N. C. Sil

Positron-atom and positron-molecule collisions

Abstract The scattering of positrons by atoms and molecules is reviewed. The recent development of monoenergetic slow positron beams and their applications to the measurement of cross sections for positrons interacting in various gases are discussed briefly. The theoretical development of positron collisions in gases is covered from the beginning. Both elastic and the inelastic processes have been included. Comparisons are made between the measured values and theoretical predictions. The deficiencies of the theoretical models and experimental apparatus are also pointed out.

Positronium formation in positron scattering from hydrogen and helium atoms: the distorted-wave approximation

The distorted-wave approximation is applied to investigate the rearrangement collision problem of positronium formation in the ground state with positron scattering from hydrogen and helium atoms for the positron energy ranges 8.704-200 eV and 20-100 eV respectively.

Electron impact excitation of positive ions in dense plasma

The authors present a generalised method for the calculations of inelastic collision strengths for scattering of electrons by ions in dense plasma taking account of the shielding of the Coulomb potential by the Debye-Huckel model. Matrix elements between arbitrary initial and final Slater orbitals are obtained in closed form. Results for the 1s-2p transition in the He+ ion due to Hatton et al. (1981) have been reproduced and new results have been presented for 1s-3d and 1s-4f transitions.

e+-He scattering using an integral form of the close-coupling approximation

An integral form of the close-coupling approximation by Ghosh and Basu (1973) and Chaudhuri at al. (1974) has been employed to investigate the processes e++He(1s2) to e++He(2s2) and e++He(1s2) to (e+e-)+He(1s). Couplings of the excited states of both the helium and positronium atoms have been neglected. Results obtained by using an approximate form of the close-coupling approximation which neglects some strong distortion effects are presented. The results in two-state close-coupling approximation do not show good agreement with the experimental findings below the positronium formation threshold. The effect of exchange of electrons is found to be appreciable in the low-energy region.

Ionization of atomic hydrogen by positron impact

A final-state wavefunction, involving three appropriate confluent hypergeometric functions, which satisfies the correct asymptotic condition is used to study positron-impact ionization of atomic hydrogen. Results are reported for the double as well as triple differential cross sections and compared with the available theoretical findings. The double differential cross section curves for different ejection angles show the presence of a cusp-like structure at the region of equal energy sharing by the outgoing positron and the ejected electron. This is in conformity with the findings of Mandal et al. (1986) at zero ejection angle but unlike the structure exhibited by the curves of Schultz and Reinhold (1990).

Electron impact excitation of hydrogenic ions to arbitrary s states including exchange effects

A recently proposed generalised method for calculating Coulomb-Born cross sections for 1s-ns excitation of hydrogenic ions by electron impact has been extended to include the effect of electron exchange. Asymptotic scaled cross sections for n to infinity have also been studied. Some preliminary results for He+ are presented for n=2, 3, 6 and infinity .

Integrals involving products of Coulomb functions and inverse powers of the radial coordinate

Integrals of the type integral Rinfinity r-nF1(r)F2(r) dr, where F1 and F2 are Coulomb functions, are required for various problems in atomic physics. Methods are described for the evaluation of such integrals; they involve deformation of the integration contours into the complex plane.

Analytical expression for higher-order derivatives of the Lewis integral with its application to proton - hydrogen charge transfer collisions

We present a method for evaluating arbitrary higher-order derivatives of the three-denominator Lewis integral frequently encountered in collisional and structural problems of atomic and molecular physics. The method has been applied successfully to obtain analytical expressions for the charge transfer amplitude in proton - hydrogen collisions and the results for total cross sections in Jackson - Schiff (JS) and distorted-wave approximations (DWB) are presented.

Electron Capture by Protons Passing through Molecular Hydrogen

The cross section for the process p+H2=H+H2+ is calculated from low to moderate energies using the impact‐parameter formulation with a two‐state approximation. The experimentally measured cross section in this energy range shows a rise and fall with increasing energy of the incident proton; our theoretical calculation reproduces the same behavior with the maximum position slightly shifted towards the lower energy.

Ionisation of Atomic Hydrogen Under Heavy Particle Impact

Ionisation of hydrogen atoms in ground and metastable 2s states by impact of protons and antiprotons has been studied at collision energies varying from 10 to 1566keV/amu. Total and double differential cross sections are calculated employing a final continuum state wave function which incorporates distortion due to the Coulomb fields of both the projectile and the target nuclei. The present calculated results are compared with the existing experimental findings as well as other theoretical predictions.