Picosecond ground state recovery of the F-center in RbCl,KCl and KBr

Abstract

Optical pump-probe measurements on the F-center in RbCl, KCl and KBr show a ps ground state recovery, when a substantial number of F-aggregates is present. The observed de-excitation process becomes faster for a larger concentration of aggregate centers and is related to energy transfer from the excited state of the F-center to aggregate centers. This energy transfer occurs towards F₂- and N-centers in KCl. Our results are in accordance with measurements of the quantum efficiency of the F-center luminescence and with transient absorption measurements.     

Measurement of the decay and time-resolved spectra of the emission from σ-excitons in KBr in the nanosecond range,produced by heavy-ion irradiation

While energetic heavy ions are currently in increasing use in solid state physics, radiation chemistry and biology, there is still little experimental information regarding the primary physicochemical processes. These high LET (linear energy transfer) ion particles injected into condensed matter produce short life-intermediates such as excited states, ions, and radicals, at high density along the track. The density effect of these intermediates may cause damage-formation and chemical reaction to yield different results compared with low LET radiation (γ, X-ray) or photo-irradiation. Electronically excited states, among the above intermediates, are thought to be especially important precursors. Measurements of the decay and time-resolved spectra of the emission are expected to give useful information regarding the radiation action of heavy ions. The measurements were done using a KBr single crystal at 4.2 K. We reported previously [1] that heavy ion irradiated KBr results in an extraordinarily large ratio of σ-emission to π-emission 2 intensity (～ 5 times larger than in the X- or electron-irradiation). This large ratio is, as was ascertained previously, not due to apparent processes such as temperature increase of a track but to intrinsic processes which will be discussed here.     

Radiative energy transfer from Pb2+ to Eu2+ ions in NaCl,Kcl, and KBr single crystals

In the present work, the emission and excitation spectra of NaCl, KCl, and KBr doubly doped with europium and lead ions were investigated. In all cases, excitation with light lying in the A-band of the Pb²⁺ ions produces in addition to the ³P₁ → ³S_o Pb²⁺ emission, the 4f⁶ 5d (^t2g) → 4f⁷ europium emission. This fact indicates that energy transfer occurs from Pb²⁺ to Eu²⁺ ions. From the data obtained, it was determined that the energy transfer process is of a radiative nature and that it is more efficient in KCl than in either of NaCl or KBr. A possible explanation for this fact is given.     

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.     

The role of F A1:Ag+ defects in laser light generation and coadsorption of CO and halogen atoms at the KCl and KBr surface sites. First principles calculations

F _A1:Ag⁺ color center at the low coordination (100) and (110) surface sites of KCl and KBr thin films play an important role in providing tunable laser oscillation and adsorbatesubstrate interactions. Double-well potentials at this site are investigated using ab initio molecular electronic structure calculations. The calculated Stokes shifted (optical transition bands), opticaloptical conversion efficiencies, the probability of orientational destruction, exciton (energy) transfer and Glasner-Tompkins empirical rule suggest that laser light generation is sensitive to (i) the lattice anion, (ii) the coordination number of surface ions, and (iii) the choice of the basis set centered on the anion vacancy. The adsorbate-substrate interactions were found to be dependent on the electronegativity of the adatom and on the lattice anion. Optimised geometries and the coadsorption of CO and (F, Cl, Br, I) on KCl and KBr (100) crystals are presented. Calculated chemisorption energies for CO on the (halogen atom/defect free sites of KCl and KBr (100) crystals) showed that the coadsorption of halogen atom tends to block other adsorbate-substrate interactions at the nearest neighbour sites. Thus if halogen atom coverage increases, the CO prefers to be adsorbed on the K⁺ site of the KCl and KBr (100) surfaces and on KBr relative to KCl.      

Specific features of the temperature quenching of luminescence of self-trapped excitons in alkali halide crystals under low-temperature deformation

The activation energy of temperature quenching of luminescence of self-trapped excitons in alkali halide crystals subjected to low-temperature uniaxial deformation is evaluated experimentally. It is found that an increase in the activation energy is observed in the following series of crystals: KBr → NaCl → KI → NaBr → CsBr → RbI. The effect of enhancement of intrinsic luminescence of alkali halide crystals due to the lowering of the symmetry of the crystal lattice under low-temperature uniaxial deformation is interpreted by analyzing the observed increase in the activation energy that characterizes the height of the potential barrier separating channels of radiative and nonradiative decay (with the formation of radiation defects) of self-trapped excitons.     

