Effects of pressure on optical F-bands in rubidium halides

High pressure optical absorption measurements have been made on additively colored RbCl and RbBr at pressures up to 10 kbar and on electrolytically colored Rbl up to 4 kbar. The pressure dependence of the F-band produced in RbCl by x-irradiation in the s.c. phase is also reported. Effects of the pressure induced structure transformation in these salts are described. F-band pressure shifts are related to a semi-empirical treatment which takes account of relative ion sizes. A new Ivey law for F-centers in host crystals having the s.c. structure is presented. This relation is established using all available F-band data including those for salts transformed to the s.c. structure by the application of high pressure.     

Elektronen-Kern-Doppelresonanz-Untersuchung vonU 2-Zentren in Kaliumchlorid

U ₂-centers in alkali halides are neutral hydrogen atoms in interstitial lattice sites, as has been shown by EPR measurements. The hyperfine interactions with the proton and with the four nearest halogen nuclei are resolved in the EPR spectrum. In order to resolve hyperfine interactions with further nuclei of the surrounding lattice ENDOR measurements have been performed onU ₂-centers in KCl at 77 °K. The analysis of the ENDOR spectra gave precise values for the hyperfine and quadrupole interaction constants of the nearest neighbour chlorine and potassium nuclei. The isotropic hyperfine constant of the chlorine neighbours is 24 times larger than that of the potassium neighbours although both nuclei are on equivalent first shell lattice positions. The hyperfine interactions of second shell potassium nuclei [(1/2, 1/2, 3/2)-position] show an unexpectedly large isotropic hyperfine constant. One expects a pure magnetic dipole-dipole interaction for the outer shell nuclei because of the concentrated hydrogen wave function. Two further chlorine shells could be approximately analysed. A theoretical estimate of the hyperfine and quadrupole interaction constants was made by orthogonalizing the 1s hydrogen wave function to the cores of the surrounding ions. If one takes into account the mutual overlap of neighbouring potassium and chlorine ions, one gets the right order of magnitude of the measured constants and a value of 10.4∶1 for the ratio of the isotropic hyperfine constants of the first shell chlorine and potassium nuclei. The relatively large isotropic constant of the second shell potassium nuclei can also be explained on this basis.     

Pressure dependence of F-center hyperfine interactions

We report a study of the effects of intense hydrostatic stress (10 Kbar) on the ENDOR spectra of F-centers in several alkali halides. In LiF, KCl, and KBr, the first shell contact hyperfine interaction constant, a_I, increases with compression, while the second shell term, a_II, decreases. In LiCl, however, da_II/dP and da_I/dP are both less than zero. We explain our experimental results, including the anomaly cf LiCl, with a model involving (1) ionic motion in a non-constant spin-density gradient and (2) ion-motion-induced perturbation of the F-center envelope function.     

Temperature dependence of F-centre hyperfine interactions

We report the temperature dependence of F-centre Isotropic hyperfine energies for the first shells of KCl, KBr, NaCl, LiF and Lid, and the second shell of KCl. In all cases at least 30 ENDOR data points were taken between 1.3° and 300°K, and the Fermi contact energies were found to adhere closely to a curve of the form a(T) = A + B coth (C/T).The effects of pure lattice expansion may be removed with the aid of high pressure ENDOR data. The remaining temperature dependence can be explained in terms of a configuration co-ordinate model, with the F-electron interacting with a single effective mode of lattice vibration.     

ESR- und ENDOR-Spektren von F-Zentren in RbCl und RbBr

ESR- and ENDOR-spectra of F-centers in RbCl and RbBr have been measured, giving the hyperfine constants for 5 resp. 6 shells of neighbouring ions. The resolved structure with a characteristic distance of 36 G in the ESR-spectrum of RbCl is due to the interaction with the nearest Rb⁸⁵ nuclei. All results fit well with the properties of F-centers in other alkali halides.     

Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.     

Pressure shifts of F-center hyperfine interaction parameters in alkali chlorides

We report here calculated values of the pressure shift in the shell I hyperfine interaction constants for F-centers in KCl, NaCl and LiCl. The calculation is based on a modified version of the pseudopotential approach of Bartram et al. with electronic polarization included by means of an r-dependent polarization potential. The results agree reasonably well with the available experimental data, and the strong dependence of the pressure shift on ion size ratio is shown to be mainly due to changes in the indirect-overlap contributions to the spin density. These terms result from the requirement of mutual orthogonality of core states on neighbouring ions, and their dependence on the lattice spacing is stronger than that of other contributions to the spin density. Our calculations further indicate that although the predicted value of the pressure shift is sensitive to errors in the pseudopotential and polarization parameters, the relative importance of the indirect overlap contributions is evident in spite of such errors.     

