Spectroscopic analysis of nonequilibrium excited state chemistry in a NASA arc jet

To support the determination of total specific enthalpy of the plasma flow of the 60 MW Interaction Heating Facility at NASA Ames Research Center, spectroscopic measurements of nonequilibrium emission were performed in the freestream. In the visible to near-infrared wavelength region, the most prominent emission was from molecular N₂, and in the ultraviolet region, the spectra were dominated by emission from molecular NO. The only atomic lines observed were those of copper (an erosion product of the electrodes). The bands of the First Positive System of N₂ (B → A) differed significantly from spectra computed assuming only thermal excitation, suggesting overpopulation of the high vibrational states of the B state of N₂. Populations of these high vibrational levels (peaking at v_upper 13) of the N₂ B state were determined by scaling spectra simulated for each upper vibrational state separately. The overpopulation of the high vibrational levels is assigned to inverse predissociation of neutral atoms into the N₂ B state at v_upper > 10 via level-crossing processes with the N₂ A ^{0 5} Σ_g state. The v_upper 12 and 13 emission has been used to determine N atom densities, which agree very well between the different experiment campaigns and with the computational-fluid-dynamics simulation of these flows.