Luminescence of interstitial atomic hydrogen in KBr and RbBr doped with I-

In KBr and RbBr doped with OH^- and I^- interstitial atomic hydrogen was produced in a configuration in which it is surrounded tetrahedrally by 3 Br^- and 1 I^- ions. Excitation in the absorption bands leads to a blue-green and an infrared fluorescence band (KBr: 2.53 and 1.35 eV). The absorption spectrum consists of 3 I-type and 1 broad Br-type charge transfer bands as could be shown from the excitation spectrum. The emission occurs only from relaxed I-type charge transfer states with a transition moment along the connection line H^o-I^-. The results are discussed with a simple charge transfer model.In mit OH^- and J^- dotiertem KBr und RbBr wurde atomarer Wasserstoff auf Zwischengitterplatz in einer Konfiguration hergestellt, in welcher er tetraedrisch von 3 Br^-- und 1 J^--Ion umgeben ist. Anregung in den Absorptionsbanden führt zu einer blaugrünen und einer infraroten Fluoreszenzbande (KBr: 2,53 und 1,35 eV). Das Absorptionsspektrum besteht aus 3 jodartigen und einer breiten, bromartigen Charge-Transfer-Bande, wie mit Hilfe des Anregungsspektrums gezeigt werden konnte. Die Emission erfolgt nur aus den relaxierten, angeregten, jodartigen Charge-Transfer-Zuständen mit dem Übergangsmoment parallel zur Verbindungslinie H^o-I^-. Die Ergebnisse werden anhand eines einfachen Charge-Transfer-Modells diskutiert.     

Measurement of stored energy of KBr after 4·6°K reactor irradiation

Four KBr single crystals were irradiated at 4·6°K in the core of the Munich Nuclear Reactor for periods of 100 sec, 10 min or 1 hr, respectively. After irradiation the stored energy of the samples was measured in a differential-heat-flow calorimeter at heating rates of 0·29°K/min and 1·1°K/min. At 0·29°K/min peaks of stored energy release were resolved near 11 and 20°K and at 25, 38 and 45°K. An attempt was made to evaluate the corresponding activation energies using two different methods (see Table 1 in the text). The 25°K annealing stage is due to a first order recombination reaction. The anneling states near 20, at 38 and at 45°K do not correspond to first order reactions. Our results are compared with annealing studies on low temperature X-irradiated KBr crystals. A similar experiment was performed on KCl single crystals and is reported, together with a more detailed comparison and discussion of both experiments, in a later paper[14].     

O−−-Lücken-Dipole in Alkalihalogenidkristallen

Dielectric loss measurements are reported for KBr, KCl, and NaCl crystals containing O^?? ions. The frequency and temperature dependences can be well represented by a Debye curve. The dipole moment found immediately after quenching is interpreted as being due to O^?? ions associated with anion vacancies. Activation energies for reorientation of the ion are E₀=0.59 eV for KCl, 0.54 eV for KBr and 0.61 eV for NaCl. Fluorescence bands were found in freshly quenched KCl crystals at 470 nm and 1.05 μm and are attributed to isolated dipoles. Dielectric losses decreased and the optical absorption changed during annealing due to the collection of centers into aggregates. A detailed analysis of the kinetics for KCl indicates the formation of double dipoles as the first step. The energy of association is E_A=0.85 eV. Earlier measurements of absorption, fluorescence and photochemical coloring are reinterpreted on the basis of the proposed model in which an O^?? center is thought of as a O^?? ion associated with an anion vacancy.     

Electronic structure of the self-trapped exciton in the potassium halides

We present pseudopotential calculations of the energy levels and wave-functions of the electron associated with the self-trapped exciton in KCl, KBr and KI. Accurate values are obtained for the separation of the singlet and triplet luminescent bands, for the electronic transitions, and for the spin resonance linewidths. Lattice distortion is shown to be particularly important in KCl. Additional low-energy electronic transitions, at 0.7 eV in KCl and 0.94 eV in KBr, are predicted, but have not been seen experimentally.     

Remarkable properties of alkali halides and new techniques for determining Debye--Waller factors

We have used 70 Ci ¹⁸³Ta Mössbauer sources to measure the Debye--Waller factors of NaCl, KCl, and KBr single crystals for several of the (h00) and (nnn) Bragg reflections. We have found that the procedure used to analyze data by earlier workers leads to incorrect parameters in the Debye--Waller factor exponent, and we use a procedure that eliminates the need to introduce an empirical parameter to account for thermal expansion effects. We observe that both the cations and anions in NaCl and KBr have identical Debye--Waller factors in the [nnn] direction. We observe Q⁴ terms in NaCl and KCl, with some evidence for a Q⁴ term in KBr. The size of this contribution varies with the direction of momentum transfer and will be reported. We observe that the Debye temperature and the coefficient of the anharmonic Q² term also vary with direction of momentum transfer. We believe this to be the first definitive evidence for a non-spherical thermal cloud in a cubic crystal; the ions have a larger amplitude of oscillation in the [h00] direction than in the [nnn] direction. This effect is temperature dependent and greater at lower temperatures.     

Some luminescent properties of additively colored KBr · Ag crystals

The luminescent characteristics of additively colored KBr · Ag crystals are investigated. In particular, the decay of the phosphorescence, the flash of emission due to IR light in relation to the silver concentration, and the temperature quenching of the steady luminescence are discussed. It is shown that the investigated crystals have a complex system of shallow and deep traps. The thermal activation energy for nonradiative transitions in the B centers is found. It is concluded that the luminescence is produced by an electron-recombination mechanism.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 81–85, November, 1973.     

