The influence of key vibrational states on plasma modelling

Vibrational states play an important role in molecular plasmas; they induce gas phase reactions not available for ground state neutrals as well as surface reactions [1] or contribute to gas heating via V-T relaxation. Furthermore, the vibrational excitation process itself affects the electron kinetics and contributes to collisional energy losses. Therefore, cross-sectional data for vibrational excitation is needed to accurately simulate molecular plasmas. 

The QEC expert system which computes electron impact cross sections using the UK molecular Rmatrix code (UKRmol+) [2,3] has recently been updated to allow for the calculation of vibrational excitation processes for situations where the electron is not trapped by a resonance [3]. These cross-sections can then be used as the basis for vibrational resolution in a plasma model.

Setting up a computational model with fully resolved vibrational states can easily lead to a reaction set with thousands of individual reactions. Depending on the process parameters, some individual vibrational states might not have a significant influence on the plasma system. Identifying and removing these states simplifies the chemistry set, saving computational resources and easing the identification of important reactions. We have developed tools for identifying these species while highlighting species which contribute most to the production of desired species. With regards to vibrational states, these tools can be used to reduce the vibrational resolution to only the states significantly affecting the plasma and to identify vibrational states for which cross-sectional data are needed. These tools and the calculation of cross-sections will be presented for exemplar plasma systems, for example, in nitrogen.

[1] J.-E. Jung et al., Vac. Sci. Technol. A 2020 , 38, 023008

[2] B. Cooper et al., Atoms 2019, 7, 97

[3] Z. Masin et al, arXiv:1908.03018 [https://arxiv.org/abs/1908.03018]

[4] V. Kokoouline et al., Plasma Sources Sci. Technol. 2018, 27, 115007