Neutral Bromine Research Articles

Ab Initio Study on the Vibrational and Electronic Properties of Radiation-Induced Defects in Potassium Bromide

The vibrational and electronic properties of several basic radiation defects in potassium bromide are computed at the quantum mechanical level using a periodic supercell approach based on hybrid functionals, an all-electron Gaussian-type basis set, and the Crystalcomputer code. The exciton energy in alkali halides is sufficient to create lattice defects, such as F–H Frenkel defect pairs, resulting in a relatively high concentration of single defects and their complexes. Here, we consider eight defects: the electronic F+- and F-centers (bromine vacancy without and with trapped electrons) and their dimers; hole H-center (neutral bromine atom forming the dumbbell ion with a regular Br− ion.); VK-center (Br2− molecular ion consisting of a hole and two regular ions); and two complex Br3− defects, combinations of several simple defects. The local geometry and the charge- and spin-density distributions of all defects are analyzed. Every defect shows its characteristic features in Raman spectra, and their comparison with available experimental data is discussed.

Metal Halide Scintillators with Fast and Self‐Absorption‐Free Defect‐Bound Excitonic Radioluminescence for Dynamic X‐Ray Imaging

AbstractScintillators for radiation detection are of great significance in medical imaging, security, and nondestructive inspection. The current challenge for scintillators is to simultaneously achieve high scintillation light yield, fast radioluminescence, simple film fabrication, large X‐ray attenuation efficiency as well as stable and nontoxic compositions; no previous scintillators fulfill all the above requirements. Here, metal halide Rb2AgBr3, possessing defect‐bound excitonic radioluminescence, is shown as efficient and fast scintillators. This nontoxic and stable scintillator emits from excitons bound to neutral bromine vacancies, enjoying an efficient and spin‐allowed fast emission with minimized self‐absorption. Rb2AgBr3 thus has a high light yield (25 600 photons MeV−1), fast scintillation decay time (5.31 ns), and a record value of light yield versus decay time (4821 photons MeV−1 ns−1). The close‐space sublimation method is developed for fast and scalable fabrication of oriented Rb2AgBr3 films. The scintillator film is further integrated with commercial flat‐panel imagers, and the spatial resolution reaches 10.2 line pairs per millimeter at the modulation transfer function of 0.2, doubling the resolution of conventional CsI:Tl flat‐panel detectors. The dynamic X‐ray imaging and its use to real‐time monitoring of bone movement without ghosting effect is also demonstrated.

Hyperfine structure constants of atomic bromine (Br I)

The absorption spectrum of the neutral bromine (Br I), lying in the region from 11,300cm−1 to 12,600cm−1 has been investigated using concentration modulation absorption spectroscopy with a tunable Ti:Sapphire laser. The bromine atoms were excited by discharging the mixture of Helium and bromine vapour in a hollow discharge glass tube. The hyperfine structure spectra of 45 lines were analyzed. Hyperfine structure constants A and B were derived for 20 even and 27 odd levels amongst which constants for 12 even and 21 odd levels were newly reported.

Eu2+-doped CsBr photostimulable X-ray storage phosphors — analysis of defect structure by high-frequency EPR

Eu 2+-doped CsBr in form of needle image plates is a promising alternative to the granular Eu 2+: BaFBr X-ray storage phosphor with respect to photo-stimulated luminescence, yield and spatial resolution. However, in order to understand the corresponding mechanisms on an atomic scale, the defect structure of Eu 2+-doped CsBr photostimulable X-ray storage phosphors has to be analyzed. By means of high-frequency electron paramagnetic resonance (EPR) spectroscopy, three types of defects have been observed — an electrically neutral dimeric [Formula: see text] defect complex and a neutral trimeric [Formula: see text] defect complex (FZ-center), as well as acceptor-type F-centers [Formula: see text] that are generated by the trapping of electrons at initially neutral bromine vacancies.

Interactions between anionic and neutral bromine and rare gas atoms

High-quality, ab initio potential energy functions are obtained for the interaction of bromine atoms and anions with atoms of the six rare gases (Rg) from He to Rn. The potentials of the nonrelativistic (2)Sigma(+) and (2)Pi electronic states arising from the ground-state Br((2)P)-Rg interactions are computed over a wide range of internuclear separations using a spin-restricted version of the coupled cluster method with single and double excitations and noniterative correction to triple excitations [RCCSD(T)] with an extrapolation to the complete basis set limit, from basis sets of d-aug-cc-pVQZ and d-aug-cc-pV5Z quality. These are compared with potentials derived previously from experimental measurements and ab initio calculations. The same approach is used also to refine the potentials of the Br(-)-Rg anions obtained previously [Buchachenko et al., J. Chem. Phys. 125, 064305 (2006)]. Spin-orbit coupling in the neutral species is included both ab initio and via an atomic approximation; deviations between two approaches that are large enough to affect the results significantly are observed only in the Br-Xe and Br-Rn systems. The resulting relativistic potentials are used to compute anion zero electron kinetic energy photoelectron spectra, differential scattering cross sections, and the transport coefficients of trace amounts of both anionic and neutral bromine in the rare gases. Comparison with available experimental data for all systems considered proves a very high precision of the present potentials.

