VUV excitation of luminescence in pure and Tl+ doped KI

Excitation spectra of pure and Tl⁺ doped KI have been measured at room temperature (RT) and liquid nitrogen temperature (LNT) between 13 and 30 eV. Energy transfer between the host lattice ions and the Tl⁺ impurities, and intrinsic recombinaton of electron-hole pairs, have been studied at different temperatures and excitation energies. Energy transfer has been observed only at RT while at LNT the intrinsic recombination, V_k centers-electrons, is the dominant process.     

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Abstract

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl⁺ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl⁰ centre forming the spatially correlated defect pair with V_k centre while the 580 nm stimulation band is connected with the unperturbed Tl⁰ centres. It is concluded that the Tl⁺ luminescence at low temperature is connected with the electron recombination with the Tl²⁺ centre.     

Calculation of energies of radiative tunneling transitions between defects in alkali halides

The procedure for calculating the energies of radiative tunneling transition between spatially separated electron and hole centres in ionic crystals based on semiempirical INDO calculation of a pair of close defects and on a further account of electronic polarization of a crystal due to defects has been presented. Pairs of an electron, F, and hole (H, V_k, V₂) centres in KCl and LiF crystals are studied. The influence of close defect interaction and of polarization upon emitted photon energy has also been considered. It is shown that due to polarization this energy varies with a distance between defects in a more complicated way than $\text{e}{}^2\text{?R}$ shown in semiconductors.     

Infrared and 1H nuclear magnetic resonance studies of solvent effect on hydrogen bond in some pyridine n-oxide trifluoroacetates

The IR and ¹H NNR spectra are reported for the trifluoroacetates of five substituted pyridine N-oxides in five dry aprotic solvents of different polarity and studied with respect to the influence of the reaction field. Both the centre of gravity ($\text{V}$_H) of continuous absorption and the chemical shift (δ) show weak dependence upon solvent polarity. In the range of ε from 2.27 to 10.36 the continuous absorption is very similar. In acetonitrile (ε = 37.5) $\text{V}$_H is shifted. The magnitude and sign of the $\text{V}$_H shift depend on the proton acceptor properties of N-oxides and can be correlated with the position of the proton in the complex. The influence of the reaction field on OHO bonds is compared with the influence on OH?N?O^??HN⁺ bonds, and the differences in behavior of these complexes are related to their energy surface. The weak dependence of $\text{V}$_H and δ upon the reaction field in pyridine N-oxide trifluoroacetates is consistent with the single minimum energy surface postulated in the literature for the strong OHO bonds.     

Enhancement of shake-up structure in alkali-metal-ion exchanged forms of α-Zr(HPO4)2 by sputtering

Mg Kα ESCA spectra of several α-Zr(PO₄)₂M₂ compounds (M = Li⁺, Na⁺, K⁺, Cs⁺) have been obtained. Satellite structure is observed at ～7–8 eV from the main P 2s peak (corresponding to ～15–16 eV from the main Zr 3d_{$\text{5}\text{2}$} peak). The intensity of the satellite depends on the counter-ion intercalated. For a given counter-ion it is strongly increased by sputtering, the rate of increase being also dependent on the counter-ion. This observation is interpreted mainly in terms of electron-defect formation similar to that involved in the formation of colour centres by radiation damage, and subsequent charge-transfer shake-up of the trapped electrons to the electron-deficient phosphorus or Zr(IV) centres.     

Collisional quenching of HgBr(B2∑+12)

Rate coefficients for the quenching of HgBr(B²∑⁺_{$\text{1}\text{2}$}) by HgBr₂, H₂, CO₂, CO, O₂, N₂, Xe, and Br₂ are reported. HgBr(B²∑⁺_{$\text{1}\text{2}$}) is formed by the photolysis of HgBr₂ with the output from an ArF laser. The rate coefficients are determined by monitoring the time resolved B²∑⁺_{$\text{1}\text{2}$} → X²∑⁺_{$\text{1}\text{2}$} emission as quenching species are added. Radiative lifetimes at particular emission frequencies are also reported, and quenching mechanisms are discussed.     

