Vacancy formation energies from positron trapping measurements

The threshold temperatureT _t at which thermally generated vacancies produce measurable positron trapping is a linear function of the energy of self-diffusionQ. SinceQ is also linearly related to the vacancy formation energyE _1v ^f , a measurement ofT _t leads directly to a determination of the latter. It is possible to make a precise determination ofE _1v ^f without approaching the melting point—a major advantage in dealing with refractories or with metals having a high vapour pressure in the solid state.     

Measurements of total cross sections for positron-K and-Rb scattering

A modification of the apparatus that we have been using to measure absolute total cross sections (Q _T′s) for positron and electron scattering by alkali metal atoms has enabled us to more easily obtainQ _T′s at low energies (<10 eV). Our positron-K and-RbQ _T measurements now extend from 1 to 98 eV and we have made the correspondingQ _T measurements for electrons in the same system using the same technique (a beam transmission technique). Our measured positron-K and-RbQ _T values show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV, in sharp contrast to the results obtained in recent 5-state close-coupling approximation calculations. At higher energies (above 10 eV), we have found that the same calculations are in quite good agreement with our measured positron-K and-RbQ _T′s. Our measurements of the correspondingQ _T′s for electrons below 10 eV do not show a peak, but rather continue to increase as the energy is decreased.     

Positron scattering from hydrocarbons

The total cross sections for positron impact on hydrocarbons have been calculated using the additivity rule in which the total cross section for a molecule is the sum of the total cross section for the constituent atoms. The energy range considered is from a few eV to several thousand eV. The total cross sections for positron impact on an atom are calculated by employing a complex spherical potential which comprises of a static, polarization and an absorption potential. We have good agreement with the experimental results for hydrocarbons for positron energy ⩾100 eV. Our results also agree with the available calculations for CH₄ and C₂H₂ which employed full molecular wavefunctions beyond 100 eV. Our absorption cross sections also agree with molecular wave-function calculations for C₂H₂ and CH₄ beyond 100 eV. We have shown the Bethe plots fore ⁺−C ande ⁺−H scattering systems and Bethe parameters have been extracted. We have fitted the cross section for positron impact on hydrocarbons in the formσ _t(C _n H _m )=naE ⁻ b+mcE ⁻ d in the energy range 300–5000 eV wherea=195.0543,b=0.7986,c=371.1757 andd=1.1379 withE in eV andσ _t in 10⁻¹⁶ cm².     

Total and ionization cross-sections of N<Subscript>2</Subscript> and CO by positron impact: Theoretical investigations

In this paper we report our calculations on several important total cross-sections (TCSs) of positron impact on isoelectronic N₂ and CO molecules, treated in the complex spherical potential formalism. Basically the total (complete) cross-section Q _T consists of elastic and inelastic contributions. Our total inelastic cross-section (Q _inel) contains ionization and electronic excitations together with positronium formation. Our goal here is to bifurcate Q _inel further to deduce total ionization cross-section, using the ‘complex scattering potential–ionization contribution’ (CSP-ic) method of electron–atom/molecule scattering. The present range of positron energy is 15–2000 eV. All the resulting cross-sections are in a good general accord with the existing data. This work highlights the importance of various scattering channels in e⁺-N₂ and e⁺-CO interactions at intermediate and high energies.     

Magnetic properties of magnetite thin films close to the Verwey transition

Temperature dependence of the magnetic properties of magnetite thin film across the Verwey transition has been investigated. As the temperature is decreased, the magnetization of the film in a fixed field showed a sharp decrease close to the Verwey transition temperature (T_v). The M–H loops of the film have been recorded at various temperatures below and above T_v. It is found that film does not saturate at any temperature and saturation becomes more difficult below T_v. While cooling through T_v, the extrapolated value of magnetization to infinite field (Q), calculated from the numerical fit 4πM=Q [1−(H*/H)^1/2], does not show a drop, but the coefficient indicating difficulty in saturation (H*) shows a sharp rise as does the coercivity.     

