parameters were refined using the least-squares criteria incorporated in the digital computer simulation program eprlq. Listings of this program and sample input data are available on request.
A black box Binary Encounter Bethe (BEB) with an effective core potential (ECP) procedure is implemented, which facilitates the efficient calculation of electron impact ionization cross sections for molecules that include heavy atoms. This is available in the Quantemol electron collisions software, a user friendly graphical user interface to the UKRMol+ codes. Tests were performed for the following series of molecules:
We present R-matrix calculations of electron and positron low energy scattering from the highly polarizable pyrazine molecule. We compare integral and differential elastic cross sections with experimental results and assess the quality of the models used for describing collisions of either projectile. Static-exchange-plus-polarization models give a good description of electron scattering (including that of shape resonances), whereas both the integral and small-angle differential cross sections are underestimated for positron collisions for the same models. We discuss whether the absence of a permanent dipole moment improves the comparison with experiment for this molecule, as well as future calculations that may improve the description of polarization effects and thus positron scattering.
Graphical abstract
There are many measurements and calculations of total electron impact ionisation cross sections. However, many applications, particularly in plasma physics, also require fragmentation patterns. Approximate methods of deducing partial cross sections are tested based on the use of total cross section computed within the well-used Binary Encounter Bethe (BEB) approximation. Partial ionisation cross sections for three series of molecules including CH$_4$, CF$_4$ and CCl$_4$; SiH$_4$ and SiCl$_4$; NH$_3$ and PH$_3$, were estimated using two methods. Method one is semi-empirical and uses mass spectroscopy data to fix the partial cross sections at a single electron energy. The second is a a fully computational method proposed by Huber {\it et al.} (2019, J. Chem. Phys., 150, 024306). Comparisons with experimental results suggest that the mass spectroscopy method is more accurate. However, as Huber's method requires no experimental input, this method could be used as a first approximation when no experimental data is available. As mass spectroscopy sometimes provides incomplete datasets, a hybrid method based on the use of both methods is also explored.
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