Electron nuclear double resonance of the cubic Dy3+ center in the KZnF3 single crystal

ENDOR measurements on the¹⁹F^? nuclei in the first four shells of KZnF₃ containing Dy³⁺ ions in the cubic site are reported. The values and signs of the hyperfine and transferred hyperfine interaction parameters are determined. The local deformation of the crystal lattice in the vicinity of the impurity ion is estimated. The theoretical analysis of the THFI parameters for the first coordination shell of the F^? ions has been carried out. For the Dy³⁺ ion the influence of spin polarization of the closed 5s and 5p shells is considered for the first time. Spin polarization is shown to play a significant role in the mechanisms of rare-earth ion-ligand coupling.     

Elektronenspinresonanz- und optische Untersuchungen zur FarbzentrenreaktionF⇄F′ in KCl-Kristallen

Simultaneous measurements of optical absorption and electron spin resonance of additively colored KCl crystals have been carried out during the optical bleaching ofF-centers at ?90°C. TheF-center resonance signal does not change in its significant properties e.g. line shape, line width,g-value, and saturation behavior, but it decreases proportional to the decrease of the opticalF-center absorption band. No ESR-signal due to theF′-center could be observed. The time dependence of theF?F′-center conversion has been investigated by ESR and optical measurements. It does not give any indication of a paramagnetism of theF′-center, too. The kinetics of the thermal decay of theF′-center at ?90°C have been studied. One can interpret the time dependence of this process, if one assumes that the mean lifetime of anF′-center depends on the distance of the nearest halogen ion vacancy. The expectation of life of theF′-centers therefore increases during the thermal decay, as the mean distance between remainingF′-centers and vacancies increases.     

Photothermal bleaching of Ca colloids in additively colored CaF2

Cation colloidal particles in additively colored CaF₂ crystals have been bleached with light in the colloidal band. At 360 K the colloid band decreases and a new band in the near i.r. region is created which is probably due to highly coagulated F-centers. At 430 K the colloidal band is destroyed at the same time that a band appears at 383 nm. This band has been tentatively associated to a kind of F-center. Photochromic conversions between colloids and i.r. bands at 360 K and between colloids and 383 nm band at 430 K have been obtained. A mechanism for the bleaching of colloids has been proposed.     

Strain-optical coefficients and deformation potential constants of the valence band at K = 0 for RbCl,RbBr and RbI

Using static and ultrasonic methods, absolute values of the strain-optical coefficients, p₁₁, p₁₂ and p₄₄ are determined for RbCl, RbBr and RbI in the wavelength region 240–410 nm. Following the band-structure approach, deformation potential constants of the valence band at K = 0 are evaluated for RbBr and RbI.     

Höhere Anregungszustände von Farbzentren

In additively colored KCl, KBr, KI, RbCl, RbBr and RbI the absorption on the short-wave side of the F-band was investigated. Besides the K band three new absorption peaks L₁, L₂ and L₃ were found, all in a fixed relation to the F band. The experimental data indicate that K, L₁, L₂ and L₃ are transitions of the F electron to higher excited states, the three highest of which are conductive states. By comparing the absorption band areas the relative transition probabilities to the different excited states were obtained.     

Zur Bildungskinetik der F-Folgezentren in KCl

Following the same procedure as in part I the formation of Z₁-centers under Flight irradiation was investigated in additively colored KCl crystals doped with Ca⁺⁺ and Sr⁺⁺ ions. The measurements show that again F′-centers and empty anion vacancies are the first reaction product and determine the rate of Z₁-center formation. The temperature dependence and rate of the Z₁-center formation is in agreement with the corresponding results on F _A -center formation, demonstrating that Z₁-centers are formed as the consequence of an ionic process and not a mere electronic transfer. It is concluded that again empty anion vacancies or F′ centers diffuse randomly in the lattice (activation energy 0.6 eV) until they are captured at an impurity site. Arguments are presented that these impurities are divalent cation/vacancy complexes, so that the model of the Z₁-center should be a combination of the F-center with this complex. The independence of the Z₁-band of the nature of the divalent cation indicates the assoziation of the F-center to the vacancy-side of the complex. A characteristic delay period in the Z₁-center formation can be accounted for by a non-statistical distribution of F-centers and divalent cations to each other. This is a necessary consequence of the high temperature dynamical equilibrium between these imperfections which is frozen in by the quenching of additively colored crystals. A direct proof for this interpretation is the absence of the delay period in crystals where the F-centers are formed by x-irradiation at room temperature.     

