The formulation of a suitable nonlocal model potential for electron exchange is presented, checked with electron-hydrogen and electron-helium scattering, and applied to the study of elastic and inelastic scattering and ionization of orthopositronium ͑Ps͒ by helium. The elastic scattering and the nϭ2 excitations of Ps are investigated using a three-Ps-state close-coupling approximation. The higher (nу3) excitations and ionization of Ps atoms are treated in the framework of the Born approximation with present exchange. Calculations are reported of phase shifts and elastic, Ps excitation, and total cross sections. The present target elastic total cross section agrees well with experimental results at thermal to medium energies.

The scattering of orthopositronium ͑Ps͒ by hydrogen atoms has been investigated in a five-state coupledchannel model allowing for Ps(1s)H(2s,2p) and Ps(2s,2p)H(1s) excitations using a recently proposed electron-exchange model potential. The higher (nу3) excitations and ionization of the Ps atom are calculated using the first Born approximation. Calculations are reported of scattering lengths, phase shifts, elastic, Ps and H excitation, and total cross sections. Remarkable correlations are observed between the S-wave Ps-H binding energy and the singlet scattering length, effective range, and resonance energy obtained in various model calculations. These correlations suggest that if a Ps-H dynamical model yields the correct result for one of these four observables, it is expected to lead to the correct result for the other three. The present model, which is constructed so as to reproduce the Ps-H resonance at 4.01 eV, automatically yields a Ps-H bound state at Ϫ1.05 eV that compares well with the accurate value of Ϫ1.067 eV. The model leads to a singlet scattering length of 3.72a 0 and effective range of 1.67a 0 , whereas the correlations suggest the precise values of 3.50a 0 and 1.65a 0 for these observables, respectively.

Understanding how a tiny dilute evaporative colloidal spray droplet gets transformed into a microgranule with a characteristic morphology is crucial from scientific as well as technological points of view. In the present work, it is demonstrated that the morphology and the size distribution of the microcapsules can be tuned simply by adjusting the drying temperature. Shape and size of the capsules are quantified at four different drying temperatures. It is shown that the morphology transits gradually from sphere to toroid with increasing temperature keeping the average volume-fraction of the correlated nanoparticles nearly unaffected for the synthesized granules. A plausible mechanism for the chronological pathway of such morphological transformation is illustrated. Computer simulation corroborates the experimentally observed morphological transition. The variation in hollowness and buckling tendency of the capsules are elucidated by scattering and imaging techniques.

The low-energy scattering of ortho positronium (Ps) by H, He, Ne, and Ar atoms has been investigated in the coupled-channel framework by using a recently proposed timereversal-symmetric nonlocal electron-exchange model potential with a single parameter C. For H and He we use a three-Ps-state coupled-channel model and for Ar and Ne we use a static-exchange model. The sensitivity of the results is studied with respect to the parameter C. Present low-energy cross sections for He, Ne and Ar are in good agreement with experiment.

The scattering of ortho positronium (Ps) by H 2 has been investigated using a three-Ps-state [Ps(1s,2s,2p)H 2 (X 1 Σ + g )] coupled-channel model and using Born approximation for higher excitations and ionization of Ps and B 1 Σ + u and b 3 Σ + u excitations of H 2 . We employ a recently proposed time-reversal-symmetric nonlocal electron-exchange model potential. We present a calculational scheme for solving the body-frame fixed-nuclei coupled-channel scattering equations for Ps-H 2 , which simplifies the numerical solution technique considerably. Ps ionization is found to have the leading contribution to targetelastic and all target-inelastic processes. The total cross sections at low and medium energies are in good agreement with experiment.

We perform a three-positronium (Ps) state [Ps(1s, 2s, 2p)] coupled-channel calculation of Ps-H 2 scattering including the effect of electron exchange. At medium energies, higher excitations and ionization of Ps are treated within the framework of the first Born approximation. In both cases exchange is included using a recently proposed nonlocal model exchange potential which is free of non-orthogonality problems common in the usual antisymmetrization scheme. The present total cross sections at low and medium energies are in encouraging agreement with experiment.

Calculation for the electronic excitation of the ground state of H 2 to B 1 + u and b 3 + u states by positronium-(Ps) atom impact has been carried out using the first Born approximation considering discrete Ps excitations up to n = 6 and Ps ionization in the final state. To include the effect of electron exchange, we propose an alternative approximation scheme in the light of the Rudge approach, which takes into account the composite nature of the Ps-atom projectile.

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...