Vibronic State-Specific Modelling of High-Speed Nitrogen Shocked Flows. Part I: Kinetic Database

TL;DR

This paper develops a vibronic-specific kinetic database for nitrogen shocked flows using advanced models like FHO, compares dissociation rates with experiments, and explores modeling challenges for vibronic excitation processes.

Contribution

It introduces a physically accurate FHO model for nitrogen kinetics and assesses its applicability and limitations in high-temperature shock flow simulations.

Findings

FHO model provides rate coefficients aligning well with experimental data.

Extending Landau-Zener models to vibronic excitation is impractical.

Discrepancies up to an order of magnitude found in exponential gap law fits.

Abstract

A database of kinetic processes for nitrogen shocked flows was built using vibronic-specific state-to-state models. The Forced-Harmonic-Oscillator model (FHO), which is more physically accurate in the high temperature regime than the popular Schwartz-Slawsky-Herzfeld model (SSH), was implemented in the computation of rate coefficients for vibrational transition and dissociation of $\textrm{N}_2$ and $\textrm{N}_2^+$ by heavy particle impact. Thermal dissociation rate coefficients of $\textrm{N}_2(\textrm{X}^1\Sigma_\textrm{g}^+)$ by collisions with $\textrm{N}_2(\textrm{X}^1\Sigma_\textrm{g}^+)$ and $\textrm{N}(^4\textrm{S}_\textrm{u})$ were in their turn obtained, which were shown to agree reasonably well with state-of-the-art experimental values. The possibility of extending the well known Landau--Zener and Rosen--Zener--Demkov models (for heavy particle impact excitation of atomic particles) to heavy particle impact vibronic excitation of diatomic particles was ascertained to be impractical. As an alternative, an exponential gap law was considered. By fitting the curve that represents the law to experimentally obtained values for rate coefficients values of several vibronic transitions of $\textrm{N}_2$ reported in the literature, discrepancies of as much as one order of magnitude were obtained, evidencing some crudeness of the model. Reactions such as ionisation of $\textrm{N}_2$ by electron impact, charge exchange and dissociative recombination of $\textrm{N}_2^+$ were modeled using process cross sections or rate coefficients from the literature. A companion article describes the application of this model to nitrogen shocked flows.