Abstract
We computed electron impact ionisation cross sections (EICSs) of iron hydrogen clusters, FeH n with n = 1,2,...,10, from the ionisation threshold to 10 keV using the Deutsch-Mark (DM) and the binary-encounter-Bethe (BEB) formalisms. The maxima of the cross sections for the iron hydrogen clusters range from 6.13 × 10-16 cm2 at 60 eV to 8.76 × 10-16 cm2 at 76 eV for BEB-AE (BEB method based on quantum-chemical data from all-electron basis sets) calculations, from 4.15 × 10-16 cm2 at 77 eV to 7.61 × 10-16 cm2 at 80 eV for BEB-ECP (BEB method based on quantum-chemical data from effective-core potentials for inner-core electrons) calculations and from 2.49 × 10-16 cm2 at 43.5 eV to 7.04 × 10-16 cm2 at 51 eV for the DM method. Cross sections calculated via the BEB method are substantially higher than the ones obtained via the DM method, up to a factor of about two for FeH and FeH2. The formation of Fe-H bonds depopulates the iron 4s orbital, causing significantly lower cross sections for the small iron hydrides compared to atomic iron. Both the DM and BEB cross sections can be fitted perfectly against a simple expression used in modelling and simulation codes in the framework of nuclear fusion research. The energetics of the iron hydrogen clusters change substantially when exact exchange is present in the density functional, while the cluster geometries do not depend on this choice.
Highlights
Plasma-wall interaction (PWI) remains one of the key issues in nuclear fusion research
We compared the effect of using either all-electron basis sets or effective-core potentials for inner-core electrons in the quantum chemical calculations necessary to obtain the orbital and kinetic energies required for the BEB method on the resulting BEB-AE or BEB-ECP cross sections, respectively
The maxima of the cross sections for the iron hydrogen clusters range from: 6.13 × 10−16 cm2 at 60 eV to 8.76 × 10−16 cm2 at eV, 4.15 × 10−16 cm2 at eV to 7.61 × 10−16 cm2 at 80 eV, 2.49 × 10−16 cm2 at 43.5 eV to 7.04 × 10−16 cm2 at 51 eV for the BEB-AE, BEB-ECP and DM methods, respectively. Both BEB approaches yield cross sections substantially higher than those obtained via the DM method, most pronounced for the smallest FeH and FeH2 clusters with deviations of more than a factor of two for BEB-AE and almost a factor of two for BEB-ECP
Summary
Plasma-wall interaction (PWI) remains one of the key issues in nuclear fusion research. Besides the formation of gas-phase atomic species in various charge states, di- and polyatomic molecular species are expected to be formed via PWI processes. These compounds may profoundly disturb the fusion plasma and may lead to unfavourable re-deposition of materials and composites in other areas of the vessel [7,8,9,10]. Collisions of atoms and molecules with electrons are an important example of such processes They are mainly characterised by the respective electron-impact ionisation cross sections (EICSs). Their understanding is especially important for modelling the plasma energy balance. Apart from magnetic confinement fusion, EICS data plays a role in astrophysics and in a variety of other applications, such as low-temperature processing plasmas, gas discharges, and in chemical analysis [12]
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