Relaxation during short-range-ordering in Cu-30% Zn alloy

The internal friction measured in a vacuum torsion pendulum has been used to determine the kinetics of short range ordering in an Cu-30% Zn alloy. Curves of relative internal friction (Q^–1/Q ₀ ^–1 ) against annealing time have been determined for various annealing temperatures. Average activation energy of 1·7 eV was found for the ordering process, which equals to that derived for zinc diffusion in coarse grained copper.     

Electron impact dissociation of ND<Superscript>+</Superscript>: formation of D<Superscript>+</Superscript>

Absolute cross sections for electron impact dissociation of ND⁺ leading to the formation of D⁺ have been measured by applying the animated electron-ion beam method in the energy range from the reaction threshold up to 2.5 keV. The maximum inclusive cross section is observed to be (16.8 ± 0.8) × 10⁻¹⁷ cm² at the electron energy of 65.1 eV. The appearance energy for the D⁺ production is measured to be (4.0 ± 0.5) eV. Collected data are analyzed in details by means of an original procedure in order to determine separately the contributions of dissociative channels. A specific Monte Carlo modeling has been developed, which is proven to reconstruct adequately the dissociative ionization cross section. The present energy thresholds provide information about the ground and excited states of the molecular ion, as well as about the possible population of the vibrational levels. The reaction D₂(v) + N⁺ (or H₂(v) + N⁺) is a probable source for that population and it constitutes the first step of the molecular activated processes, so the corresponding chain of reactions has to be considered to study the chemistry of plasma sources.     

A new criterion for the metallicity of elements

A vast majority of elements are metallic in the liquid state. The latent heat of vapourization, ΔH_v, of such elements is greater than the critical value of ∼ 42 kJ mol⁻¹ (0.44 eV mol⁻) which demarcates metals from non-metals. It is shown that ΔH_v can be related to the Fermi energy as well as to the Herzfeld criterion involving atomic polarizability.     

Temperature dependence of positron annihilation in iron and vanadium

The Doppler broadened lineshape is measured as a function of temperature for iron and vanadium; vacancies produce a strong trapping effect in γ-Fe, but a weak effect in V. Threshold temperaturesT _t for trapping are 1280±25 and 1370±30 K. Empirical linear relationships betweenT _t , self-diffusion energyQ ^SD and monovacancy formation energyH _1v ^F are discussed and used to determineH _1v ^F values for γ-Fe and V.     

Isotopic effects of the N(2D) + H2 → NH + H reaction: a quantum time-dependent wave packet investigation

Based on the potential energy surface reported by Li and co-workers (J. Comput. Chem. 34 1686–1696 (2013)), the dynamics calculations of N(²D)?+?H₂(v ₀?=?0, j ₀?=?0) reaction and its isotopic variants HD and D₂ are studied using time-dependent wave packet method in the collision energy range of 0.01–1.0?eV. Dynamics properties such as reaction probability, differential cross section, and integral cross section are studied at state-to-state level of theory. Present values are compared with available theoretical and experimental results. The results indicate that the integral cross sections of N(²D)?+?D₂ reaction are in general good agreement with the experimental data at collision energy below 0.15?eV. The rotational state-resolved integral cross sections of N(²D)?+?H₂/HD/D₂ reactions are compared with experimental values for the first time, with the obtained values being in good agreement with the experimental data.     

Measurements of total and positronium-formation cross sections for the scattering of positrons by alkali atoms

We present a review of our recent measurements of total cross sections (Q _T's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (Q _Ps's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQ _Ps measurements, our approach involves setting upper and lower limits onQ _Ps using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQ _T's andQ _Ps's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQ _T's andQ _Ps's for K which also appears in our measurements. The absence of such a peak in our measuredQ _T's andQ _Ps's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQ _T which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQ _T measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb.     

