Search Results (92 total)

Detailed calculations have been carried out for the direct electron-impact excitation and resonant excitation cross sections from the ground state to the individual magnetic sublevels of (2p_3∕2⁻¹3s_1∕2)₂ of highly charged ions of neonlike barium by using a fully relativistic distorted-wave method. The contributions of resonant excitations to the linear polarization of the magnetic quadrupole (M2) line [(2p_3∕2⁻¹3s_1∕2)₂-2p^{6  1}S₀] have been investigated systematically. It is found that the 4l5l^′, 4l6l^′, and 5l5l^′ resonances have significant depolarizing or enhancing effects on the linear polarization. These results agree very well with the experimental measurements by Takács [Phys. Rev. A 54, 1342 (1996)].

The charge transfer and excitation cross sections for protons colliding with Na(3s) atoms are calculated in the incident energy region from 1.7  eV  to  5  keV by a quantum-mechanical molecular-orbital close-coupling (QMOCC) method and from 1  to  100  keV by a two-center atomic-orbital close-coupling method. The present QMOCC charge transfer cross section for low energy (<40  eV) is in agreement with some other calculations, but disagrees with the experimental measurements [Z. Phys. A 313, 155, 1983]. For the higher energies, the cross sections calculated by the two methods are in harmony with the experimental and other theoretical data.

The higher spin levels in neutron-rich ¹⁰⁴Nb have been investigated from a study of the prompt γ rays in spontaneous fission of ²⁵²Cf with the Gammasphere detector array. A new collective band has been identified for the first time. This band is proposed as a semidecoupled band based on the configuration π5/2^-[303]⊗ν1/2^-[541]. The other structural characteristics are discussed.

Density functional theory calculations are carried out to investigate the electronic properties of molecular junctions formed by amine- and thiol-terminated alkane monolayers sandwiched between two metal (Au, Ag) electrodes. Based on extensive analysis of molecular monolayers of varying densities, we establish a relationship between the alignment of the molecular energy levels and the interface dipoles, which shows that the band alignment (BA) in the limit of long, isolated chains is independent of the link group and can be computed from a reference system of noninteracting molecule+metal molecule metal electrodes. The main difference between the amine and thiol linkers is the effective dipole moment at the contact. This is very large, about 4.5 D, for amine linkers, leading to a strong dependence of the BA on the monolayer density and a slow convergence to the isolated molecule limit. Instead, this convergence is relatively fast for S anchors due to the very small, ∼0.2 D, effective dipoles at the contacts.

The high-spin spectroscopy of neutron-rich ¹⁰⁹Ru is studied by measuring the prompt γ rays from the spontaneous fission fragments of ²⁵²Cf with 102 Compton-suppressed Ge detectors in the Gammasphere detector array. Previous band structures are confirmed and the ground state band is extended. A positive parity band based on a 332.5 keV level is newly identified. This band is proposed as a single-neutron excitation band built on the 7/2⁺[404] Nilsson orbital. Some band structural characteristics are discussed.

Collision dynamics of He²⁺+H(1s) ionization in a hot, dense (Debye) plasma is studied by the two-center atomic orbital close-coupling (AOCC) method in the energy range 5–300  keV∕u. The target AO basis contained all n≤6 bound states and 117 quasicontinuum states, while that on the projectile contained all n≤4 bound states. The eigenenergies and wave functions of all these states were obtained variationally as a function of the screening parameter in a Debye-Hückel potential and used in AOCC dynamics calculations. With increasing the screening, more and more discrete states enter the target and projectile continuum and increase the ionization cross section, particularly at low collision energies. The ionization to target continuum is the main ionization channel, while the ionization to the projectile continuum becomes important only for large screenings and collision energies below ∼50  keV∕u.

Collision dynamics of the He²⁺+H(1s) system imbedded in a Debye plasma is studied by the two-center atomic orbital close-coupling (AOCC) method in the energy range 5–300  keV∕u. The atomic orbitals and electron binding energies of atomic states are calculated within Debye-Hückel approximation of the screened Coulomb potential and used in AOCC dynamics formalism to calculate the state-selective electron capture and excitation cross sections. The basis contained 174 orbitals centered on the target (all n≤6 discrete states and 117 quasicontinuum states) and 20 orbitals centered on the projectile (all n≤4 discrete states). It is demonstrated that the screening of Coulomb interactions in the system progressively reduces the number of available excitation and electron capture channels when the screening parameter increases. The screening of Coulomb interactions introduces changes also in the values of direct and exchange couplings, thus affecting the magnitude and energy behavior of the cross sections. The control of dynamics of collision processes in a Debye plasma by varying the plasma screening of Coulomb interactions in the collision system is discussed.

