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.
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