Abstract
Population models are a prerequisite for performing qualitative analysis of population densities measured in plasmas or predicting the dependence of plasma emission on parameter variations. Models for atomic helium and hydrogen as well as molecular hydrogen in low-pressure plasmas are introduced. The cross-sections and transition probabilities used as input in the atomic models are known very accurately, and thus a benchmark of these models against experiments is very successful. For H2, in contrast, significant deviations exist between reaction probabilities taken from different literature sources. The reason for this is the more complex internal structure of molecules compared to atoms. Vibrationally resolved models are applied to demonstrate how these deviations affect the model results. Steps towards a consistent input data set are presented: vibrationally resolved Franck–Condon factors, transition probabilities, and ionization cross-sections have been calculated and are available now. Additionally, ro-vibrational models for selected transitions are applied successfully to low-density, low-temperature plasmas. For further improving the accuracy of population models for H2, however, it is necessary to establish a comprehensive data set for ro-vibrationally resolved excitation cross-sections based on the most recent calculation techniques.
Highlights
Helium and atomic as well as molecular hydrogen are present in different kinds of plasmas, ranging from astrophysics to plasma-processing devices and fusion experiments
Based on a set of potential energy curves for the hydrogen molecule taken from the literature vibrationally resolved Franck–Condon factors (FCF) and Einstein coefficients have been calculated for all states up to the principal quantum number p = 4 in H2 and its isotopomeres (D2, T2, HD, DT) [42]
In order to investigate the dependence of the scaling factors on the plasma parameters, calculations for two different values of Tvib have been performed: with decreasing Tvib from 4500 K to 3000 K the relative changes in the scaling factors are below 6%
Summary
Helium and atomic as well as molecular hydrogen are present in different kinds of plasmas, ranging from astrophysics to plasma-processing devices and fusion experiments. While in equilibrium plasmas the (local) thermodynamic equilibrium is fulfilled, for nonequilibrium plasmas, corona or collisional radiative (CR) models have to be applied Such models describe the excited state population densities in a zero-dimensional approximation and they can be used either for backward or for forward calculations. Forward calculations allow for known plasma parameters predicting the population densities of excited states The latter information can be useful; for example, for predicting the photon emission of atomic lines as well as molecular bands, and the impact of this radiation on surfaces [6]. Efforts are undertaken to improve and enlarge the available set of reaction probabilities for molecular hydrogen
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