The Spallation Neutron Source ring injection dump beam line has been suffering high beam losses since its commissioning. In order to understand the mechanisms of the beam losses, we have performed 3D simulation studies of the beam line. The 3D models consist of three injection chicane dipoles and one injection dump septum magnet. The 3D particle trajectories in the models are computed. We then extend particle optics calculations to the injection dump. Our studies have clearly shown some design and operation problems, which cause beam losses in the injection dump beam line. These include incorrect chicane dipole settings, incorrect position of a chicane dipole, too small aperture of injection dump septum, and inadequate focusing downstream. This paper reports our findings and the remedies to the injection beam loss problems.

The high-spin levels in the neutron-rich ¹⁴³La nucleus have been investigated from the study of prompt γ rays in the spontaneous fission of ²⁵²Cf with the Gammasphere detector array. A new level scheme with spin up to 27/2 ħ has been established, and the collective bands have been identified for the first time. A set of opposite-parity bands connected by ΔI=1E1 crossing transitions gives evidence for octupole correlations. The results of B(E1)/B(E2) branching ratios indicate that the octupole correlations in ¹⁴³La are strong. The reflection-asymmetry shell model is applied to calculate the levels of the octupole bands of ¹⁴³La, and the calculated results are in good agreement with the experimental ones.

The ionization process in collisions of He²⁺ with C^q+ (q=0–5) is investigated using a continuum-distorted-wave eikonal-initial-state approximation. Total and single- and double-differential cross sections for 1s and 2s electrons are calculated for projectile energies from 30  keV∕u  to  10  MeV∕u. From the double-differential cross sections at an electron-ejection angle of θ=0°, the variation of ionization mechanisms with q has been studied and their dependences on the projectile energies and the electron subshells are also discussed. It is found that in the whole energy range considered, the absolute values of soft collision (SC) and binary encounter (BE) peaks decrease with increasing q. For the lower incident energies, the electron capture to the projectile continuum (ECC) peak, as well as SC and BE peaks, decreases with increasing q. For the higher incident energies (>1  MeV∕u), the absolute value of the ECC peak increases with increasing q, so that the crossings of cross sections appear for C^q+ with different q. This can be explained by the matching of velocities between the projectile and the initially bound active electron in the target. Finally, by comparing the ionization rate coefficients for K-shell electrons of C^q+ by ion and electron impacts in an environment of inertial controlled fusion (ICF), the possibility to diagnose the He²⁺ distribution in ICF by the characteristic Kα spectrum of C^q+ is investigated.

The Spallation Neutron Source ring employs doublet quadrupoles and dipole correctors in its straight sections. The electromagnetic quadrupoles have a large aperture, small aspect ratio, and relatively short iron-to-iron distance. The corrector is even closer to one of the quads. There have been concerns on the magnetic fringe field and interference in the doublet magnets and their assemblies. We have performed 3D computing simulations to study magnetic field distributions in the doublet magnets. Further, we have analyzed the particle optics based on the z-dependent focusing functions of the quads. The effect of the magnetic fringe field and interference, including the third-order aberrations, on the particle motion are investigated. The lens parameters and the first-order hard edge models are derived and compared with the parameters used in the ring lattice calculations.

Charge transfer processes due to collisions of ground state Si³⁺(3s  ¹S) ions with atomic hydrogen are investigated using the quantum-mechanical molecular-orbital close-coupling (MOCC) and classical-trajectory Monte Carlo (CTMC) methods. The MOCC calculations utilize ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained from Herrero [J. Phys. B 29, 5583 (1996)] which were calculated with a full configuration-interaction method. Total and state-selective single-electron capture cross sections are obtained for collision energies from 0.01  eV∕u to 1  MeV∕u. Total and state-selective rate coefficients are also presented for temperatures from 2×10³  K to 10⁷  K. Comparison with existing data reveals that the total CTMC cross sections are in good agreement with the experimental measurements at the higher considered energies and that previous Landau-Zener calculations underestimate the total rate coefficients by a factor of up to two. The CTMC calculations of target ionization are presented for high energies.

