Electron-impact study of the S2molecule using theR-matrix method

Abstract

The present study deals with the calculation of elastic [integrated and differential cross section (DCS)], momentum-transfer, excitation, and ionization cross sections for electron impact on S${}_{2}$ molecules using the R-matrix method. The target states are represented by including correlations via a configuration-interaction technique. We used a double zeta plus polarization Gaussian basis set contracted as (12,8,1)/(6,4,1) for S atoms. The results of the static exchange, correlated one-state, and 20-state close-coupling approximations are presented. We have detected a stable anionic bound state ${}^{2}{\ensuremath{\Pi}}_{g}$ of S${}_{2}$${}^{\ensuremath{-}}$ having the configuration $1{\ensuremath{\sigma}}_{g}^{2}\ensuremath{\cdots}$$5{\ensuremath{\sigma}}_{g}^{2}$ $1{\ensuremath{\sigma}}_{u}^{2}\ensuremath{\cdots}4{\ensuremath{\sigma}}_{u}^{2}$ $1{\ensuremath{\pi}}_{u}^{4}$ $1{\ensuremath{\pi}}_{g}^{4}$ $2{\ensuremath{\pi}}_{u}^{4}$ $2{\ensuremath{\pi}}_{g}^{3}$. The vertical electron affinity value is 1.42 eV, which is comparable with the experimental value of $1.67\ifmmode\pm\else\textpm\fi{}0.015$ eV. We detected two shape resonances, both of ${}^{2}{\ensuremath{\Pi}}_{u}$ symmetry in the excitation cross sections of the ${}^{1}{\ensuremath{\Delta}}_{g}$ and ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ excited states. The dissociative nature of these resonances is explored by performing scattering calculations in which the S-S bond is stretched. These resonances support dissociative attachment, yielding S and S${}^{\ensuremath{-}}$. We have also predicted six resonances of various symmetries (${}^{2}{A}_{u}$, ${}^{2}{B}_{1g}$,${}^{4}{A}_{u}$, ${}^{4}{B}_{1g}$) in the $X{\phantom{\rule{0.16em}{0ex}}}^{3}{{\ensuremath{\Sigma}}_{g}}^{\ensuremath{-}}$ $\ensuremath{\rightarrow}$ $B{\phantom{\rule{0.16em}{0ex}}}^{3}{{\ensuremath{\Sigma}}_{u}}^{\ensuremath{-}}$ transition. We have calculated the DCS, in a correlated one-state model, by using the polydcs program of Sanna and Gianturco. The data from the momentum-transfer cross section, generated from DCS, are used to compute effective collision frequencies over a wide electron temperature range (200--30 000 K). The ionization cross sections are calculated in the binary-encounter Bethe model in which Hartree-Fock molecular orbitals at a self-consistent level are used to calculate kinetic and binding energies of the occupied molecular orbitals. We have included up to $g$-partial wave ($l=4$) in the scattering calculations. For this molecule we have used a Born-closure top-up procedure to account for the higher partial waves for the convergence of the cross section for the dipole-allowed excitation from the ground state. We have also evaluated the scattering length of the S${}_{2}$ molecule, which is equal to 2.615${a}_{0}$.