O−−-Lücken-Dipole in Rubidiumhalogenidkristallen

Dielectric loss measurements are reported for RbCl, RbBr, and RbJ crystals containing O^??ions. The frequency and temperature dependence can be approximated by a Debye curve. The dipole moment is interpreted as being due to O^?? ions associated with anion vacancies. Activation energies for reorientation of the dipole are 0.60 eV for RbCl, 0.56 eV for RbBr, and 0.50 eV for RbJ. The deviations of the experimental dielectric loss curves from a Debye curve are interpreted. The dielectric losses decrease and the center uv absorption changes during annealing due to association of the centers into aggregates. A detailed analysis of the kinetics for RbBr indicates the formation of O^??-pair centers as the first step. The activation energy for association isE ₂=0.79 eV.     

Anomalous Phases in Cavity-Free 240 GHz Pulsed ENDOR Spectra of 1.44 GeV Xe-Irradiated LiF

Paramagnetic centers generated by swift heavy ion irradiation of LiF crystals could be identified as electrons trapped at regular anion vacancy sites (F centers). Well-resolved electron-nuclear double resonance (ENDOR) spectra resulting from the hyperfine interaction with ⁷Li and ¹⁹F nuclei located in six different shells could be recorded. In order to preserve the millimeter-sized crystals, a cavity-free setup was used for the ENDOR experiments at an electronic Larmor frequency of 240 GHz. Apparently even under conditions of extremely high local energy loss in the ion track, the local density of persistent F centers is still sufficiently low to prevent distortions of the ionic crystal. The spread of hyperfine coupling constants was less than 5 %. Neither in electron paramagnetic resonance (EPR) nor in ENDOR spectra there was evidence for different types of paramagnetic centers. When performing ENDOR by applying the radiofrequency pulse directly after the 3-pulse Mims-type microwave sequence, an anomalous ENDOR effect was observed. The observed “positive” and “negative” ENDOR response can be attributed to efficient hole and anti-hole formation in the inhomogeneously broadened EPR spectrum and can be used to determine the sign of hyperfine coupling constants.     

On the colloidal absorption bands in rubidium chloride and rubidium bromide crystals coloured with sodium

Colloidal absorption bands in rubidium chloride and rubidium bromide crystals coloured with sodium have been investigated. After a certain heat treatment of the RbCl and RbBr crystals containing F-centers, colloidal bands were observed at 585 mμ for RbCl and at 600 mμ for RbBr. The measured positions of these bands were compared with those calculated by means of the theory ofDoyle. Measurements of the areas of the absorption curves were also carried out at different temperatures.     

Structure and conformation of nitroxyl spin-label compounds in frozen solutions by electron nuclear double resonance spectroscopy

The structure and conformation of carboxylic acid, formyl, and propenoic acid derivatives of the nitroxyl spin-label 2,2,5,5-tetramethyl-1-oxypyrroline have been determined by electron nuclear double resonance (ENDOR) spectroscopy. From ENDOR spectra of the spin-label compounds in frozen solutions, we have assigned the resonance absorption features for each class of protons. The ENDOR spectra were analyzed on the basis of their dependence onH ₀. The maximum and minimum ENDOR shifts for each proton were shown to correspond to axially symmetric principal hyperfine coupling (hfc) components, from which the dipolar contributions were estimated to calculate electron-proton separations. Conformational analysis on the basis of torsion angle search calculations constrained by the ENDOR determined electron-proton distances revealed that in all three spin-label compounds the side chains are in a planar conformation with respect to the oxypyrrolinyl ring. In the carboxylic acid and formyl derivatives the C=O group is in as-trans conformation with respect to the vinyl group of the spin-label, while in the spin-labeled propenoic acid the conformation is found to be all planartrans-s-cis.     

π-electron character observed through the anisotropy in the ENDOR spectra of undoped (CH)x

Electron nuclear double resonance (ENDOR) measurements were performed on stretched films of undoped cis-rich (CH)_x at 77°K. The anisotropy of the hyperfine coupling was directly observed for the first time. The anisotropy shows the π-electron character and is consistent with the assumption of the bond alternation kink in the π-electron system in undoped (CH)_x.     

ESR and endor studies of the VK center in SrF2

The results of electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) studies of a self-trapped hole (V_K center) in SrF₂ are reported. The g-factor and hyperfine interaction constants were determined for the flourine nuclei forming the center and for those in the nearest three different sites. The values of hyperfine interaction constants are intermediate to the known values for CaF₂ and BaF₂.     

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.