Ultracold radiative charge transfer in hybrid ion-atom traps

We present ab initio quantum chemistry calculations for elastic scattering and the radiative charge transfer reaction process and collision rates for trapped ytterbium ions immersed in a quantum degenerate rubidium vapor.
The collision of the ion (or ions) with the quasiatom is the key mechanism to transfer quantum coherences between the systems. We use first-principles
quantum chemistry codes to obtain the potential surfaces and coupling terms for the two-body interaction of Yb^+ with Rb. We find that the low energy collision has an inelastic radiative charge transfer process in agreement with recent experiments.
The charge transfer cross section agrees well with the semiclassical Langevin model at higher energies but is dominated by resonances at submillikelvin temperatures.

Photoionization cross sections for the trans-iron element Se + from 18 to 31 eV

Absolute photoionization cross-section calculations are presented for Se + using large-scale close-coupling calculations within the Breit--Pauli and Dirac--Coulomb R -matrix approximations. The results from our theoretical work are compared with recent measurements (Esteves 2010 PhD Thesis publication number AAI3404727, University of Reno, NV, USA; Sterling et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 025701; Esteves et al 2011 Phys. Rev. A 84 013406) made at the advanced light source (ALS) radiation facility in Berkeley, CA, USA. We report on results for the photon energy range 18.0--31.0 eV, which spans the ionization thresholds of the 4 S o 3/2 ground state and the low-lying 2 D o 5/2,3/2 and 2 P o 3/2,1/2 metastable states. Metastable fractions are inferred from our present work. Resonance energies and quantum defects of the prominent Rydberg resonances series identified in the spectra are compared for the 4p → n d transitions with the recent ALS experimental measurements made on this complex trans-iron element.

Electron-impact ionization of diatomic molecules using a configuration-average distorted-wave method

Electron-impact ionization cross sections for diatomic molecules are calculated in a configuration-average distorted-wave method. Core bound orbitals for the molecular ion are calculated using a single-configuration self-consistent-field method based on a linear combination of Slater-type orbitals. The core bound orbitals are then transformed onto a two-dimensional (r,θ) numerical lattice from which a Hartree potential with local exchange is constructed. The single-particle Schrödinger equation is then solved for the valence bound orbital and continuum distorted-wave orbitals with S-matrix boundary conditions. Total cross section results for H2 and N2 are compared with those from semiempirical calculations and experimental measurements.

Experimental and theoretical photoionization cross sections for Se + from 18 eV to 31 eV

Absolute Se photoionization cross-section measurements and Dirac-Coulomb R -matrix calculations are reported for the photon energy range 18.0 eV – 31.0 eV, which spans the ionization thresholds of the 4 S 0 3/2 ground state and the low-lying 2 D 0 3/2,5/2 and 2 P 0 1/2,3/2 metastable states. The determination of the photoionization and recombination properties of n -capture element ions is motivated by their astrophysical detection and the importance of their elemental abundances in testing theories of nucleosynthesis and stellar structure.

Single-photon single ionization of W+ ions: experiment and theory

Experimental and theoretical results are reported for photoionization of Ta-like (W+) tungsten ions. Absolute cross sections were measured in the energy range 16–245 eV employing the photon–ion merged-beam setup at the advanced light source in Berkeley. Detailed photon-energy scans at 100 meV bandwidth were performed in the 16–108 eV range. In addition, the cross section was scanned at 50 meV resolution in regions where fine resonance structures could be observed. Theoretical results were obtained from a Dirac–Coulomb R-matrix approach. Photoionization cross section calculations were performed for singly ionized atomic tungsten ions in their 5s²5p⁶5d⁴(⁵D)6s ⁶Dj.  J = 1/2, ground level and the associated excited metastable levels with J = 3/2, 5/2, 7/2 and 9/2. Since the ion beams used in the experiments must be expected to contain long-lived excited states also from excited configurations, additional cross-section calculations were performed for the second-lowest term, 5d⁵⁶Sj, J = 5/2, and for the 4F term, 5d³6s^{2 4}Fj, with J = 3/2, 5/2, 7/2 and 9/2. Given the complexity of the electronic structure of W+ the calculations reproduce the main features of the experimental cross section quite well.

Ionization of Atoms by Slow Heavy Particles, Including Dark Matter

Atoms and molecules can become ionized during the scattering of a slow, heavy particle off a bound electron. Such an interaction involving leptophilic weakly interacting massive particles (WIMPs) is a promising possible explanation for the anomalous 9σ annual modulation in the DAMA dark matter direct detection experiment [R. Bernabei et al., Eur. Phys. J. C 73, 2648 (2013)]. We demonstrate the applicability of the Born approximation for such an interaction by showing its equivalence to the semiclassical adiabatic treatment of atomic ionization by slow-moving WIMPs. Conventional wisdom has it that the ionization probability for such a process should be exponentially small. We show, however, that due to nonanalytic, cusplike behavior of Coulomb functions close to the nucleus this suppression is removed, leading to an effective atomic structure enhancement. We also show that electron relativistic effects actually give the dominant contribution to such a process, enhancing the differential cross section by up to 1000 times.

Comparison of DES predictions of turbulent flow and heat transfer in a 45° ribbed duct

Numerical predictions of the turbulent flow and heat transfer of a stationary duct with square ribs 45° angled to the main flow direction are presented. The rib height to channel hydraulic diameter is 0.1, the rib pitch to rib height is 10. The calculations have been carried out for a bulk Reynolds number of 50,000. The flows generated by ribs are dominated by separating and reattaching shear layers with vortex shedding and secondary flows in the cross-section. The hybrid RANS-LES approach is adopted to simulate such flows at a reasonable computation cost. The capability of the various versions of DES method, depending the RANS model, such as DES-SA, DES-RKE, DES-SST, have been compared and validated against the experiment. The significant effect of RANS model on the accuracy of the DES prediction has been shown. The DES-SST method, which was able to reproduce the correct physics of flow and heat transfer in a ribbed duct showed better performance than others.

