Total (elastic and inelastic) scattering cross sections for several positron-molecule systems at 10–5000 eV:H2,H2

Abstract

We report calculations on the total (elastic plus inelastic) positron-scattering cross sections from several diatomic and polyatomic molecules (H&, H20, NH3, CH4, N2, CO, C2H2, O2, SiH4, CO&, N20, and CF4) where experimental data are available. The impact energy (E) range is 10 -5000 eV. A local spherical complex optical potential (SCOP) is calculated for each positron-molecule system from the target charge density [p(r)], which in turn is determined from the corresponding molecular wave function at the Hartree-Fock level. The real part of the SCOP is composed of the repulsive static and attractive positron-correlation-polarization potential of Jain [Phys. Rev. A 39, 2437 (1990)]. The imaginary component of the SCOP, the so-called absorption potential, is derived semiempirically as a function of p(r), E, and the mean excitation energy. The resulting complex optical potential is treated exactly in a variable-phase approach to yield complex phase shift and the total-cross-section quantities. In this intermediateand high-energy region, the small contribution due to the nonspherical nature of the target is neglected. In addition, we fit the total-cross-section values to a simple analytic formula. For molecules possessing a permanent dipole or quadrupole moment, the present results are reliable only roughly above 50 eV. PACS number(s): 34.80.i, 34.90.+ q, 61.80.Fe

Total electron-scattering cross sections of pyrimidine

“…In the SCOP model, V opt ( r) is at first expanded around the center of mass of the molecule using symmetry-adapted functions [22]:…”

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

“…None of these calculations are correctly able to predict the rise in the total cross section values as the positron energy is decreased below the positronium formation threshold. The reason for this shortcoming in the work of Baluja and Jain [6] may be the use of a variation of electron absorption potential for positron scattering. The absorption potential used in the present work, Eqs.…”

“…The functions f 4 , f 6 , and f 8 are the higher-ᐉ contributions from the Born phase shifts for the dipole ͑ϳ1/r 4 ͒, quadrupole ͑ϳ1/r 6 ͒, and octopole ͑1/r 8 ͒ parts of the asymptotic polarization potential, respectively. The closed-form expressions for these functions are [20] FIG.…”

“…The elastic cross sections calculated by Gibson agreed reasonably well with the corresponding experimental data below the threshold for positronium formation. In the intermediate-energy range (from positronium formation threshold to about 1000 eV), positron-hydrogen molecule scattering cross sections have been calculated using a local spherical complex potential [6], using the additivity rule of the independent-atom model [4] and using the coupled-channel calculations [7]. None of these calculations are correctly able to predict the rise in the total cross section values as the positron energy is decreased below the positronium formation threshold.…”

“…Second, no good model absorption potential specifically designed for positron scattering has existed until only recently [5]. Having no viable option, previous positron-molecule collision calculations, using the model potential approach, were carried out either using model absorption potentials that were designed for electron scattering or modifying those electron absorption potentials in purely empirical ways [6].…”