arXiv subjects

P. K. Biswas

Publications and source records attributed to P. K. Biswas.

95 records · Page 6Linked to original sources

arXiv2000

Positronium atom scattering by H_2 in a coupled-channel framework

The scattering of ortho positronium (Ps) by H$_2$ has been investigated using a three-Ps-state [Ps(1s,2s,2p)H$_2($X $^1Σ_g^+$)] coupled-channel model and using Born approximation for higher excitations and ionization of Ps and B $^1Σ_u^+$ and b $^3Σ_u^+$ 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 target-elastic and all target-inelastic processes. The total cross sections at low and medium energies are in good agreement with experiment.

physics.atm-clus↗

arXiv2000

Scattering of positronium by H, He, Ne, and Ar

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 time-reversal-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.

physics.atom-ph↗

arXiv1999

Positronium-hydrogen-atom scattering in a five-state model

The scattering of ortho-positronium (Ps) by hydrogen atoms has been investigated in a five-state coupled-channel 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 ($n\ge 3$) 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 -1.05 eV which compares well with the accurate value of -1.067 eV. The model leads to a singlet scattering length of 3.72$a_0$ and effective range of 1.67$a_0$, whereas the correlations suggest the precise values of $3.50a_0$ and 1.65$a_0$ for these observables, respectively.

physics.atm-clus↗

arXiv1999

Electron exchange model potential: Application to positronium-helium scattering

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 ortho positronium (Ps) by helium. The elastic scattering and the $n = 2$ excitations of Ps are investigated using a three-Ps-state close-coupling approximation. The higher ($n\ge 3$) excitations and ionization of Ps atom are treated in the framework of 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.

physics.atm-clus↗

arXiv1999

Low-energy quenching of positronium by helium

Very low-energy scattering of orthopositronium by helium has been investigated for simultaneous study of elastic cross section and pick-off quenching rate using a model exchange potential. The present calculational scheme, while agrees with the measured cross section of Skalsey et al, reproduces successfully the parameter $^ 1Z_{\makebox{eff}}$, 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.

physics.atm-clus↗