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.     

Vaccum-ultraviolet low-tenperature spectroscopic study of free excitons in alkali iialide crystals under hydrostatic pressure

Abstract

Effect of pressure up to 14 kbar on free exciton reflection bands of alkali bromide crystals in VUV spectral region at 77 K was studied for the first time, In CsBr a strong resonance coupling of three exciton levels (an exciton analog of pressure-scanned Fermi resonance) was observed and computer-calculated. In NaBr, KBr and RbBr the increase of the spin-orbit splitting of the valence band was found, Besides, the exciton binding energy was estimated to decrease with pressure in NaI, KI and RbI.     

Excitons perturbed by self-trapped excitons in alkali iodine crystals

A strong transient optical absorption band on the exciton shoulder has been observed in pure alkali iodine crystals following pulsed electron irradiation. The ultraviolet transition is ascribed to creation of a second exciton in the vicinity of a self-trapped exciton. The perturbation caused by the proximity of the self-trapped exciton lowers the energy needed to form the second exciton by 0.14 eV for KI and 0.12 eV for RbI.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Two-photon absorption spectroscopy of valence excitons in alkaline-earth fluorides

The 2 P state energy of the valence exciton in SrF 2 has been obtained to be 11.1 v eV at 298 v K through two-photon absorption (TPA) spectroscopy. The luminescence of self-trapped excitons was monitored as the signal of TPA. The exciton binding energy is estimated to be 0.9 v eV. This value of binding energy is comparable not only to those obtained through TPA in other alkaline-earth fluorides of CaF 2 and BaF 2 , but also to those of alkali fluorides. The TPA coefficient of SrF 2 was about 5 ‐ 10 m 9 v cm/W for 5.55 v eV-photons. The value is in the same order of magnitude as those measured at 2 P state of the exciton in CaF 2 and BaF 2 crystals.     

Exciton states in alkali halides

Recent experiments based on modulation spectroscopy have shown that it is possible to detect exciton levels in alkali halides up to n = 4. Therefore we worked out numerical calculations in order to predict the whole exciton series in KI and RbI. In our calculations the deep exciton levels are treated by considering the actual hole-electron interaction, whereas the effective mass approximation is used for the shallow exciton levels. The direct and exchange terms of the hole-electron interaction have been evaluated by performing three and four center integrals, the Wannier wave functions appearing in such integrals being approximated by suitable gaussian expansions of atomic orbitals.It is shown that by allowing the exciton state to extend up to 42 shells of neighbors it is possible to predict the exciton levels up to n = 2, the n = 3, 4 excitons being accounted for by the effective mass approximation. Similar computations performed for excitons in solid rare gases were found in excellent agreement with the experimental data and confirmed the reliability of our method.     

Exciton resonances in insulators

The optical excitations of an insulator are studied theoretically above the direct absorption threshold. Taking into account two conduction bands and using aGreen s function approach, we find exciton resonances which are shown to be describable byFano's configuration-interaction formalism. A formula for the optical absorption is derived and compared with recent experimental optical spectra of the alkali halides.     

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.     

Zur Theorie des Exzitons in den Alkalihalogeniden

Following the many-body treatment of exciton byHaken andSchottky we represent the electronic polarization of the lattice induced by the excess electron and the hole in the Hamiltonian as well as in the wave function by introducing localized pair states and their coupling with the exciton particles. By application of a unitary transformation the pair states may be eliminated and one obtains the effective Schrödinger equation of an exciton consisting of particles dressed with polarization clouds. The coupling coefficients are determined by a minimum condition for the polarization energy. The effective interaction law between electron and hole and their effective masses are investigated explicitly. By transformation of the Schrödinger equation into a system of difference equations and solution of the corresponding secular equation one obtains an expression for the binding energy of the lowest exciton state in the alkali halides, which represents a connection of the “excitation” and “transfer” models. For the iodides of Na, K and Rb the binding energies were calculated using an approximative estimate for the various matrix elements involved. They are in good agreement with the experimentally observed energy differences between the first absorption peak and the small shoulder which indicates the position of the band edge.     

Wannier exciton in microcrystals

A simple variational calculation is presented of the ground state properties of the electron-hole system confined in three-dimensional quantum wells with spherical shape. As the radius of the wall is reduced to a few times the effective Bohr radius of the bulk exciton, the character of the ground state changes abruptly but continuously from that of a Wannier exciton confined as a quasiparticle to that of the electron and the hole confined as individual particles. The optical experiments on CuCl microcrystals in alkali halides and in silicate glasses are briefly discussed.     

Self-trapped exciton in alkali fluorides: ODMR study

The optical detection technique is used to observe magnetic resonance in the triplet state ³B_1u of the self-trapped exciton in LiF, NaF, KF. The strong anisotropy of the hyperfine interaction in the x?y plane is related to the more localized character of the electronic wavefunction. The magnetic dipolar contribution to the fine structure can be described by a Mollwo-Ivey type law in alkali fluorides and chlorides.     

