J. M. Wadehra

A quasifree model for the absorption effects in positron scattering by atoms

Equations (1) and (4) of this letter, which essentially describe our model absorption potential, featured typographical errors. The equations should read as follows:

Scattering of low-energy electrons and positrons by atomic beryllium: Ramsauer–Townsend effect

Total cross sections for the scattering of low-energy electrons and positrons by atomic beryllium in the energy range below the first inelastic thresholds are calculated. A Ramsauer-Townsend minimum is seen in the electron scattering cross sections, while no such effect is found in the case of positron scattering. A minimum total cross section of 0.016 a.u. at 0.0029 eV is observed for the electron case. In the limit of zero energy, the cross sections yield a scattering length of -0.61 a.u. for electron and +13.8 a.u. for positron scattering.

Scattering of intermediate-energy positrons by C, N, O atoms and the corresponding diatomic molecules: elastic and total cross-sections

Elastic and total elastic plus absorption cross-sections for the scattering of positrons by carbon, nitrogen and oxygen . atoms and the corresponding diatomic molecules C , CN, CO, N , NO and O in the energy range from 100 to 5000 eV 22 2 are presented. Parameter-free interaction potentials along with the additivity rule are used in the calculations. Good agreement with the experimental data is obtained wherever such comparisons can be made. q 1999 Elsevier Science B.V. All rights reserved.

Scattering of low- to intermediate-energy positrons from molecular hydrogen

Using a complex model potential, we have calculated the total, integrated elastic, momentum transfer, absorption, and differential cross sections for positrons scattered from molecular hydrogen. The widely available software package GAUSSIAN is used to generate the radial electronic charge density of the molecule which is used to produce the interaction potentials. The quasifree absorption potential, previously developed and used for positron-atom scattering, is extended to positron scattering from molecular targets. It is shown that this model potential approach produces accurate results even into the low-energy regime.

Resonant contributions to dissociation of H2 by low-energy electron impact

A resonance theory of electron-impact vibrational excitation of diatomic molecules is extended to the case where the final vibrational level of the molecule lies in the continuum. The extended theory is applied to resonant dissociation of molecular hydrogen by low-energy electron impact. Theoretical cross sections for dissociation of ground-state H2 via the X 2 Sigma u+ and B 2 Sigma g+ resonances are presented and compared with theoretical cross sections obtained by other authors using non-resonant methods. An important aim of the present work is to study the effect of initial vibrational excitation on the dissociation cross section. It is found that the B 2 Sigma g+ resonance contributes significantly to the total dissociation cross section for all values of vi, for incident energies between 12 eV and 18 eV, while the effect of the X 2 Sigma u+ resonance becomes appreciable only for larger values of vi.

Evaluation of the second Born amplitude as a two-dimensional integral for H++H(1s) to H(1s)+H+

The second Born amplitude (with the exact propagator replaced by the free particle propagator) for the electron capture reaction H++H(1s) to H(1s)+H+ is reduced to a two-dimensional integral. The integrand has several singularities, which are dealt with. Results of the numerical integration of the cross section for various energies in the 25-200 keV range are presented.

Dissociative electron attachment to H2 molecules involving the 2Σg+ resonant Rydberg electronic state

Dissociative attachment process involving a resonant Rydberg-excited 2Σ+g electronic state of the H2− molecular ion: influence of the vibrational excitation of the target molecule and isotopic effect.

Differential cross-section surfaces for low-energy scattering of electrons and positrons from rare-gas atoms

The differential cross sections for scattering of electrons and positrons from He, Ne, Ar, Kr, and Xe at projectile energies below the inelastic thresholds are calculated using a model potential approach in which the interaction between the projectile and the target atom is partitioned into static, exchange (for electrons), and correlation-polarization parts. Two different forms of the parameter-free correlation-polarization potential are suggested; in both cases the correlation-polarization potential is determined by smoothly matching the asymptotic form of the polarization potential (∼1/r4) to the correlation potential at the outermost orbital radius of the target atom. The results of angular distributions are presented in the form of contours of constant differential cross sections as well as in the form of differential cross section surfaces in three-dimensional plots. Both of these presentations display the locations of the principal maxima and minima of the differential cross sections as well as the critical points in a very useful manner.

The second Born contribution of long-range forces to higher partial-wave phaseshifts

An exact analytic evaluation of the second Born contribution of the long-range potentials, which fall off as r-n as r to infinity , to the phaseshifts of higher partial waves (2l>2n-5) is presented. This expression agrees, for n = 4, with the second term in the energy expansion of the phaseshifts obtained previously by Ali and Fraser (1977). The expression can be used for predicting higher partial-wave phaseshifts as well as for determining the phaseshifts from experimental scattering data.