The theory of diffusion-limited recombination of donor—acceptor pairs

The asymptotics of the quasi-steady kinetics of diffusion-limited recombination of donor-acceptor (DA) pairs has been developed. The theory is based on the probability of DA pair recombination according to the relation w₀ exp [-(r/r_B)], where w₀ is constant, r is the separation between defects and r_B is the half Bohr radius of the more diffuse wavefunction. The reaction rate equation (for the thermal activation mechanism) includes the reaction constant as follows: k = 4πr_B[E_a/kT + ln (w₀r^2_B/D₀) + 1.54 + 2 K₀ (α)], α = 2r₀(w₀/D)$\text{1}\text{2}\text{,}$ where E_a is the activation energy, and K₀, I₀ are the modified Bessel functions. The quasi-steady recombination radius is introduced. The theory developed has been compared with the zone one. The interpretation of the activation energy and frequency factor have been presented for the diffusion model. An extension of the theory developed by taking into account Coulomb interaction between defects has also been presented. The applicability of the theory to the recombination of V_k centers with electronics centers is alkali halides has been shown. It is assumed that the theory is applicable to other centers of hole nature (H, proton, low symmetry exciton i.e. V_k + electron).     

Calculation of the γγ → ϱ0ϱ0 cross section at high energies

The processes with the cross sections not decreasing with energy become important at high energies. The simplest processes of this kind are γγ → V_i⁰V_j⁰ where V⁰ = ?⁰, ω, ?, ….. We calculate their cross sections in the high-energy small angle region s ? |t| ? μ². The cross section γγ → ?⁰?⁰ at high energies (s ? 10 GeV²) exceeds those of γγ → ππ, ?⁺?^? considerably. At $\text{s ? 10}{}^4\text{GeV}{}^2$ (this is the characteristic energy for the VLEPP and SLC colliders) and $\text{|t| ? 2}\text{GeV}{}^2$, the ratio $\text{(}\text{d}\text{σ/}\text{d}\text{t)(γγ → ?}{}^0\text{?}{}^0\text{)/(}\text{d}\text{σ/}\text{d}\text{t)(γγ → μ}{}^{\mathrm{+}}\text{μ}{}^{\mathrm{?}}\text{) ? 70}$.     

Limit cycle in a bound exciton recombination model in non-equilibrium semiconductors

A non-equilibrium semiconductor model involving the processes of photogeneration of electron-hole pairs (e-h) (rate G), stimulated creation of excitons from e-h (rate constant C) and decay of excitons on recombination centres (rate constant k) is analyzed in this paper for steady states and limit cycle behaviour. Considering the exciton decay to be similar to enzymatic processes in chemical reactions obeying a Michaelis-Menten law, and choosing units such that k = 1 = N, where N is the concentration of recombination centres, the model represents a 2-parameter (C and G) 2-dimensional (exciton and electron-hole concentrations x, n) dynamical system with a unique steady state (x₀,n₀) which is unstable in the region (l ? G)³?4C, the equality sign corresponding to the bifurcation curve in parameter space. In the region (l ? G)³ > 4C the system displays a unique stable limit cycle which is obtained in analytical form by employing a two-time-scales method for parameters in the neighbourhood of the bifurcation curve. The limit cycles are tilted ellipses with angular frequency $\text{}\text{?}$gw of the order of 10⁶ s^?1. In a realistic semiconductor situation G$\text{\$}\text{?}$10^?3.     

Spin-parity assignments for 66Cu levels via the 68Zn(d, α)66Cu reaction

The ⁶⁸Zn($\text{d}$, α)⁶⁶Cu reaction populating low-lying states in ⁶⁶Cu has been studied at θ_lab = 4° using deuteron beams in the energy range 9.0 to 10.5 MeV. Tensor analyzing powers were calculated and natural- or unnatural-parity assignments were made for thirteen states in ⁶⁶Cu. By combining these results with existing limits unambiguous J^π assignments of 2⁺, 2⁺, 2⁺ and 1⁺ have been made for the levels at 186, 465, 822 and 1344 keV respectively. The previous tentative assignments to seven other levels have been confirmed while that for the 1247 keV level has been shown to be incorrect. The identification of the quartet of levels based on the $\text{π(}\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)v(}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ configuration has been confirmed.     

