Band In KBr Research Articles

Magneto-optical measurements on the C band in KBr: Tl +

The 1 A 1 g → 1 T 1 u transition of thallium center in KBr has been investigated using the technique of circular dichroism. The agreement between the experimental and theoretical spectra is excellent and seems to support the mechanism of the Jahn-Teller effect.

V centres produced in KBr and KCl at 290 K by electron irradiation

Optical absorption techniques have been used to study the effects of plastic deformation at 80 and 290 K on V centres in KBr and KCl produced by 2 MeV electron irradiation at 290 K. It is suggested that new V centres present in deformed crystals are associated with halogen interstitials trapped at dislocations (Vx band) and at impurities dispersed during dislocation motion (Vs band). The V3 band in KBr and the 6.4 eV band in KCl present in undeformed material decrease during deformation of irradiated crystals. The Vx band appears in two stages on warming after deformation at 80 K. It is proposed that moving dislocations ionize or dissect interstitial defects in their path. The reaction products become mobile at higher temperatures and can diffuse to dislocations to produce Vx absorption.

Pressure Shift of theUBand in KBr

The pressure derivative of the $U$-band peak position in KBr was determined to be +27\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ eV ${\mathrm{bar}}^{\ensuremath{-}1}$ and +30\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ eV ${\mathrm{bar}}^{\ensuremath{-}1}$ for ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ impurities, respectively. These are equal, within the uncertainty of \ifmmode\pm\else\textpm\fi{}3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ eV ${\mathrm{bar}}^{\ensuremath{-}1}$ for each coefficient. The shift of the peak of the $U$ band with temperature is shown to arise almost entirely from thermal expansion.

Dichroism ofVBands in Potassium and Rubidium Halides

Ultraviolet absorption bands which are homologous to the ${V}_{2}$, ${V}_{3}$, and ${V}_{4}$ bands in KBr have been studied in the potassium and rubidium chlorides, bromides, and iodides. They are generated by x irradiation above liquid-nitrogen temperature, and they are apparently intrinsic in the sense that no tendency to saturate has been observed as a function of radiation dose. The predominant $V$ band which accompanies the $F$ band depends on the conditions of irradiation. The effect of illumination in the bands with polarized uv light shows that the ${V}_{4}$ centers can be optically reoriented by $〈100〉$-polarized light absorbed in the band at 100 K. The dichroism is washed out by warming the crystal from 100 to 200 K. The ${V}_{2}$ band can be preferentially bleached by $〈100〉$ light absorbed in the band at 100 K. Again the dichroism is washed out upon warming to 200 K. The ${V}_{3}$ band is not affected at all by light absorbed at 100 K. Above room temperature, isotropic bleaching occurs. In addition, in KCl an irreversible thermal transformation of some of the ${V}_{4}$ centers into ${V}_{2}$ and also of some ${V}_{2}$ into ${V}_{3}$ occurs when the crystal is warmed above the temperature of irradiation. The peak wavelengths of these $V$ bands lie between 200 and 360 nm, and are strongly dependent on the halide, but nearly independent of the alkali species. The close grouping of their peak wavelength positions, and the dependence on halogen species, indicate that they may all be ${{X}_{3}}^{\ensuremath{-}}$ centers, in accordance with a suggestion of Hersh. The ${V}_{4}$ and ${V}_{2}$ centers show a $〈100〉$ symmetry axis. The irreversible thermal conversions observed in KCl suggest that the three centers differ by ionic constituents and not merely by their state of ionization.

Modulated Absorption in Thallium-Doped Kbr

By means of an electric field the triplet structure of the $A$ band in KBr:T1 is shown. A quadratic Stark effect is also observed.