Absorption spectra of excitons in alkali-halides in the vacuum-ultra-violet region

On the basis of the localized excitation model, the spectra of K⁺ - 3p electron excitation in potassium halides are studied theoretically by using methods of the ligand field theory and a good agreement with the experiment is obtained. Some discussions are given on the applicability of the Frenkel exciton model to the VUV spectra of other alkali halides.     

The luminescence of unactivated alkali iodides

Two shorter wave-length emission bands have been observed in activated alkali iodides at — and light excitation apart from activator emission. The shortest wave-length band, observed only at low temperatures, is excited in exciton absorption bands and it may be due to exciton emission.This emission has greater intensity in unactivated crystals grown from the solution. We have observed this emission on CsI, KI and RbI crystals.The intermediate emission band is excited in a narrow excitation band, situated on the sharp slope of the fundamental absorption in crystals and it can apparently be ascribed to structural defects.We wish to thank M. D. Galanin for interest in this work and L. M. amovski for providing us with crystals.     

Two- and three-photon spectroscopy of solids under high pressure

Abstract

The present article reviews measurements under hydrostatic pressure on the tetrahedrally-bonded semiconductors CuCI, CuBr, ZnO, ZnS, ZnSe, ZnTe, Cds, and AgGaS₂, by two-photon spectroscopy and on the alkali halides NaC1, KBr, KI, and RbI by two- and three-photon spectroscopy. It is shown that these nonlinear techniques yield a much higher precision than linear spectroscopy in the determination of the pressure dependence of the electronic band gap. Additionally, it is often possible to determine the pressure dependence of other parameters of the electronic band structure like exciton binding energy, spin-orbit coupling, crystal-field interaction, and Luttinger parameters. In the case of the alkali halides with their rather large band gaps there is a further advantage of three-photon spectroscopy, namely that pressure-cell windows need only be transparent for photons of one third of the band gap energy.     

Interstitial-exciton interaction in KBr:Na

The intensity of luminescence of the alkali halides submitted to an ionizing radiation decreases when color centers are created. The action of the interstitial as a quenching center is directly shown in KBr:Na. At LNT, the crystal has an emission band (430 nm), due to the exciton localized close to a sodium, which disappears according to a kinetics anticorrelated with that of the formation of the H_A (interstitials stabilized by Na) unde ionizing radiation.     

Luminescence of lithium hydride crystal induced by UV-light

Emission and excitation spectra of a lithium hydride crystal were measured at 8 K. The emission bands at 3.65 and 2.7 eV were excited by the light above the absorption edge at 4.8 eV. The high energy emission at 4.6 eV was excited by the light above 6.0 eV. The emissions correspond to those from the relaxed exciton in alkali halides.     

Effect of uniaxial strain on the structure of self-localized excitons in alkali halide crystals

We have used luminescence spectroscopy to establish the effect of low-temperature (100 K) elastic uniaxial strain on the configuration of a self-localized exciton (SLE) in alkali halide crystals (AHCs) at the instant of radiative relaxation. In face-centered (fc) alkali halide crystals, redistribution of the luminescence intensity occurs from the asymmetric SLE configuration to the symmetric SLE configuration (type III → II → I), while conversely in body-centered (bc) alkali halide crystals the redistribution occurs in favor of the asymmetric (polarized) SLE configuration (type I → II). External strain along the 〈100〉 direction leads to effective slip of the anions in the alkali halide crystals along the 〈110〉 direction, coinciding with the direction of compression of the SLE, which promotes creation of preferentially the symmetric SLE configuration, while strain along the 〈110〉 direction, acting perpendicularly to the length of the SLE, leads to elongation of the SLE, which promotes creation of the asymmetric SLE configuration with a higher degree of polarization. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 67–72, January–February, 2007.     

Electronic structure of the self-trapped exciton in rare gas solids

The Rydberg-like series of the self-trapped exciton R₂¹ in rare gas solids (Ar, Kr and Xe) are obtained by solving the effective mass equation which incorporates different corrections, including the central cell correction. The results are in good agreement with the recent transient optical absorption data in which the electron is excited into higher excited states. The origin of the luminescence bands is interpreted by analogy with a similar structure of the self-trapped excitons in alkali halides.     

Optical and structural properties of thallium halide microcrystals in a porous matrix

Thallium halide microcrystals were grown from water solutions in a porous matrix obtained from alkali borosilicate glass by etching, and their absorption and luminescence spectra were studied. Constraining the crystal size was found to affect the stability of some crystal modifications. The exciton radii were determined, and the dimensions of microcrystals in which size quantization effects become observable were estimated from the absorption spectra of bulk TlBr and TlCl crystals.