Charge-density wave instability and nonlinear transport in Tl0.3MoO3 a new blue molybdenum oxide bronze

Single crystals of Tl_0.3MoO₃ blue bronze were grown by the temperature gradient flux technique for the first time. Tl_0.3MoO₃ crystalizes with monoclinic symmetry, space group C2, C2/m or Cm, $\text{a = 18.486(1)}\text{A}\text{?}$, $\text{b = 7.5474(6)}\text{A}\text{?}$, $\text{c = 10.0347(7)}\text{A}\text{?}$, β = 118.377(6)° and appears to be isostructural with the K_0.3MoO₃ blue bronze. The physical properties of Tl_0.3MoO₃ are similar to those of the K₃MoO₃ and Rb₃MoO₃ phases. The resistivity of Tl_0.3MoO₃ is highly anisotropic and its temperature susceptibility (4.2–300K) decreases sharply with decreasing temperature near the transition seen in the resistivity. The onset of another transition <60K is also indicated by the susceptibility data. Non-linear current-voltage (I–V) characteristics observed at ～70K above applied threshold voltages of the order of ～300mV/cm suggest the sliding of charge density condensate.     

ESR of Vk centers in CsI

V_k centers (self trapped hole I^?₂) have been obtained in CsI doped with Tl⁺ or Na⁺ by X-irradiation at low temperature and studied by ESR. Spin-Hamiltonian parameters are calculated and compared with those obtained for I^?₂ in the other alkali iodides, as well as the energy of the forbidden transition ²Σ⁺_u→²Π_u and the s and p components of the fundamental state.     

On the asymptotic behaviour of the mass operator near the Fermi energy

By taking due account of momentum conservation, it is shown that, when ω is near the Fermi energy ω_F, the imaginary part of the mass operator $\text{M(}\text{k}\text{, ω)}$ for an infinite Fermi system behaves like (ω ? ω_F)^p(k) where the exponent p(k) ? 2 depends on the interval in which $\text{|}\text{k}\text{|}$ is lying. In particular, the commonly asserted quadratic behaviour (ω ? ω_F² is shown to be true only for $\text{|}\text{k}\text{| ? 3k}{}_{\mathrm{<mtext>F</mtext>}}$. It is explicity assumed that the Fermi system admits a perturbative type treatment.     

Study of inclusive neutral current interactions of neutrinos and antineutrinos

We report on results from a study of hadron-energy distributions for ν and $\text{ν}$ inclusive neutral current interactions. There is no significant variation of the neutral to charged current total cross-section ratios R_ν and $\text{R}\text{ν}$ with neutrino energy. The space-time structure of neutral currents is dominated by V?A, with a significant admixture of V+A. The Weinberg-Salam model is in agreement with all data if sin²θ_w=0.24±0.02.     

Photoelectron-photon coincidence studies of the A$̃and B$̃excited electronic states of X-CC-CN+, X = CH3, CD3, Cl,Br, I

Fluorescence quantum yields and lifetimes of the above given cations in selected levels within their lowest excited electronic states have been measured by a photoelectron—photon coincidence technique. These data, obtained under collision-free conditions, lead to the radiative and non-radiative rate constants as a function of the internal energy. The symmetry of the A$\text{\$}\text{?}$state is ²A₁ (X = CH₃, CD₃), ²Σ⁺ (X = Cl), but ²Π (X = Br, I) and the corresponding k_r values for these two groups, 1–2 × 10⁶s^?1 and 2 × 10⁷s^?1 respectively, reflect the different nature of the transitions. Other essential features of the results are discussed.     

Spin-lattice relaxation of the Vk-center in LiF

The angular and temperature dependences of the spin-lattice relaxation (SLR) rate of V_k-centers in LiF doped with Mg or Ag have been investigated. In the temperature interval 4.2–100 K the results can be fitted by the formula $\text{τ}{}^{\mathrm{?1}}\text{= A(θ)T + Be}{}^{\mathrm{<mtext>?\Delta </mtext><mtext>T</mtext>}}$ with A(0°) = 0.11 sec^?1K^?1, A(90°) = 1.3 sec^?1K^?1, B = 3 × 10⁵sec^?1 and Δ = (175 ± 15)K.A mechanism for the SLR is considered, assuming the modulation of the hyperfine interaction by phononinduced transitions between the ground and excited states of the resonant molecular vibrations of the V_k-center. This mechanism is found to explain the value, the temperature dependence and the isotropy of τ^?1 in the interval T = 20–100 K.The one-phonon SLR mechanisms of the V_k-center in the T < 10 K region are discussed.     