Infrared Absorption of the Hydroxyl Ion in Alkali Halide Crystals

The O---H stretching band and its immediate sidebands have been studied for O${\mathrm{H}}^{\ensuremath{-}}$ impurities in NaCl, KCl, RbCl, KBr, KI, and NaBr from 1.4 to 300\ifmmode^\circ\else\textdegree\fi{}K. Sideband splittings range from 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ in NaCl:O${\mathrm{H}}^{\ensuremath{-}}$ to 37 ${\mathrm{cm}}^{\ensuremath{-}1}$ in KBr:O${\mathrm{H}}^{\ensuremath{-}}$, and correlate with dips in the low-temperature thermal conductivity and with far-infrared absorption bands. Preliminary results on the electric-field-induced dichroism in KCl:O${\mathrm{H}}^{\ensuremath{-}}$ and RbCl:O${\mathrm{H}}^{\ensuremath{-}}$ give a dipole transition moment perpendicular to the O---H axis for the main sideband. There is evidence that several sidebands in NaCl:O${\mathrm{H}}^{\ensuremath{-}}$ are themselves combination bands with one or more 2-3-${\mathrm{cm}}^{\ensuremath{-}1}$ tunneling levels. The first overtone yields values of ${\ensuremath{\omega}}_{e}=3790$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{e}{x}_{e}=85.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for O${\mathrm{H}}^{\ensuremath{-}}$ in KBr. These values explain very well the O${\mathrm{H}}^{\ensuremath{-}}$/O${\mathrm{D}}^{\ensuremath{-}}$ isotope shift of the main stretching band in KBr and KCl. The main-band oscillator strengths are small, on the order of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ for most crystals, but this value is consistent with a theoretical calculation of the dipole moment by Cade. The main bands broaden rapidly with increasing temperature, possibly indicating a prominent lifetime-limiting role played by the sideband levels. The sideband levels themselves are tentatively suggested to be due to a center-of-mass resonance in KBr:O${\mathrm{H}}^{\ensuremath{-}}$ and to a librational resonance or fast tunneling level in KCl:O${\mathrm{H}}^{\ensuremath{-}}$.

Optical, Electrical, and EPR Studies of KBr Crystals Doped with Cadmium

The peak positions, half-widths, and oscillator strengths of Cd (and Zn) bands in KBr and other uncolored alkali halides are given. In uncolored KBr, Cd introduces three optical absorption bands at 202, 215, and 280 nm, respectively. Two new bands, ${D}_{1}$ and ${D}_{2}$, at 250 and 285 nm, respectively, are observed in the irradiated KBr: Cd crystals. The growth of absorption coefficients ${\ensuremath{\alpha}}_{F}$ and ${\ensuremath{\alpha}}_{M}$ at the peaks of $F$ and $M$ bands, respectively, was studied as a function of radiation dose. The value of $S$ in the relation ${\ensuremath{\alpha}}_{M}=S{{\ensuremath{\alpha}}_{F}}^{2}$ is suppressed by a factor \ensuremath{\sim}100 in crystals containing more than 20 ppm of Cd. In the crystals irradiated for short times, the $M$ band cannot be produced even by optically bleaching the $F$ band. The second-stage growth of the F band is also suppressed considerably by the Cd impurity, which supports the view that the second-stage $F$ centers are necessary for the formation of $M$ centers. The ${D}_{1}$ and ${D}_{2}$ bands observed in x-irradiated crystals are absent in the additively colored crystals. In the crystals colored additively at 460\ifmmode^\circ\else\textdegree\fi{}C, an ultraviolet band at 330 nm and a band at 750 nm are observed in addition to the F band. The $F$ band can be converted to the 750-nm band by optical or partial thermal bleaching of the crystal. In a crystal colored at 560\ifmmode^\circ\else\textdegree\fi{}C the 750-nm band is absent, and only the 330-nm band and a very stable $F$ band are present. On heating the crystals to 600\ifmmode^\circ\else\textdegree\fi{}C, the 330-nm band, $F$ band, and 750-nm band disappear, and a new band at 300 nm appears. The crystal containing the 300-nm band shows an EPR absorption line with Lorentzian shape and a $g$ value of 1.987, and enhanced conductivity above 500\ifmmode^\circ\else\textdegree\fi{}C. These results suggest that the centers responsible for this band are metallic colloidal particles of Cd. The equilibrium concentration of $F$ centers in crystals doped with 28 ppm of Cd and colored additively at 560\ifmmode^\circ\else\textdegree\fi{}C is 15% less than that in a pure crystal. At 560\ifmmode^\circ\else\textdegree\fi{}C the rate of diffusion of $F$ centers in the Cd-doped crystals is less by a factor \ensuremath{\sim}5 as compared to the rate in a pure crystal, and is less by a factor \ensuremath{\sim}50 as compared to rate the of diffusion of 330-nm band centers.