The adhesion properties and catalytic activity of rutile TiO₂(110)-supported Au₇ nanoclusters in different oxidation states are investigated by means of density functional theory. The calculations cover both surface science conditions of reduced TiO₂ and real catalyst conditions of oxidized (alkaline) TiO₂ supports. Large adhesion energies of Au₇ are found only when modeling real catalysts where the cluster becomes cationic with Au⁺ ions in Au-O or Au-OH bonds. The full catalytic cycle for oxidation of CO by O₂ over Au₇ on alkaline TiO₂(110) is calculated and found to involve only small activation barriers. In the presence of the CO reductant, the Au⁺ sites are capable of cycling between bonding of atomic and molecular oxygen. We confirm our findings by comparison of calculated and experimental infrared stretch frequency data for adsorbed CO.

We have performed 3D computing simulations to study the magnetic field distribution of the injection chicane dipoles in the SNS accumulator ring. The simulations yield the performance characteristics of the magnets and generate the magnetic field data in three dimensional grids for further beam tracking study. Based on the simulation data, a 3D multipole expansion of the chicane dipole field, consisting of the generalized gradients and their derivatives, has been made. The harmonic and pseudoharmonic components in the expansion give much insight into the magnet physics and can fit directly into theoretical frame work of beam optics. The expansion is quasianalytical by fitting numeric data into interpolation functions. A 5th-order representation of the magnetic field is generated, and the effects of even higher-order terms on the field representation are discussed.

Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO₂ oxide structure forms at the steps of the Pt(332) surface after O₂ exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.

The ionization process due to collision of He²⁺ with C⁵⁺(1s) is investigated using a continuum-distorted-wave eikonal-initial-state approximation. Total, single-, and double-differential cross sections are presented for collision energies from 30  keV∕u to 10  MeV∕u. A good agreement is obtained between the current total cross section and the previous predictions. The ionization mechanism of electron capture to the projectile continuum (ECC) is found to be important in the lower collision energy range, whose contributions to the total cross section are evaluated to be approximately 12, 9, and 7 % for the projectile energies of 125  keV∕u,500  keV∕u, and 1  MeV∕u respectively. These large ECC contributions could be interpreted by the velocity matching between the projectile ion and the electron initially bound to the target.

Using the Oak Ridge National Laboratory ion-atom merged-beams apparatus, the absolute total electron-capture cross section has been measured for collisions of Ne⁴⁺ with hydrogen and deuterium at relative energies in the center-of-mass frame between 0.10 and 1006 eV/u. Comparison with previous measurements shows large discrepancies between 80 and 600 eV/u. For energies below ∼1  eV∕u, a sharply increasing cross section is attributed to the ion-induced dipole attraction between the reactants. Multichannel Landau-Zener calculations are performed between 0.01 and 5000 eV/u and compare well to the measured total cross sections. Below ∼5  eV∕u, the present total cross section calculations show a significant target isotope effect. At 0.01 eV/u, the H:D total cross section ratio is predicted to be ∼1.4 where capture is dominated by transitions into the Ne³⁺ (2s²2p²3d) configuration.

We report evidence for direct CP violation in the decay B⁰→K⁺π^- with 253  fb^-1 of data collected with the Belle detector at the KEKB e⁺e^- collider. Using 275×10⁶ BB̅ pairs we observe a B→K^±π^∓ signal with 2140±53 events. The measured CP violating asymmetry is A_CP(K⁺π^-)=-0.101±0.025(stat)±0.005(syst), corresponding to a significance of 3.9σ including systematics. We also search for CP violation in the decays B⁺→K⁺π⁰ and B⁺→π⁺π⁰. The measured CP violating asymmetries are A_CP(K⁺π⁰)=0.04±0.05(stat)±0.02(syst) and A_CP(π⁺π⁰)=-0.02±0.10(stat)±0.01(syst), corresponding to the intervals -0.05<A_CP(K⁺π⁰)<0.13 and -0.18<A_CP(π⁺π⁰)<0.14 at 90% confidence level.

We have searched for the lepton flavor violating decay τ^-→μ^-η using a data sample of 84.3   fb^-1 accumulated with the Belle detector at KEK. The η meson was detected through the decay modes: η→γγ and π⁺π^-π⁰. No signal candidates are found, and we obtain an upper limit for the branching fraction B(τ^-→μ^-η)<3.4×10^-7 at the 90% confidence level.

