The O +( 4S)+N 2(X 1Σ g+)→NO +(X 1Σ +)+N( 4S) reaction as a source of highly rotationally excited NO + in the thermosphere

Abstract

Observations of daytime NO + radiance near 4.3 μm by the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) indicate that NO + rotation is in non-local thermodynamic equilibrium (NLTE). The present work investigates the possibility of the nonthermal O ++N 2 reaction as a source of rotational NLTE NO + emission using standard chemical dynamics techniques. Calculations have been performed to determine the reaction cross section and the final NO + vibrational–rotational distributions of the O ++N 2 reaction as a function of initial translational energy. The potential energy surface was constructed based upon limited, reliable ab initio information. The calculated reaction cross section, which shows a very strong translational energy dependence, is in agreement with the available experimental data. The calculations also indicate that vibrational excitation of N 2 is as efficient as initial translational energy in promoting reaction, again in agreement with experimental evidence. The calculated average final translation energy is consistent with experiment for energies below 2 eV, indicating that the NO + internal energy is well characterized by the calculations. The results indicate that the NO + product from nonthermal collisions is vibrationally cold with substantial rotational excitation consistent with recent analyses of Earthlimb spectra.