Transient center photodecomposition in potassium bromide

We recently demonstrated that sub-band gap photoexcitation of KBr near 6.4 eV leads to desorption of hyper-thermal neutral bromine atoms without a significant thermal velocity component. In contrast, a considerable thermal velocity component is observed at higher excitation energies and is attributed to formation and subsequent diffusion of transient centers from within the bulk crystal to the surface. Excitation with a nanosecond laser at 3.5 eV also produces neutral bromine emission with a seemingly thermal velocity profile, but a near three-photon power dependence. Furthermore, the relative Br ∗ yield is greatly enhanced. We propose an explanation for these results based on formation and secondary excitation of transient defect centers in KBr by individual nanosecond pulses. Photodecomposition of newly formed transient centers leads to emission of Br and Br ∗ in a “near thermal” velocity distribution. These results are likely general for alkali halides and are consistent with a recently described theoretical model.

Stark effect in some lines of neutral bromine

This work presents experimental results of profile parameters for prominent lines of neutral bromine, for which serious discrepancies between experimental values of different authors as well as between experimental and calculated values were observed. The observed discrepancies between shock tube and earlier arc experiments are very large. Our results of profile parameters, obtained also in a wall-stabilized arc, show acceptable agreement with the earlier arc measurements. It is obvious, that the shock tube experimental result is out of the range of acceptable inaccuracy and should be remeasured. The theoretical calculations based on different assumptions give also strongly divergent results. The doubtful results of calculations for intercombination lines (e.q. 4472 A ̊ ) could be expected, but the observed factor 2 or higher discrepancy for the other lines shows the need to find better theoretical approaches for profile parameter calculations. For generating the bromine spectrum a mixture of argon and bromine vapors was introduced in the central part of a wall-stabilized arc. The arc was operated at currents of the range of 25– 60 A . The amount of bromine vapors was changed by adjusting the thermostat temperature. The spectra were recorded using a grating spectrograph (PGS2), an optical multi-channel analyzer OMA4 and a IBM-PC type computer. The plasma diagnostics was based on measured emission coefficients of neutral argon and bromine lines and the measured half-width of the Ar I 4300 A ̊ line (LTE was assumed). Profiles of diverse types were fitted to data sets of experimental points. Griem profiles agreed normally better with the experimental results than Lorentz or Voigt profiles.

Evidence for a surface exciton in KBr via laser desorption

We demonstrate that direct photoexcitation of the single crystal KBr surface leads to desorption of hyperthermal neutral bromine atoms. We have produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We argue that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provide strong support to a theoretical excitonic emission model previously described in the literature.

TRANSITION PROBABILITIES FOR Br I LINES EMITTED FROM A WALL-STABILIZED CASCADE ARC

A plasma was created in a wall-stabilized cascade arc at atmospheric pressure in a mixture of argon and bromine vapors. Arc currents between 40 and 60 A were used. The spectrum of neutral bromine was recorded in the range between 3300 and 10 400 Å, applying a spectrometer of medium dispersion, equipped with an Optical Multichannel Analyzer (CCD detector). From measured line intensities, corrected for self-absorption of radiation and the life time of the corresponding upper level absolute values of transition probabilities of 5 Br I lines, were determined. For additional 28 Br I lines transition probabilities were obtained in the absolute scale too, using the measured intensities of these lines relative to selected reference lines and the determined plasma temperature. The agreement with some other experimental data determined by a shock tube experiment is unsatisfactory. The accuracy of the new data depends mainly on the accuracy of the life time data taken from the literature.

Properties of the complex salt obtained by doping the poly( N-vinylcarbazole) with bromine

Poly( N-vinylcarbazole) (PVK) powder has been doped by bromine at room temperature and also at 370, 420 and 470 K for 24–72 h. It is shown, by electron spin resonance (e.s.r.), optical absorption, X-ray photoelectron spectroscopy (XPS), that a charge transfer complex (CT complex) is formed between PVK and Br. However, some bromine is found to react with PVK, even after doping at room temperature, by addition or substitution, since not only bromine anion, which evidences the presence of the charge transfer complex, but also neutral bromine, has been detected by CPS. After heating, our results show that PVK is transformed into a new, more insoluble polymer. This polymer is amorphous, while some traces of carbazole and NH 4Br are present in the powders. The increase of the conductivity is probably related to the formation of some π bonds in the new amorphous polymer.

Ab initio calculations on bromine oxide and dioxides and their corresponding anions

Neutral bromine oxides and dioxides as well as their corresponding anions have been studied by means of ab initio molecular orbital calculations. To test the importance of static and dynamic correlation in these systems both single-configuration-based methods [MP2, QCISD, and QCISD(T)] and multiconfiguration-based methods (CASSCF and CASMP2) have been used. Equilibrium geometries and harmonic vibrational frequencies have been obtained for BrO and the two bromine dioxide isomers (OBrO and BrOO). For the corresponding anionic species, excellent agreement has been obtained for the predicted geometries at QCISD(T) and CASMP2 levels, while frequencies obtained at QCISD(T) agree to within 10 cm−1 with the available experimental data. An analysis of the charge density shows that the nature of the BrO bond is very different within OBrO and BrOO, and that the BrO charge density is reinforced in OBrO relative to BrO itself.