Measurement of the Raman polarizability anisotropy for the v = 1 pure rotational Raman transition in H 2 by rotational Raman spectroscopy

A combination of stimulated Raman pumping and rotational Raman spectroscopy is used to accomplish the first measurement of the polarizability anisotropy γ_11,13 (355 nm) for the S₁₁ (1) transition in molecular hydrogen H₂. Saturation of the Q₀₁(1) transition connecting the |X¹ Σ⁺ _g, v = 0, J = 1 > state to the |X¹ Σ⁺ _g, v = 1, J = 1 > state in H₂ by stimulated Raman pumping is the critical element in this experiment. The observed intensities of the rotational Raman lines for these states allow an estimate of γ_11,13 (355 nm) as 0.358 ± 0.004 ų. A comparison of this value to that obtained from fundamental ab initio calculations in H₂ also is possible for the first time.     

Theoretical study of ground and excited state potential energy surfaces for the Ca+-H2 complex

The potential energy surfaces of the Ca⁺-H₂ complex are calculated using the internally contracted multireference CI method (ICMR CI) and complete active space SCF (CAS SCF) reference wave functions. The calculations involve both the ground and the excited states correlating to (3d)²D and (4p)²P Ca⁺ terms and are carried out for C_∞v and C_2v configurations. Anisotropy of the potential surfaces has also been analysed by computing the interaction energy for some representative points as a function of the angle between the H₂ molecular axis and the Ca⁺—centre of mass of H₂ bond axis. The calculations have revealed the existence of a conical intersection of the lowest excited (3d)²B₂ potential surface with the ground state one. The obtained global energy minimum of the (3d)²B₂ potential surface lying 0.683 eV below the asymptote indicates a possible stabilization of the Ca⁺-H₂ complex towards formation of an exciplex in the (3d)Ca⁺-H₂(v = 0) collision process. The dependence of the vibrational energy levels of H₂ on the distance from Ca⁺ in the C_2v configuration has also been studied.     

The Stark effect in molecules with strong nuclear quadrupole coupling

The Hamiltonian for a rotating molecule containing one nuclear quadrupole in a uniform electric field is H ₀ + H_E + H_Q , where H ₀ describes the unperturbed rotator, H_E the interaction with the electric field and H_Q the coupling of the nuclear quadrupole to the rotating molecule. The energy is evaluated, up to and including terms in H_EH_Q in symmetric tops in which the quadrupolar nucleus is on the axis, and H_E ² H_Q in linear molecules, by treating H_E and H_Q as perturbations to H ₀. Dipole moments determined through studies of the Stark effect in microwave spectra can be significantly affected by the higher-order terms, even in weak electric fields, if the nuclear quadrupole coupling energy is not small relative to the separation of the rotational energy levels.

Dipole moments determined from Stark splittings of the J = 2←J = 1 lines are reduced by approximately 1 per cent in AsF₃ and from 1·647 to 1·618D in CH₃I.     

Electric field gradients created by near neighbour impurity atoms in a cubic silver host

The electric quadrupole interactions produced by near neighbour impurity atoms of Cu, Au, Zn, In, Sn and Bi on¹¹¹Cd probe nuclei in a cubic Ag lattice were studied by the TDPAC method. The effects of the type of impurity and its concentration have been investigated. The results show the presence of a high-frequency interactionv _Q ^h superimposed to a smeared out low frequencyv _Q ^l . The high frequency interaction, in the range 20 to 600 MHz, is attributed to impurity atoms located in nearest neighbour sites, while the low frequency interaction, in the range 2 to 12 MHz, is generated by impurities distributed at various different atomic distances from the probe nuclei. Bothv _Q ^h andv _Q ^l increase with impurity concentration leaving the ratiov _Q ^h /v _Q ^l almost constant. The results show that the high frequencyv _Q ^h is linearly dependent on the solute valence, and a logarithmic function of the impurity concentration, in the range 0.5 to 4.5 at. %. Large size effects have been observed in CuAg and BiAg alloys. Instead for ZnAg and SnAg, thev _Q ^h andv _Q ^l variation is attributed basically to charge effects.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de Estudos e Projetos (FINEP).     