Gamow-Teller resonance in β+ decay of 147mDy and spin-isospin current renormalization

The total absorption spectrum of γ-rays following β⁺ decay of ^147mDy has been investigated. The decay energy Q_ε = 7.18 (10) MeV and an improved value of half-life $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 55.7 (7)}\text{s}$ are determined. The intensity of the transitions from the isomeric state is 31 (3)%, the internal conversion being included. More than 40% of decays populate ¹⁴⁷Tb levels with the excitation energy E_ex ? 4.4 MeV. The β⁺ strength function S_β⁺ exhibits a pronounced narrow maximum at E_ex = 4.84 MeV and some structure at higher energy. The total probability of ^147mDy β⁺ decay corresponds to the value $\text{log}\text{?t = 3.67 (8)}$.The calculation of S_β carried out in the random-phase approximation gives the correct resonance energy. The coefficient of the spin-isospin current renormalization found from the comparison of the calculated β-decay probability with the experimental one is |;G_A/g_v|; = 0.7±0.1.     

Radiation induced defects in KBr:Tl+ crystals

The defects produced in KBr:Tl⁺ crystals during x-irradiation at 77 K were studied using thermoluminescence (TL), thermally stimulated currents (TSC), and absorption and emission spectra. Three main glow peaks at 165, 193 and 258 K were observed both in the TL and in the TSC curves. A variety of irradiation induced absorption bands were observed in the UV, visible and infrared up to about 2 microns. The 165 K TL peak was found to emit only the 440 nm band assigned to thallium dimers, while the peaks at 193 and 258 K exhibited the UV bands at 310 and 365 nm as well as the 440 nm band.The defects produced during the irradiation were the V_k hole center, the Tl° and the Tl⁺₂ electron centers. Smaller concentrations of Tl²⁺ and (Tl⁺)⁺₂ centers were also produced.An analysis of the results including measurements on lightly and heavily doped crystals enabled to draw conclusions on the nature of the defects and on the recombination processes involved. A close correlation has been found between the temperatures at which changes in the various absorption bands take place and the temperatures of the TL peaks. The analysis enabled also a full classification of the absorption bands.     

D-state effects in (d, t) and (d, 3He) reactions

The influence of the deuteron and triton [³He] D-state on the form factor required in the DWBA analyses of the ($\text{d}$, t) [($\text{d}$, ³He)] reactions has been studied. The wave functions of Phillips are used for three-nucleon systems because they are designed to yield the correct asymptotic form for the (d, t) and (d, ³He) overlaps. The zero-range parameters D₀ and D₂ which enter into the calculation of the cross section and the tensor analysing power in the DWBA with the local energy approximation are calculated and found to yield values consistent with experimental data.     

Proton scattering from 9Be between 6 and 30 MeV and the structure of 9Be

Differential cross-section excitation functions at lab scattering angles 86.9°, 120.0°, 140.0° and 160.0° were measured for ⁹Be(p, p_o)⁹Be, ⁹Be(p, p₂)⁹Be and ⁹Be(p, d₀)⁸Be at proton lab energies from 6 to 15 MeV in 100 keV steps. A broad anomaly was observed in the ⁹Be(p, p₀)⁹Be excitation functions. Differential cross-section angular distributions were measured for ⁹Be(p, p₀)⁹Be and ⁹Be(p, p₂)⁹Be at lab energies of 13.0, 14.0, 15.0, 21.35 and 30.3 MeV and for ⁹Be(p, d₀)⁸Be at 13.0, 14.0, 15.0 and 21.35 MeV. Angular distributions of polarization analysing powers for ⁹Be($\text{p}$,p₀)⁹Be, ⁹Be(p, p₂)⁹Be and ⁹Be($\text{p}$, d₀)⁸Be were measured at 8.0, 11.0, 12.0, 13.0 and 15.0 MeV. A spherical optical-model (SOM) analysis of the elastic scattering angular distribution data from 13.0 to 30.3 MeV showed that an energy dependence of only V_R and W_s (volume real and surface imaginary depths) is sufficient to reproduce the measurements. Coupled-channels (CC) analyses were made with a quadrupole-deformed optical-model potential and strong coupling of $\text{3}\text{2}{}^{\mathrm{?}}$, $\text{5}\text{2}{}^{\mathrm{?}}$ and $\text{7}\text{2}{}^{\mathrm{?}}$ levels of a $\text{K =}\text{3}\text{2}$ ground-state rotational band of ⁹Be. The ⁹Be(p, p₀)⁹Be and ⁹Be(p, p₂)⁹Be data from 13.0 to 30.3 MeV were analyzed simultaneously at each energy, varying only V_R and W_s with energy, for a potential deformation of β = 1.1. Both SOM and CC analyses indicated the same energy dependence in V_R, while W_s averaged 3.5 MeV lower in CC than in SOM, with both energy dependences consistent with previous analyses of nucleon scattering from 1p shell nuclei.