The eigenphase formulation of fine-structure transition cross sections

The eigenphase formulation of Blatt and Biedenharn is applied to fine-structure transitions in *P atoms colliding with ‘S perturbers. Consideration is given to the limit of weak spin-orbit interaction. If the eigenphases are equal to the phaseshifts for elastic scattering by the molecular potentials then the expression for the total cross section reduces to the expression derived in the elastic approximation. However, a numerical comparison for the Li(2p ’P) + He(’S) system shows that the elastic molecular phaseshifts are not good approximations to the eigenphases. Hence the elastic approximation cannot be reliable.

Particle Image Velocimetry Investigation of a finite amplitude pressure wave

Particle image velocimetry is used to study the motion of gas within a duct subject to the passage of a finite amplitude pressure wave. The wave is representative of the pressure waves found in the exhaust systems of internal combustion engines. Gas particles are accelerated from stationary to 150 m/s and then back to stationary in 8 ms. It is demonstrated that gas particles at the head of the wave travel at the same velocity across the duct cross section at a given point in time. Towards the tail of the wave viscous effects are plainly evident causing the flow profile to tend towards parabolic. However, the instantaneous mean particle velocity across the section is shown to match well with the velocity calculated from a corresponding measured pressure history using 1D gas dynamic theory. The measured pressure history at a point in the duct was acquired using a high speed pressure transducer of the type typically used for engine research in intake and exhaust systems. It is demonstrated that these are unable to follow the rapid changes in pressure accurately and that they are prone to resonate under certain circumstances.

On the observation of the fine structure effect in non- relativistic (e, 2e) processes

The spin asymmetry arising in an (e,2e) process using spin- polarized incoming electrons with non-relativistic energies is shown to be dominated by the fine structure effect if a suitable kinematical regime is chosen. Calculations in the distorted wave Born approximation (DWBA) for both the triple differential cross-section and the spin asymmetry are presented for the inner shell ionization of argon. This process would provide an accessible target for existing experimental set-ups.

Ionization from the outer shell of Ar by proton impact

Recent results for proton-argon total ionization cross sections [Kirchner Phys. Rev. Lett. 79, 1658 (1997)] show large disagreement between theory and experiment for energies below 80 keV. To address this problem we have employed a recently developed theoretical method with a more pragmatic approach to the charge screening both in the initial and final channels. The target is considered as a one-electron atom and the interactions between this active electron and remaining target electrons are treated by a model potential including both short- and long-range effects. In the final channel the usual product of two continuum distorted wave functions each associated with a distinct electron-nucleus interaction is used. New results in the present calculation show good agreement in total cross sections for the energy range 10-300 keV with the measurement of Rudd [Rev. Mod. Phys. 57, 965 (1985)].

Electron-impact ionization of atomic hydrogen close to threshold

A systematic study of the ionization of atomic hydrogen by electron impact from 0.3 eV to a few eV above the ionization threshold has been carried out using a semiclassical-quantal calculation. Differential and integrated cross sections are presented at 0.3 eV above the energy threshold. Triple- differential cross sections (TDCS) are presented at constant theta(12) geometry where theta(12)=180degrees and 150degrees. Good agreement is achieved with the measurement [Roder, Phys. Rev. Lett. 79, 1666 (1997)] and calculations based on exterior complex scaling at 2 eV and 4 eV above threshold. Results of triple-differential cross sections are also presented at 0.3, 0.5, and 1.0 eV above threshold at both theta(12)=180degrees and 150degrees. At theta(12)=180degrees the small local maximum in the TDCS around theta(1)=90degrees reported by Pan and Starace [Phys. Rev. A 45, 4588 (1992)] at 0.5 eV above threshold is not observed in our calculation at energies down to 0.3 eV above threshold. The shape of our double differential cross sections seems to disagree qualitatively with the available calculations as we found two local maxima around 15degrees and 165degrees in our calculation. Single differential cross sections in our formulation appear naturally as a function of total excess energy E and, therefore, constant for all combinations of individual electron energies E-1 and E- 2 with E=E-1+E-2. Total ionization cross sections are also compared with measurement and available theoretical calculations and found to be in reasonably good agreement up to 10 eV above ionization threshold.

Electron impact ionization of hydrogen-like molybdenum ions

The electron impact ionization cross sections of hydrogen-like molybdenum ions were measured with an electron beam ion trap at the electron energies of 49.4, 64.4 and 79.6 keV The results are 2.82(22) x 10(-23), 3.13(29) x 10(-23) and 3.23(51) x 10(-23) cm(2), respectively. These results are compared with the experimental results measured previously. The agreement with the results obtained with, scaling formulae is also discussed.

Electron-impact ionization of hydrogen-like iron ions

Electron-impact ionization cross sections have been determined for hydrogen like iron ions at selected electron energies between 1.45 and 4.3 times the threshold energy. The cross sections were obtained by measuring the equilibrium ionization balance in an electron beam ion trap. This ionization balance is obtained from x-ray measurements of radiative recombination into the K-shell of hydrogen-like and bare iron ions. The measured cross sections are compared with distorted-wave calculations and several semiempirical formulations.

Measurement of charge exchange and X-ray emission cross sections for solar wind-comet interactions

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.