R-matrix calculations on molecules of astrophysical interest using Quantemol-N

We have carried out a series of ab initio R-matrix calculations at the static exchange and close-coupling levels of approximation on molecules of astrophysical interest. These include the polar triatomics HCN and HNC (hydrogen isocyanide) and their isotopologues DCN and DNC, the diatomics CS (carbon monosulphide) and SiO (silicon monoxide), the weakly polar CO molecule and the non-polar CH4 molecule. With the exception of CO, all the calculations presented here were carried out using the software ‘Quantemol-N’ which provides an intuitive user-friendly interface to the UK polyatomic R-matrix codes. A chapter is devoted to the discussion on the software: how to prepare an R-matrix calculation using it, its present capabilities and future development. The ultimate aim of this thesis is to demonstrate the need to account for electron-induced chemistry in any astrophysical model. We seek to show that in the case of polar molecules, namely, those molecules with large dipole moments, electron collisions are the dominant mechanism of rotational excitation in comets and other astrophysical bodies. Specifically, we will show that electron-impact excitation rate coefficients are several orders of magnitude higher than the corresponding atom-molecule ones. The thesis concludes with a summary of the key findings and opportunities (and where necessary improvements) that may arise from them. All the scattering equations presented here used atomic units

R-matrix calculations on molecules of astrophysical interest using Quantemol-N.

We have carried out a series of ab initio R-matrix calculations at the static exchange and close-coupling levels of approximation on molecules of astrophysical interest. These include the polar triatomics HCN and HNC (hydrogen isocyanide) and their isotopologues DCN and DNC, the diatomics CS (carbon monosulphide) and SiO (silicon monoxide), the weakly polar CO molecule and the non-polar CH4 molecule. With the exception of CO, all the calculations presented here were carried out using the software ‘Quantemol-N’ which provides an intuitive user-friendly interface to the UK polyatomic R-matrix codes. A chapter is devoted to the discussion on the software: how to prepare an R-matrix calculation using it, its present capabilities and future development. The ultimate aim of this thesis is to demonstrate the need to account for electron-induced chemistry in any astrophysical model. We seek to show that in the case of polar molecules, namely, those molecules with large dipole moments, electron collisions are the dominant mechanism of rotational excitation in comets and other astrophysical bodies. Specifically, we will show that electron-impact excitation rate coefficients are several orders of magnitude higher than the corresponding atom-molecule ones. The thesis concludes with a summary of the key findings and opportunities (and where necessary improvements) that may arise from them. All the scattering equations presented here used atomic units.

Electron impact calculations of total elastic cross sections over a wide energy range – 0.01 eV to 2 keV for CH₄, SiH₄ and H₂O

In this paper we report the results of a new theoretical methodology for determining the total elastic electron scattering cross section, Qel, over a wide range of incident energies between 0.01 eV and 2 keV. We have combined results from the UK molecular R-matrix code using Quantemol-N software to determine Qel for incident energies between 0.01 eV and the ionization threshold of the target with calculations based on the spherical complex optical potential formalism for higher energies up to 2 keV. We present results for three selected molecular targets; CH4, SiH4 and H2O as exemplars of the methodology. The present results were found to be in good agreement with previous experimental and theoretical results. The total elastic cross sections for such a wide energy range are reported perhaps for the first time

Elastic scattering of low-energy electrons by CH3CN and CH3NC molecules

Rotationally-summed elastic cross sections for electrons collisions with the isomers acetonitrile and methyl isocyanide are reported. Theoretical differential and integral cross sections are calculated using the UK molecular R-matrix codes in the energy range from 1 eV to 10 eV. The dynamic interaction is represented within a static-exchange plus polarization model based on the use of an extensive orbital sets. Both molecule have a large permanent dipole moment and a Born closure procedure is used to get more reliable cross sections. These molecules show low-energy, π∗ resonances at 2.4 and 2.7 eV for CH3CN and CH3NC, respectively; and very broad σ∗ ones at about 6 eV. Our results suggest that electron collisions with CH3CN / CH3NC show similar properties to those found for electron collisions with HCN / HNC

UKRmol: a low-energy electron- and positron-molecule scattering suite

We describe the UK computational implementation of the R-matrix method for the treatment of electron and positron scattering from molecules. Recent developments in the UKRmol suite are detailed together with the collision processes it is enabling us to treat