PdCO分子结构与势能函数

用密度泛函理论的B3LYP方法,对钯原子采用LANL2DZ收缩价基函数,碳原子和氧原子采用AUG-cc-pVTZ基组,对PdC,PdO和PdCO体系的结构进行优化,计算表明：PdC分子基态为¹Σ⁺态,键长为R_e=0.17285nm,离解能为4.919eV.PdO分子基态的平衡核间距为0.18546nm,其电子态为³Π,离解能为2.455eV,并拟合得到Murrell-Sorbie势能函数;PdCO分子有两 关键词： PdCO 分子结构 势能函数     

Excitations in a quantum Hall ferromagnet with strong Coulomb interaction

A quantum Hall ferromagnet is considered at integer fillings ν, provided typical Coulomb interaction energy E _c is large compared to the cyclotron energy ω_H. Low-energy collective modes consist of a magnetoplasmon exciton and a gapless spin exciton. All charged excitations have a gap. The activation energy gap for a pair of charged topological excitations—skyrmion and antiskyrmion—is small, i.e., Δ< vω_H. The electric charge of a skyrmion is the multiple q=eνQ, where Q is the integer topological charge.     

O3+与H2碰撞中非解离电荷转移过程的全量子计算

应用全量子的分子轨道强耦合方法,研究了基态的O³⁺(2s²2p ²P)与氢分子碰撞的非解离电荷转移过程,计算了不同方位角(25°,45°,89°),能量分别为100,500,1000和5000eV/u时的单电子俘获的振动分辨的态选择截面及相应的微分截面.分子轨道强耦合计算中采用了自旋耦合价带理论计算的三原子分子势能面和径向耦合矩阵元.对氢分子的自身转动,采用无限阶的冲量近似方法;对体系的电子运动同H₂或H^{+< 关键词： 非解离电荷转移过程 全量子的分子轨道强耦合方法 无限阶冲量近似 振动冲量近似国家自然科学基金(批准号：10604011 10574020)和国家高技术研究发展计划(863)惯性约束聚变领域资助的课题.}     

Electron affinities of the bromine oxides BrOn,n = 1-4

Bromine oxides are of significant interest due to their importance in atmospheric chemistry. Density functional theory (DFT) methods have been used in conjunction with a DZP++ (double-ζ plus polarization with diffuse functions) basis set to study the molecular geometries and total energies of BrOn and BrO^- _n (n = 1-4). The adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vert) of the bromine oxide and the vertical detachment energy (VDE) of each anion are reported. Harmonic vibrational frequencies and zero point energies are also reported. Five different DFT methods were employed for comparison. Among these, the BHLYP method predicts the geometries and the vibrational frequencies in best agreement with available experimental data, while the the other methods do better in predicting the limited number of energetic quantities determined observationally. The predicted adiabatic electron affinities are 2.38 eV (BrO, experiment 2.35 eV), 2.36 eV (BrO₂), 3.35 eV (BrOO), 4.32 eV (BrO₃), 2.91 eV (BrOOO) and 5.28 eV (BrO₄). The electronic ground state of BrOO^- is a triplet (³A^") state. Predicted Br–;O bond distances range from 1.61 (BrO₃) to more than 2 Å for Br...O₂ for the neutral molecules; and from 1.61 Å for BrO^- to 1.82 Å for BrO^- to more than 2 Å for Br^-...O₃ among the anions. The BrOO isomer (C_s symmetry) is predicted to lie 17–18 kcal mol^-1 below the C_2v symmetry OBrO structure. The asymmetric ³A^" anion BrOO^- analogously lies below OBrO^-, in this case by 40–41 kcalmol_-1. BrOOO (C_s symmetry) is predicted to lie 42-45 kcalmol-1 below the symmetric C_3v BrO structure. Finally the asymmetric BrOOO- anion (C₁ symmetry) is predicted to lie 10-13kcal mol-1 below symmetric C_3v BrO^- ₃.     

Excited state positronium formation in low density gases

Excited state Ps atoms formed in low density Ne, Ar and H₂ gases have been observed for the first time. The maximum yield was estimated to be ≈ 5.7 × 10^?2 excited Ps atoms per stopped positron of energy ≈ 16 eV in H₂. This is about 14 times greater than previous maximum yields.