We report quantum-mechanical close-coupling calculations for rotational state resolved cross sections for p-H 2 +HD collisions. The low temperature limit of p-H 2 +HD is investigated, which is of significant astrophysical interest in regard to the cooling of primordial gas and the interstellar media. Sharp resonances have been reproduced in the cross sections of some transition states at very low kinetic energies, E

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Very-low-energy scattering of orthopositronium by helium has been investigated for the simultaneous study of elastic cross section and pickoff quenching rate using a model exchange potential. The present calculational scheme, while it agrees with the measured cross section of Skalsey et al., reproduces successfully the parameter 1 Z eff , the effective number of electrons per atom in a singlet state relative to the positron. Together with the fact that this model potential also leads to an agreement with measured medium energy cross sections of this system, this study seems to resolve the long-standing discrepancy at low energies among different theoretical calculations and experimental measurements. ͓S1050-2947͑99͒09006-X͔ PACS number͑s͒: 34.10.ϩx, 36.10.DrStudies on positronium-͑Ps-͒ impact scattering have gained momentum these days due to the availability of the ortho-Ps beam in the laboratory and its vast applicational potential coupled with the present inadequate and inconclusive understanding of its interaction dynamics with matter ͓1͔. Ps scattering by neutral targets has posed a challenge to theoreticians on a proper accounting of experimental data as most existing theoretical works disagree with the major experimental trend.The discrepancy figures prominently in the Ps-He system where there are many theoretical and experimental studies. The medium-energy experimental cross section shows a declining trend with decreasing energy ͓2͔ from a peak around 20 eV for Ps-He scattering. A similar trend is also observed in Ps-H 2 and Ps-Ar systems ͓2,3͔. This trend, which is supported by the recent measurement of Skalsey et al. ͓4͔, could not be reproduced in most theoretical predictions ͓5-9͔. At low energies, these theories and experiments ͓2-4,10,11͔ on the Ps-He system are inconsistent with each other and also among themselves. For illustration, the zero-energy cross sections calculated on Ps-He by different authors vary from 3.3 Å 2 ͓12͔ to 16.54 Å 2 ͓13͔ while the measured values range from 2.3Ϯ0.4 Å 2 ͑at 0.915 eV͒ ͓4͔ to 11Ϯ3 Å 2 ͑between 0 and 0.3 eV͒ ͓10͔. Pointing out the very reactive nature of Ps scattering and its associated convergence difficulties, a prescription for the generation of a nonlocal model exchange potential has been advocated recently and applied successfully to different electron-impact ͑targets H, He͒ and Ps-impact ͑targets: H ͓14,15͔, He ͓16͔, H 2 ͓17͔, Ar, Ne ͓18͔͒ scattering problems using static exchange to three-Ps-state models. The three-Ps-state calculation for Ps-He predicts a lower zero-energy cross section of 2.42 Å 2 ͓16͔.In this work we shed light on the above-mentioned discrepancy in the Ps-He system in conjunction with a determination of the parameter 1 Z eff , which denotes the effective number of electrons per atom in a singlet state relative to the positron. The incident ortho-Ps(1s) atom in a triplet state with a lifetime of 142 ns can decay into three photons and is more stable than its para counterpart in a singlet state with a lifetime of 0.125 ns for a two-photo...

Low-energy muon-transfer cross sections and rates in collisions of muonic atoms with hydrogen isotopes are calculated using a six-state close-coupling approximation to coordinate-space Faddeev-Hahn-type equations. In the muonic case satisfactory results are obtained for all hydrogen isotopes and the experimentaly observed strong isotopic dependence of transfer rates is also reproduced. A comparison with results of other theoretical and available experimental works is presented. The present model also leads to good transfer cross sections in the well-understood problem of antihydrogen formation in antiproton-positronium collision.

We perform dynamical calculations of direct muon-transfer rates from thermalized muonic hydrogen isotopes to bare nuclei C 6ϩ and O 8ϩ . For these three-body charge-transfer reactions with Coulomb interaction in the final state we use two-component integrodifferential Faddeev-Hahn-type equations in configuration space using close-coupling approximation. To take into account the high polarizability of the muonic hydrogen due to the large charge of the incident nuclei, a polarization potential is included in the elastic channel. The large final-state Coulomb interaction is explicitly included in the transfer channel. The transfer rates so calculated are in good agreement with recent experiments.