Time-dependent R-matrix theory applied to two-photon double ionization of He

We introduce a time-dependent R-matrix theory generalized to describe double-ionization processes. The method is used to investigate two-photon double ionization of He by intense XUV laser radiation. We combine a detailed B-spline-based wave-function description in an extended inner region with a single-electron outer region containing channels representing both single ionization and double ionization. A comparison of wave-function densities for different box sizes demonstrates that the flow between the two regions is described with excellent accuracy. The obtained two-photon double-ionization cross sections are in excellent agreement with other cross sections available. Compared to calculations fully contained within a finite inner region, the present calculations can be propagated over the time it takes the slowest electron to reach the boundary.

Electron-induced hydrogen loss in uracil in a water cluster environment

Low-energy electron-impact hydrogen loss due to dissociative electron attachment (DEA) to the uracil and thymine molecules in a water cluster environment is investigated theoretically. Only the A'-resonance contribution, describing the near-threshold behavior of DEA, is incorporated. Calculations are based on the nonlocal complex potential theory and the multiple scattering theory, and are performed for a model target with basic properties of uracil and thymine, surrounded by five water molecules. The DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster. This growth is due to two effects: the increase of the resonance lifetime and the negative shift in the resonance position due to interaction of the intermediate negative ion with the surrounding water molecules. A similar effect was earlier found in DEA to chlorofluorocarbons. 

Similarity between Positronium-Atom and Electron-Atom Scattering

We employ the impulse approximation for a description of positronium-atom scattering. Our analysis and calculations of Ps-Kr and Ps-Ar collisions provide a theoretical explanation of the similarity between the cross sections for positronium scattering and electron scattering for a range of atomic and molecular targets observed by S. J. Brawley et al. [Science 330, 789 (2010)].

Positron scattering and annihilation on noble-gas atoms

Positron scattering and annihilation on noble-gas atoms is studied ab initio using many-body theory methods for positron energies below the positronium formation threshold. We show that in this energy range, the many-body theory yields accurate numerical results and provides a near-complete understanding of the positron–noble-gas atom system. It accounts for positron-atom and electron-positron correlations, including the polarization of the atom by the positron and the nonperturbative effect of virtual positronium formation. These correlations have a large influence on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter Zeff (effective number of electrons participating in annihilation) rise steeply along the sequence of noble-gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.

Positronium-atom scattering at low energies

A pseudopotential for positronium-atom interaction, based on electron-atom and positron-atom phase shifts, is constructed, and the phase shifts for Ps-Kr and Ps-Ar scattering are calculated. This approach allows us to extend the Ps-atom cross sections, obtained previously in the impulse approximation [I. I. Fabrikant and G. F. Gribakin, Phys. Rev. Lett. 112, 243201 (2014)], to energies below the Ps ionization threshold. Although experimental data are not available in this low-energy region, our results describe well the tendency of the measured cross sections to drop with decreasing velocity at v < 1 a.u. Our results show that the effect of the Ps-atom van der Waals interaction is weak compared to the polarization interaction in electron-atom and positron-atom scattering. As a result, the Ps scattering length for both Ar and Kr is positive, and the Ramsauer-Townsend minimum is not observed for Ps scattering from these targets. This makes Ps scattering quite different from electron scattering in the low-energy region, in contrast to the intermediate energy range from the Ps ionization threshold up to v ∼ 2 a.u., where the two are similar.

Rifampicin and sodium fusidate reduces the frequency of methicillin-resistant Staphylococcus aureus (MRSA) isolation in adults with cystic fibrosis and chronic MRSA infection

Nosocomial transmission of methicillin-resistant Staphylococcus aureus (MRSA) to patients with cystic fibrosis (CF) frequently results in chronic respiratory tract carriage. This is an increasing problem, adds to the burden of glycopeptide antibiotic use in hospitals, and represents a relative contraindication to lung transplantation. The aim of this study was to determine whether it is possible to eradicate MRSA with prolonged oral combination antibiotics, and whether this treatment is associated with improved clinical status. Adult CF patients (six male, one female) with chronic MRSA infection were treated for six months with rifampicin and sodium fusidate. Outcome data were examined for six months before treatment, on treatment and after treatment. The patients had a mean age of 29.3 (standard deviation=6.3) years and FEV(1) of 36.1% (standard deviation=12.7) predicted. The mean duration of MRSA isolation was 31 months. MRSA isolates identified in these patients was of the same lineage as the known endemic strain at the hospital when assessed by pulsed-field gel electrophoresis. Five of the seven had no evidence of MRSA during and for at least six months after rifampicin and sodium fusidate. The proportion of sputum samples positive for MRSA was lower during the six months of treatment (0.13) and after treatment (0.19) compared with before treatment (0.85) (P<0.0001). There was a reduction in the number of days of intravenous antibiotics per six months with 20.3+/-17.6 on treatment compared with 50.7 before treatment and 33.0 after treatment (P=0.02). There was no change in lung function. Gastrointestinal side effects occurred in three, but led to therapy cessation in only one patient. Despite the use of antibiotics with anti-staphylococcal activity for treatment of respiratory exacerbation, MRSA infection persists. MRSA can be eradicated from the sputum of patients with CF and chronic MRSA carriage by using rifampicin and sodium fusidate for six months. This finding was associated with a significant reduction in the duration of intravenous antibiotic treatment during therapy.

Ultracold, radiative charge transfer in hybrid Yb ion – Rb atom traps

Ultracold hybrid ion–atom traps offer the possibility of microscopic manipulation of quantum coherences in the gas using the ion as a probe. However, inelastic processes, particularly charge transfer can be a significant process of ion loss and has been measured experimentally for the ${\rm Y}{{{\rm b}}^{+}}$ ion immersed in a Rb vapour. We use first-principles quantum chemistry codes to obtain the potential energy curves and dipole moments for the lowest-lying energy states of this complex. Calculations for the radiative decay processes cross sections and rate coefficients are presented for the total decay processes; ${\rm Y}{{{\rm b}}^{+}}(6{\rm s}{{\;}^{2}}{\rm S})+{\rm Rb}(5{\rm s}{{\;}^{2}}{\rm S})\to {\rm Yb}(6{{{\rm s}}^{2}}{{\;}^{1}}{\rm S})+{\rm R}{{{\rm b}}^{+}}(4{{{\rm p}}^{6}}{{\;}^{1}}{\rm S})+h\nu $ and ${\rm Y}{{{\rm b}}^{+}}(6{\rm s}{{\;}^{2}}{\rm S})+{\rm Rb}(5{\rm s}{{\;}^{2}}{\rm S})\to {\rm YbR}{{{\rm b}}^{+}}({{X}^{1}}{{\Sigma }^{+}})+h\nu $. Comparing the semi-classical Langevin approximation with the quantum approach, we find it provides a very good estimate of the background at higher energies. The results demonstrate that radiative decay mechanisms are important over the energy and temperature region considered. In fact, the Langevin process of ion–atom collisions dominates cold ion–atom collisions. For spin-dependent processes [1] the anisotropic magnetic dipole–dipole interaction and the second-order spin–orbit coupling can play important roles, inducing coupling between the spin and the orbital motion. They measured the spin-relaxing collision rate to be approximately five orders of magnitude higher than the charge-exchange collision rate [1]. Regarding the measured radiative charge transfer collision rate, we find that our calculation is in very good agreement with experiment and with previous calculations. Nonetheless, we find no broad resonances features that might underly a strong isotope effect. In conclusion, we find, in agreement with previous theory that the isotope anomaly observed in experiment remains an open question.