is through nonradiative modes. The presence of alkali and halogen impurities greatly influences the decay in the powders. It is suggested that the decay involves dislocation motion and such experiments can give important information about defect interactions.

Luminescence induced by tunnelling recombination between neutral silver atoms and Vk centres in alkali halides

AbstractThe decay curves of the tunnelling recombination luminescence between the neutral silver atom and the Vk centre in NaCl and KCl induced by irradiation with an electron pulse are obtained in the temperature range between 77 and 580 K. It is shown that the decay at 77 K follows a t−1 dependence for NaCl but a slower time dependence for KCl. The temperature dependence of the decay curve for NaCl is found to be such that the increase of temperature makes the decay slower at a delay time depending on temperature and enhances the decay rate at a longer delay time. On the other hand, the decay rate for KCl becomes faster with increasing temperature. The time dependence of the decay at 77 K and the temperature dependence of the decay are explained in terms of the anisotropic character of the tunnelling recombination.

A self‐consistent model for the electronic structure of the U‐center in alkali halides

AbstractA simple one‐orbital per site model Hamiltonian for the U center in alkali halides with rock‐salt structure is presented, with correlation effects introduced via an Anderson type Hamiltonian. The Cluster‐Bethe lattice method is used to determine the local density of states, yielding both localized and extended states. A one‐electron approximation is assumed and the problem is solved self consistently in the Hartree‐Fock scheme. The optical excitation energy is in fair agreement with experiment. The present approach is compared with other models previously used to describe this center and the results indicate that it adequately incorporates the relevant features of the system indicating the possibility of its application to other physical situations.

Relaxed excited states of Tl +—Like centers in alkali halides, polarization effect of Ga + center

The linear polarization of the A T emission is another clue to the symmetry of the 3T 1u excited states. Besides, it yields quantitative information about dynamical processes such as feeding rates and relaxation or tunneling between the different sublevels 3T 1u.

Theoretical survey ofFcenters in alkali halides with NaCl structure. II. Emission and the relaxed excited state

The emission energy and relaxed excited state is analyzed for the $F$ center in 16 alkali halides, in a static point-ion lattice with ion-size correction, and including lattice distortion. The emission energies follow a Mollwo-Ivey relation, disagreeing with experimental values, and the relaxed excited state is highly localized. The failure on these points indicates the need for further refinement of the model and method.

Modes of vibrations due to (U- center + additive cation impurities) in alkali-halides

Abstract Local and resonant modes due to hydride ions in various alkali halides containing additive cation impurities have been computed by the Green function technique. Local vibrations due to U -centers in alkali halides have O h symmetry. When one of the six nearest neighbour cations is replaced by an additive impurity, the site symmetry of the system is lowered from O h to C 4 v . The phonon Green functions matrix is analysed according to the irreducible representation of the point group symmetry pertaining to the substitutional impurity. We have considered the vibrations of the hydride ion and all its six nearest neighbours. Analytical expressions have been derived for various modes of vibrations. Using group theory the 21 × 21 matrix has been block diagonalized into various irreducible representations. The effect of mass changes and the changes in short-range force constants have been taken into account. The computed results of the localized modes have been compared with the available experimental results. Good agreement has been found. Theoretical results on resonant modes are also displayed, which will be of use in future experiments on these systems.

Binding Energies of Positron to F and F′ Colour Centres in Alkali Halides

AbstractThe binding energy of a positron to a F centre is calculated for twenty alkali halides, and that to a F′ centre for nine alkali halides.

Cluster-Bethe-lattice treatment for theU2center in alkali halides

A cluster-Bethe-lattice calculation for the electronic density of states of a ${U}_{2}$ center in the rocksalt structure is presented for a simple one-orbital Hamiltonian. The parameters involved in the Hamiltonian are given values based on qualitative arguments for Na, K, Rb chlorides and bromides and the proton contact term is calculated for these sets of parameters, in fair agreement with experimental data.

Vibronic Theory for Bound Polaron with Application to Excited States of F-center in Alkali Halides

A systematic approach to the vibronic problem of bound polaron is proposed taking advantage of spherical symmetry of the system. The interaction modes of LO phonon classified according to the angular momentum quantum number are introduced and the Hamiltonian is rewritten in the form which is manifestly invariant under rotation. The theory is applied to the analysis of the transient absorption spectra due to the transition from the relaxed excited state to higher excited states of the F -center in alkali halides. A good fit between the theoretical prediction and the experiment is obtained and the final states of the transition are assigned to be 3 p state and its phonon side bands perturbed through vibronic interactions.

Vacancy‐induced splitting of excited states and the structure of Sn2+V centres in alkali halides

AbstractThe polarization characteristics are calculated for the A and C emissions of Sn2+V centres in alkali halides, taking into account the rearrangement in the electronic level system of the excited sp states in the course of relaxation. The light absorption processes are considered in the approximation of a weak crystal field. The emission processes are described by a model in which the interaction of the optical electrons of impurity ions with Eg lattice vibrations (JTE) is assumed to be the principal, and the spin–orbit interaction and the interaction with T2g vibrations are taken into account as additive corrections. It is also assumed that in Sn2+V centres the influence of the cation vacancy V is small as compared with JTE. Comparing the results of the calculation with the experimental data, the order of three tetragonal minima on the adiabatic potential energy surface (APES) of the 3P state of Sn2+V centres is established which is consistent with the order of the corresponding minima of 1P state. It is shown that in both excited states Z minima are the lowest ones. The vacancy‐induced splitting of the relaxed excited A state is found to be about 10−2 eV. The limit values are determined for the polarization degrees of A and C emissions at various temperatures, the parameters are found and the structure is discussed for the Sn2+V centres in alkali halides.