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 g of S 2 − having the configuration 1σ 2 g · · ·5σ 2 g 1σ 2 u · · · 4σ 2 u 1π 4 u 1π 4 g 2π 4 u 2π 3 g . The vertical electron affinity value is 1.42 eV, which is comparable with the experimental value of 1.67 ± 0.015 eV. We detected two shape resonances, both of 2 u symmetry in the excitation cross sections of the 1 g and 1 + 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 − . We have also predicted six resonances of various symmetries ( 2 A u , 2 B 1g , 4 A u , 4 B 1g ) in the X 3 g − → B 3 u − 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 2.615a 0 .
Cross sections for rotational excitation and de-excitation of the HeH+ ion by an electron impact are computed using a theoretical approach that combines the UK R-matrix code and the multi-channel quantum defect theory. The thermally-averaged rate coefficients derived from the obtained cross sections are fitted to an analytical formula valid for a wide range of temperatures.
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