Optical and thermal stability, half-widths and oscillator strengths of F-aggregate bands in highly pure KBr crystals

The decay and growth of F, F-aggregate and colloidal centres in additively coloured crystals have been studied (i) by bleaching the additively coloured crystal with white or F light, (ii) by annealing the optically bleached crystal at room temperature in the dark and (iii) by heating the coloured and `coloured and optically bleached' crystal up to 400 °c. The thermal stability of the centres can be described by the equations colloids, and colloids F. N centres are stable at room temperature and disappear completely on heating the crystal to 100 °c. Correction in the F-band height due to the over-lapping absorption of the excited states of the M centres is evaluated. Peak positions and half-widths of the M, R2, N and colloid bands are determined in highly pure and `pure' KBr crystals at room temperature. The variation of the half-width W of the M band in KBr with temperature is found to vary as The changes in the absorption coefficients at the peaks of the F and F-aggregate bands observed in the optical bleaching experiments, the room-temperature annealing experiments and the thermal bleaching experiments are used to obtain the oscillator strengths of the F-aggregate centres in KBr. By taking the oscillator strength of the F centre as 0.48, the values 0.11, 0.43 and 0.06 are obtained for the oscillator strengths of the M, R and N centres, respectively.

Zeeman Effect of the A Absorption Bands in Indium-Doped Alkali Halides

Zeeman effects of the A absorption bands in KBr:Tl, KI:Tl, KCl:Pb and KBr:Pb are studied by means of circular dichroism, and g -factors of the excited states relevant to these absorption bands are found to be 0.6∼0.8.

Color Center Studies in Alkali Halides by Pulsed Electron Beam Irradiation

The formation and decay of color centers in alkali halides have been studied using a pulsed electron beam. The F and V -type bands reach maximum heights in optical density at the end of the radiation pulse. V k and V 1 bands in KBr appear to show the first order decay kinetics; their room temperature life times are 1.3 and 3.8 µsec, respectively. A fraction of F and F ' bands bleach by the annihilation with migrating V k centers. The activation energies for the disappearance of H , V 1 and V k centers in KBr are found to be 0.09, 0.25 and 0.39±0.02 eV, respectively. A fraction of H centers in KCl decays with a temperature independent time constant (∼0.13 µsec) which is due to the tunneling with nearby F centers. In crystals containing U -centers (H - ions) the F band continues to grow for several microseconds after the pulse. Absorption bands which decay with the same time constant as the F band growth have been found at 775 mµ (KBr) and 725 mµ (KCl).

Zeeman Effect of the A Absorption Bands in Tl- and Pb-Doped Alkali Halides

Zeeman effects of the A absorption bands in KBr:Tl, KI:Tl, KCl:Pb and KBr:Pb are studied by means of circular dichroism, and g -factors of the excited states relevant to these absorption bands are found to be 0.6∼0.8.

Photochemically Produced V Bands in KBr and KCl

The properties of V bands produced by X-irradiation at temperatures above 78°K have been investigated in KBr and KCl crystals. Preferential bleaching with polarized light revealed that many V bands have complicated composite structures in spite of the simplicity of their shapes. The observed dependence of shapes and positions of V bands on the duration of irradiation and the crystal temperature during irradiation are attributed to their composite nature. Some of component bands were found to be destroyed by prolonged X-irradiation. The band which had been called V 4 in KCl was found not to be one corresponding to the V 4 band in KBr.

