Low-energy positron scattering from singlet and triplet metastable He(
1s2s) is investigated using the
R-matrix propagation method in hyperspherical coordinates. Elastic and positronium-formation cross sections are reported, and near-threshold resonance structures are analyzed in terms of eigenphase sums and the time-delay matrix. For the triplet target, the calculated cross sections are in agreement with available convergent-close-coupling results and display the expected threshold behavior. Beyond the known
S-wave features, Feshbach resonance series in higher partial waves extending up to highly excited atomic thresholds are systematically identified. The time-delay matrix further uncovers hidden resonances that produce obvious structures in the positronium-formation cross sections.
We investigate ground-state positronium formation cross sections in - scattering using simplified second-order perturbative Born series. The second-order angle-integrated capture cross sections are also found to be in very good agreement with the experimental results of Fromme et al and Diana et al in the energy region . Present second-order differential cross sections show a qualitative difference over the first-order results and agree well with the recent measurement of Tang and Surko at 80 and 100 eV.
We investigate the elastic and inelastic scattering of positrons by sodium atoms in both the ground state, Na(
3s), and excited states, Na*(
3p,
4s,
3d), using the hyperspherical coordinate method with a model potential to represent the atomic core. The threshold behavior of positronium (Ps) formation cross sections is analyzed as the positron impact energy
E approaches zero. Within this framework, we derive a generalized expression for partial-wave Ps-formation cross sections at low positron energies, which applies to both ground-state and excited-state sodium targets. Our results confirm that the total threshold behavior follows the expected power-law dependence:
σPs∝E−1/2 for exothermic reactions and
σPs∝Ea for endothermic reactions, where
a>0. Furthermore, we find that Ps-formation cross sections for positron scattering from excited Na states are significantly larger than those from the ground state in the low-energy region. A notable enhancement of Gailitis-Damburg oscillations is observed above the Ps(
n=2) threshold, which may account for the increase observed in experimental data. Incorporating contributions from excited sodium targets improves agreement with experimental results and may help resolve discrepancies between theoretical predictions and measurements.
The cross section for antihydrogen formation in collision of an antiproton with ground-state positronium has been calculated using the adiabatic hidden-crossing method. At collision velocities below the cross section maximum (3 keV) it is shown that the adiabatic approach provides a very simple description of the positron transfer process, in that capture occurs by a two-state mechanism involving coupling between the 1sσ and 2pσ potential curves only. Good numerical agreement with previous estimates is obtained. 1.
The close-coupling equations for the positron-hydrogen system restricted to s-wave scattering from the l=0 levels of hydrogen and positronium are written as a set of coupled momentum space Lippmann-Schwinger equations. The matrix element for positronium formation is evaluated by integrating over the multipole projected components of the momentum space wave functions. Cross sections generated in the coupled static approximation provide a verification of the method. With a larger basis, including the 1s and 2s levels of hydrogen and the 1s and 2s levels of positronium, the broad s-wave resonance at 2.6 Ryd shifts position to 3.17 Ryd. An additional resonance is observed in the cross section at an energy of 1.10 Ryd.
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 $v\ensuremath{\sim}2$ a.u., where the two are similar.