NIST–JANAF Thermochemical Tables for the Bromine Oxides

The thermodynamic and spectroscopic properties of the bromine oxide species have been reviewed. Recommended NIST–JANAF Thermochemical Tables are given for six gaseous bromine oxides: BrO, OBrO, BrOO, BrOBr, BrBrO, and BrO3. Sufficient information is not available to generate thermochemical tables for any condensed phase species. Annotated bibliographies (over 280 references) are provided for all neutral bromine oxides which have been reported in the literature. There are needs for additional experimental and theoretical data to reduce the uncertainties in the recommended values for these six species. Of all the species mentioned in the literature, many have not been isolated and characterized. In fact some do not exist. Throughout this paper, uncertainties attached to recommended values correspond to the uncertainty interval, equal to twice the standard deviation of the mean.

Discrete structure in the 3d photoabsorption spectra of neutral, singly ionized and doubly ionized bromine.

The 3d photoabsorption spectra of Br, ${\mathrm{Br}}^{+}$, and ${\mathrm{Br}}^{2+}$ have been recorded photographically in the 60--90 eV region using the dual-laser-produced plasma technique. In each case intense 3d\ensuremath{\rightarrow}4p transitions were observed while 3d\ensuremath{\rightarrow}np (n\ensuremath{\gtrsim}4) features were also obtained in the spectrum of neutral bromine. The transitions were classified with the aid of multiconfiguration Hartree-Fock calculations. The excited states are predicted to decay by direct autoionization involving 4s or 4p electrons and the rates for the different processes and resulting linewidths were also calculated. From these data theoretical cross sections for the observed features were also computed. \textcopyright{} 1996 The American Physical Society.

Stark broadening and shift of neutral bromine lines

The authors report results of an experimental study of the Stark broadening and Stark shift of thirteen and five neutral bromine lines respectively in a plasma of a wall-stabilised arc. An electron density of around 3*1016 cm-3 was determined from the width of the Hbeta line while an electron temperature of 9700 K was derived from plasma composition data. The agreement with semiclassical calculations and with results of two simplified approaches is within the limits of the estimated errors of experiment and theory. Large discrepancy between theory and experiment is detected only for the multiplets originating from the (1D2)5p 2Po energy level.

Trapping of Holes in Silver Bromide Single Crystals

AbstractThe reaction between a deposit of silver and neutral bromine formed by holes migrating to the surface of a silver bromide crystal is used to investigate the concentration decay of holes in the interior of the crystal. The effective lifetime of photoexcited holes in a thin surface layer is shown to be at least an order of magnitude larger than the bulk lifetime. Thermal annealing of the samples substantially increases the bulk trapping rate whilst leaving the surface rate unchanged. The observations are explained in terms of a layer of adsorbed bromine formed at the surface of the crystal during exposure. It is shown that an effective bromine acceptor is necessary for the formation of a stable surface latent image. No relation is found between the lifetimes of photoexcitedle ectrons and holes, which suggests that these carriers do not recombine with one another. The nature and concentration of the trapping centres and the effect of annealing the crystals is discussed.

Analysis of the Spectrum of Neutral Atomic Bromine (Br I).

The spectrum of the neutral bromine atom, Br I, has been newly investigated by using electrodeless discharge tubes as light sources. The observations have led to a list of wavelengths and estimated intensities for 1253 spectral lines in the range 1067 to 24100 Å. The number of known energy levels has been increased to 123 even and 128 odd levels, as compared with the 27 even and 33 odd levels previously known. All predicted energy levels of the 4s24p4ns, up, nd, nf electron configurations from 0 to ~93250 K have been discovered. The observations in the vacuum ultraviolet establish that the positions of all the levels lying above those of the ground configuration as given in the compilation Atomic Energy Levels, Vol. II (1952) should be increased by 6.7 K. All but 26 faint lines of Br I have been classified. A total of 67 levels has been ascribed to the 4s2 4p* nf configurations. It is demonstrated that the nf configurations exhibit almost pure pair coupling. The very regular (3P2)nf[5]°11/2 series yields for the principal ionization energy of Br I the value 95284.8 K.

Chemical Effects of Nuclear Recoil. Alkali Bromates

Summary An attempt is made to explain some of the discrepancies which have been observed when studying the retention of bromine in neutron irradiated bromates. In particular the influence of solvent, irradiation time and thermal annealing is investigated in potassium bromate, lithium (natural abundance) bromate and lithium (depleted in 6Li) bromate. Experiments show that a considerable amount of neutral bromine is formed upon recoil, and that the abnormal behaviour of lithium bromate is due to the consequences of 6Li(n, ɑ)T reactions. An explanation is put forward on the basis of an oxygen free intermediate recoil bromine fragment. The isotope effect on bromine recoil is confirmed.