The Hd-center interaction with a Rb+ ion during irradiation and thermal annealing in kBr

The role of Rb⁺ ions on defect formation in KBr has been studied. The impurity suppresses colorability due to X-raying at 6 K, but does not result in the formation of any centers characteristic to Rb⁺ ions at this temperature. A new optical absorption band peaked at 3.19 eV is produced only by thermal annealing of irradiated KBr:Rb. This band is annealed in parallel to the annealing of the F band in a stage at 55 K, obeying second-order rate equation with an activation energy of 0.102 eV. This band is ascribed to the H_A(Rb⁺)-center. Calculation is made on the elastic interaction energy between the H-center and a Rb⁺ ion, to show that the interaction along 〈110〉 is repulsive, whereas that along 〈001〉 is attractive. Based on this result of calculation, the structure and the formation mechanism of the H_A(Rb⁺) center, and origin of suppression of colorability at 6 K are discussed. The difference in the interaction of the interstitial atom with Rb⁺ during its dynamical motion and thermal motion is emphasized.     

Optical absorption of the F center as function of the temperature

Abstract

The temperature shift of the F absorption band for KBr and KCl single crystals was experimentally and theoretically determined. The theoretical calculations are based on the pseudopotential method using different wave functions and assumed a linear thermal expansion coefficient for the crystalline lattice. The change of F-center absorption energy due to collisions with the thermally vibrating lattice is calculated. This contribution is small. The measurements agree fairly well with the results obtained using the present model.     

Lyoluminescence mechanism of additively and gamma coloured alkali halides in fluorescent and chemiluminescent solutions

Lyoluminescence of gamma irradiated and additively coloured NaCl, NaBr, KCl and KBr crystals when dissolved in fluorescent and chemiluminescent aqueous solutions is investigated. Spectral analysis of the emitted light has shown that lyoluminescence spectra are similar to the fluorescence spectra of these solutions. Luminescence takes place when a fluorescent acceptor is directly excited by a liberated F-centre (hydrated electron e^-_aq) or indirectly by an energy transfer from a released excited halide ion ∥X^-∥¹ (solvated hole after recombination with hydrated electron). Oxygen, in general, has a marked quenching effect on luminescence. In chemiluminescent systems, its presence is essential to provoke luminescence in a sequence of events leading to a fluorescent end product.     

Charge transfer excited states of hydroxyl ion impurities in alkali halide crystals

A theoretical investigation of the u.v. absorption anisotropy factor g and the transition energies of OH^? molecule in alkali halide crystals is presented in terms of a charge transfer model and the free OH^? ground wave-function. Comparison of calculated and observed transition energies for OH^? in KBr and NaCl shows reasonable agreement between theory and experiment. For KCl:OH^?, a reasonable result for the transition energy is also obtained through a confinement of the free K4s function used for the one-electron transfer function. The model fails, however, to explain the value of about 0·3 for the anisotropy factor which is observed in a number of host crystals.     

Optical and structural characterization of KBr crystals doped cadmium bromide (CdBr2)

In this paper we demonstrate the presence of CdBr₂ and cadmium aggregates in KBr matrix during Czochralski growth of KBr crystals. The chemical decomposition of CdBr₂ due to high temperature of crystallisation and reformation of cadmium bromide seems to be responsible for this effect.     

溴化钾质量对红外光谱测试结果的影响

溴化钾(KBr)在中红外区段没有吸收峰,所以常作为红外光谱测试样品的载体。如果溴化钾试剂中混有杂质,则杂质产生的吸收峰会对样品测试的红外光谱形成干扰和影响。对国内外多家厂商的溴化钾试剂分别进行了红外光谱测试,就溴化钾试剂质量对红外光谱测试的影响进行了分析和论述。分析表明,红外光谱实验所选用的溴化钾纯度至少应当在99%以上。     

FTIR原位研究PCL薄膜的结晶过程

利用带热台的傅里叶变换红外光谱仪 (FTIR) ,原位跟踪了聚ε 己内酯 (PCL)在硅基片及KBr单晶基片上的结晶和熔融过程。发现在结晶过程中 ,由于结晶后分子间的自由体积缩小 ,分子间相互作用增强而导致了CO峰位的漂移及C—O—C ,C—H结晶峰的出现。并通过结晶峰与非晶峰对比计算出了相对结晶度。对不同浓度的PCL/THF溶液成膜的FTIR测试表明 ,薄膜厚度越小 ,结晶度越低。这可能是由于基板及几何维数的限制导致。另外发现 ,相同条件下 ,PCL在KBr基板上成膜后的结晶度要大于在硅片上成膜的结晶度。     

Thermoluminescence and optical studies on X-irradiated terbium-doped potassium bromide crystals

This paper reports the thermoluminescence (TL) and other optical studies made on terbium-doped KBr crystals X-irradiated at room temperature. Photoluminescence studies confirm the presence of terbium ions in the KBr matrix in their trivalent form. Formation of Z₁ centres on F-bleaching of X-irradiated crystals is observed. The characteristic emission due to Tb³⁺ ions in the spectral distribution under optically stimulated emission and thermoluminescence emission confirm the participation of the Tb ions in the recombination process. TL process has been identified to be due to the thermal mobilization of electrons produced during the colouration process.