Madhumita Basu

Positron-hydrogen scattering at intermediate energies using the close-coupling approximation

Two expansion schemes: (a) H(1s), H(2s), H(2p), Ps(1s) and (b) H(1s), H(2s), H(2p), Ps(1s) are employed to investigate e+-H scattering using the close-coupling approximation at intermediate energies. Results for elastic, 1s-2s and 1s-2p excitation processes are reported in the energy range 54.4-200 eV. The angular correlation parameters lambda , R and I are also calculated. Results are compared with elaborate theoretical predictions. The effects of the capture channel are found to influence the scattering parameters.

Positron-hydrogen scattering including short- and long-range correlations

An integral form of the close-coupling approximation has been employed to investigate positron-hydrogen scattering. Two coupling schemes (1s-2s-2p-Ps and 1s-2s-2p-Ps) are employed and the present method includes the short- and long-range correlations partially. The calculations have been carried out for low (<or=3) partial waves and up to the incident energy 10.0 eV. The effect of the positronium formation channel is found to be significant in the energy region considered. Low-order elastic phaseshifts and ground-state capture cross sections are found to be very encouraging. The model proves to be a systematic way to attack the e+-H system.

Positron-hydrogen scattering below the first excitation threshold

A five-state (H(1s, 2s, 2p, 3d), Ps(1s)) CCA has been employed to study positron-hydrogen scattering in the energy range k=0.1 to 0.85 au. The elastic d-wave phaseshifts are found to be in excellent agreement with those of existing reliable predictions. The effect of the positronium formation channel is found to be significant. The differential and total cross sections are reported by combining the present CCA results (l)2) and nearly exact numbers obtained using variational methods.

Positronium formation in the n=2 states in e+-H2 scattering

The n=2 excited state capture cross sections in e+-H2 scattering have been calculated using the first Born approximation. The integrated and differential capture cross sections are predicted in the energy range 30-1000 eV. The total Ps formation cross sections ( sigma TPs= sigma rgPs+ Sigma 2sPs+ sigma 2pPs) are found to be in good agreement with the measured values in the energy range 50-150 eV.

Ionisation of a lithium atom by positron impact

The total ionisation cross sections of the lithium atom by positron impact are calculated by using two distorted wave models. The lithium atom is considered to be a one-electron atom. The results using the first Born approximation (FBA) have also been reported. The distorted wave results differ very little from those of the FBA.

The sensitivity of the final-state wavefunction in positron impact ionisation

The effect of the choice of final state wavefunction of the triple differential cross section (TDCS) for ionisation of hydrogen and helium atoms by positron impact has been studied. Three models for the wavefunction of the final state have been investigated. The result for electron impact ionisation have been included for relative comparison. The TDCS for positron impact ionisation using these models have been reported for the first time. The choice of a double Coulomb wavefunction for the final channel may be a good choice at high incident energy for asymmetric (Ehrhardt) geometry. A dramatic difference between the results of the first Born and double Coulomb approximations has been found for positron impact ionisation near threshold energies.

Total ionization cross section in electron-hydrogen scattering

The total ionization cross section in electron-hydrogen scattering in the energy range 20.4–68.0 eV has been calculated by a method in which the initial state of the system is treated by a distorted wave polarized orbital method and the final state is described by (1) a product of two unscreened Coulomb functions and (2) a product of a plane wave and a Coulomb function. The corresponding two sets of results using a plane wave in the incident channel have also been reported. The present results where both the electrons are represented by Coulomb waves are in closer agreement than other theoretical predictions With measured values.

Low energy positron hydrogen atom scattering using CCA

The article reports low order phase shifts using CCA with different coupling schemes. The phase shifts are found to be systematically improved with the increase of target states. The effect of Ps formation is found to be significant in the energy range considered. The CCA predictions including the rearrangement channel are found to be encouraging.

Resonance just above the Ps-formation threshold in positron-hydrogen scattering

The close-coupling approximation using five basis sets: (a) H(1s)+Ps(1s), (b) H(1s,2s)+Ps(1s), (c) H(1s,2s)+Ps(1s,2s), (d) H(1s,2s,2p)+Ps(1s,2s) and (e) H(1s,2s,2p)+Ps(1s,2s,2p) has been employed to investigate positron-hydrogen scattering below the first excitation threshold. The present basis sets (d) and (e) have confirmed the existence of two consecutive resonances just above the Ps-formation threshold, as obtained by Basu et al. (1989) using four-state (H(1s,2s,2p)+Ps(1s)) CCA, in the elastic channel.

The effect of exchange and dressing in laser-assisted e--H scattering

In the framework of first-order theory, differential cross sections for e--H scattering in the presence of a laser field and have been calculated. In the calculation, the dressed wavefunction of Byron et al. (1984). The effect of exchange was taken into account explicitly. Both the dressing and the exchange effects are sensitive to the direction of the laser field. In a sufficiently intense laser field, the effect of exchange is virtually absent, i.e. the incident electron seems to be distinguishable from the target electron.