Total (elastic+absorption) cross sections for e-CH4 collisions in a spherical model at 0.10-500 eV.

Abstract

Electron-${\mathrm{CH}}_{4}$ scattering is investigated in a wide energy range (0.1--500 eV) by using a previously proposed spherical model [A. Jain, J. Chem. Phys. 81, 724 (1984)]. The study is divided into three energy regions: the 0.1--1.0-eV region, where a well-known minimum occurs in the total cross section [Ramsauer-Townsend (RT) effect], between 2 and 20 eV, with a d-wave broad structure around 7--8 eV, and from 20 to 500 eV, where inelastic channels (mainly ionization and dissociation) dominate over the elastic process. It is shown that a simple model, in which the total optical complex potential is spherical, is capable of reproducing qualitative features in the total, differential, and momentum-transfer cross sections in the present energy region.The present results are almost equivalent to more rigorous close-coupling calculations at low energies (El20 eV), where the rotationally elastic channel dominates. Below 20 eV, the total optical potential is real and consists of three spherical terms, namely, a static term calculated accurately from near-Hartree-Fock one-center methane wave functions, a parameter-free polarization potential of Jain and Thompson (however, below 1 eV we employ a cutoff-type phenomenological polarization potential), and a local-exchange interaction in the asymptotically adjusted version of the Hara free-electron-gas exchange (AAHFEGE) potential (however, below 1 eV, we consider just the HFEGE form). At and above 20 eV, we employ a complex optical potential with the same real part as in the 2--20-eV region, while the imaginary part is an energy-dependent absorption potential calculated from target electron density and short-range static-exchange potential in the quasifree model with Pauli blocking [Staszewska et al., J. Phys. B 16, L281 (1983)].Two versions of this absorption potential are used; one with undistorted charge density and the other with a polarized density. The final complex total optical potential is treated exactly in a partial-wave analysis to yield various cross sections. The results are compared with the available experimental and other theoretical data. The absorption cross sections are not sensitive to various forms of real part of the optical potential, and in general, reduce the elastic cross sections significantly.