Investigations of Reactive Processes at Temperatures Relevant to the Hypersonic Flight Regime

Reactions involving nitrogen N and oxygen O atoms dominate the energetics of the reactive air flow around spacecraft when reentering the atmosphere in the hypersonic flight regime. One of the important determinants for the chemistry in such flows are the thermal rate coefficients for reactive processes. In the present project, rate coefficients are determined for reactions involving O3P and NO2Pi. For this, a potential energy surface PES for the ground state of the NO2 molecule is constructed based on high-level ab-initio calculations and interpolated using the reproducible kernel Hilbert space RKHS method and Legendre polynomials. The global PES of NO2 in the ground state is constructed by smoothly connecting the surfaces of the grids of various channels around the equilibrium NO2 geometry by a distance-based switching function. The rate coefficients are calculated from Monte Carlo sampling. The results indicate that at high temperatures primarily the lowest electronic state is relevant and zero-point effects are not relevant. The rate coefficient for O2 production at 20,000 K becomes comparable to within a factor of around three to the rate coefficient of the oxygen exchange channel for the same temperature. The computational approach outlined and pursued is generic for small-molecule reactions and can be applied to other reactions relevant to the hypersonic flight regime.