We report the results of a study of charged B decays to the D^±π^∓π^∓ and D^*±π^∓π^∓ final states using complete D^(*) reconstruction. The contributions of two-body B→D^**π decays with narrow (j=3/2) and broad (j=1/2) D^** states have been determined and the masses and widths of four D^** states have been measured. This is the first observation of the broad P-wave D₀^*0 and D′₁⁰ mesons. The analysis is based on a data sample of 65 million BB̅ pairs collected in the Belle experiment.

We report improved measurements of branching fractions for B→Kπ, π⁺π^-, π⁺π⁰, and KK̅ decays based on a data sample of 85.0 million BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB e⁺e^- storage ring. This data sample is almost three times larger than the sample previously used. We observe clear signals for B→Kπ, π⁺π^-, and π⁺π⁰ decays and set upper limits on B→KK̅ decays. The results can be used to give model-dependent constraints on the CKM angle φ₃, as well as limits on the hadronic uncertainty in the time-dependent analysis of the angle φ₂.

Charge transfer due to collisions of ground state O³⁺(2s²2p  ²P^o) ions with molecular hydrogen are investigated using the quantum-mechanical molecular-orbital (QMO) coupled-channel method. The QMO calculations utilize ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained with the spin-coupled valence-bond approach for a representative range of orientation angles and diatom internuclear separations. Vibrationally resolved cross sections for nondissociative single electron capture are obtained for energies between 0.1 eV∕u and 10 keV∕u for H₂ in its ground vibrational level using the infinite order sudden approximation (IOSA). Two further approximations are considered in which the electronic radial couplings are assumed to be independent of the diatom stretching. In the first case, vibrational motion is taken into account by multiplying the electronic radial couplings by Franck-Condon (FC) ionization factors while in the second, vibrational motion is completely neglected. We refer to these two approaches as the vibrational sudden approximation (VSA) and the electronic approximation (EA), respectively. In the latter, the resulting cross sections for electronic transitions are multiplied by FC factors to obtain relative vibrationally resolved cross sections which are independent of the collision energy (the centroid approximation). Comparison with existing experimental data for total and electronic state-selective cross sections shows best agreement with IOSA and VSA, but discrepancies for EA. The triplet-singlet electronic cross section ratio reveals a departure at low collision energies from the statistical value.

Using the Oak Ridge National Laboratory ion-atom merged-beam apparatus, absolute total electron-capture cross sections have been measured for collisions of Ne³⁺ ions with hydrogen (deuterium) atoms at energies between 0.07 and 826  eV∕u. Comparison to previous measurements shows large discrepancies between 50 and 400  eV∕u. Previously published molecular-orbital close-coupling (MOCC) calculations were performed over limited energy ranges, but show good agreement with the present measurements. Here MOCC calculations are presented for energies between 0.01 and 1000  eV∕u for collisions with both H and D. For energies below ∼1  eV∕u, an enhancement in the magnitude of both the experimental and theoretical cross sections is observed which is attributed to the ion-induced dipole attraction between the reactants. Below ∼4  eV∕u, the present calculations show a significant target isotope effect.

We have performed a search for the lepton-flavor-violating decay τ→μγ using a data sample of 86.3   fb^-1 accumulated by the Belle detector at KEK. No evidence for a signal is seen, and we set an upper limit for the branching fraction of B(τ→μγ)<3.1×10^-7 at the 90% confidence level.

We report the first observation of a b→u type charmless baryonic B decay, B⁺→pp̅ π⁺, as well as b→s type B⁰→pp̅ K⁰ and B⁺→pp̅ K^*+ decays. The analysis is based on a 78  fb^-1 data sample recorded on the Υ(4S) resonance with the Belle detector at KEKB. We find B(B⁺→pp̅ π⁺)=(3.06_-0.62^+0.73±0.37)×10^-6, B(B⁰→pp̅ K⁰)=(1.88_-0.60^+0.77±0.23)×10^-6, and B(B⁺→pp̅ K^*+)=(10.3_-2.8-1.7^+3.6+1.3)×10^-6. We also update B(B⁺→pp̅ K⁺)=(5.66_-0.57^+0.67±0.62)×10^-6 and present an upper limit on B(B⁰→pp̅ K^*0) at the 90% confidence level. A common feature of the observed decay modes is threshold peaking in baryon pair invariant mass.