The role of vibrational and rotational excitation in electron-impact N₂ dissociation at non-equilibrium Earth entry conditions

Uncertainty studies reveal a significant impact of e- and heavy-impact dissociation of N₂ on the resulting uncertainty on the heat flux impinging on a vehicle entering the Earth's atmosphere. Typically, aerothermal flow models use a single macroscopic rate coefficient for the N₂ dissociation across the shock layer. For simplicity, a two-temperature model is employed, wherein the rotational distribution is assumed to keep pace with the rapid translational excitation due to the shock (T_r = T_t), while vibrational modes are assumed to lag and are therefore assigned a separate temperature (T_v). These simplifications are further affected by the introduction of √(T_t^.T_v) in the Arrhenius formula.



In this work we show that rate coefficients derived from R-matrix calculations indicate that vibrationally excited N₂ dissociates at vastly faster rates and electron-impact excitation plays an important role in populating the most reactive states. Moreover, rotationally excited N₂ exhibits a lower barrier to dissociation as well.

The macroscopic rate assumptions are examined using an isochoric-isothermal state-to-state model to show that the T_v and T_r evolutions affect the dissociation under non-equilibrium entry conditions. This provides electron-impact chemistry informed uncertainty boundaries for the aerothermal flow studies. Dissociation of vibrationally excited N₂ due to electron impact is included here and proves to be significantly more impactful than dissociation due to heavy particle collisions.