Photochemistry of V centers in alkali halides X-Rayed near 200°K

Photochemical and dichroic studies were made of the V 4 band in KBr and of the V band at 240 mμ in KCl both of which are produced most prominently by X-irradiation around 200°K. It was found that H centers are produced by the photo-excitation of these V bands at low temperatures and that both of them are excited most strongly by light polarized along a 〈100〉 direction. Measurements were also made of the recovery of the photochemically destroyed V 4 band in KBr. It is suggested that these V bands correspond to the same center which is associated with an interstitial halogen atom. Plausible models are proposed and it is concluded that a di-inter-stitial halogen molecule or a Cl − 3 molecule (Christy's V 3 center) are consistent with the experimental facts.

Color Centers Produced in KCl and KBr by Prolonged X Irradiations at Low Temperatures

The principal purposes of this study were to observe the effect of an x irradiation of extended duration on the production of color centers in KCl and KBr at low temperatures and to examine the implications with regard to possible color-center models. Growth curves were determined at 5\ifmmode^\circ\else\textdegree\fi{}K for the $F$, $K$, $M$, and $H$ bands in KCl and KBr and for a new band at 245 m\ensuremath{\mu} in KCl and at 278 m\ensuremath{\mu} in KBr, called the ${H}^{\ensuremath{'}}$ band. Growth curves were determined at 80\ifmmode^\circ\else\textdegree\fi{}K for the $F$, $K$, and $M$ bands in KCl and KBr. The growth of the ${V}_{4}$ band in KBr and of a band having its absorption maximum at 240 m\ensuremath{\mu} in KCl, designated as the $V(240)$ band, was also followed at 80\ifmmode^\circ\else\textdegree\fi{}K. The ratio of the absorptions at the peak of the $F$ and $K$ bands was found to remain constant indicating that both of these bands arise from transitions in the same center. The concentration of $M$ centers was found to depend quadratically on the $F$-center concentration. This result gives conclusive support to the model proposed by van Doorn and Haven that the $M$ center consists of a pair of associated $F$ centers. The $F\ensuremath{-}\mathrm{t}\mathrm{o}\ensuremath{-}H$ ratio was observed to be constant for short x-ray exposures, supporting the viewpoint that the two centers are produced as complementary defects; however, for prolonged exposures, this ratio is no longer constant. The number of $F$ centers is in constant ratio to the combined concentrations of $H$ and ${H}^{\ensuremath{'}}$ centers provided that the oscillator strength of the ${H}^{\ensuremath{'}}$ band is suitably chosen. The linear dependence of the generation of both ${V}_{4}$ centers in KBr and $V(240)$ centers in KCl on $F$-center concentration suggests that the ${V}_{4}$ and $V(240)$ centers, rather than ${V}_{1}$ centers, are fundamentally related to the mechanism of formation of $F$ centers near liquid-nitrogen temperature for a prolonged irradiation.

Recombination of Vacancies and Interstitials in KBr at Low Temperatures

Measurements and a detailed analysis were made of the thermal annealing of the $\ensuremath{\alpha}$ band in KBr x rayed at temperatures below 20\ifmmode^\circ\else\textdegree\fi{}K. Four distinct annealing stages at 11, 17, 19, and 21\ifmmode^\circ\else\textdegree\fi{}K were resolved, the first three stages corresponding to first order reactions and the fourth stage to a second order reaction. The activation energies for the annealing stages were about 0.015, 0.03, 0.04, and 0.06 eV, respectively. The first three stages were interpreted as a correlated recombination of close pairs of bromide vacancies and interstitials, and the fourth stage as the recombination of more distant defect pairs through a free migration of an interstitialcy. The activation energy 0.06 eV is interpreted as the free migration energy of a bromide-ion interstitialcy. Interaction energies between a vacancy and an interstitial ion in several configurations were calculated and possible annealing sequences for the first three annealing stages are proposed. The observed low pre-exponential factor for the first stage is also discussed.

F Bands in KBr–KI Mixed Crystals

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