Appl Phys B (2010) 101: 689–700 DOI 10.1007/s00340-010-4145-0 NO excitation and thermal non-equilibrium within a flat plate boundary layer in an air plasma D. Studer · P. Boubert · P. Vervisch Received: 2 April 2010 / Revised version: 18 June 2010 / Published online: 20 July 2010 © Springer-Verlag 2010 Abstract Optical emission spectroscopy experiments are carried out by recording the radiation from the γ transitions of nitrogen monoxide in an air inductively coupled plasma in interaction with a water-cooled metallic flat plate at mod- erate pressure. The calibrated results allow to derive the vi- brational and rotational temperatures of the NO(A 2 Σ + ) ex- cited state as well as its densities in the free jet and within the boundary layer by comparison with calculated spectra. Those results are compared with previous ones concerning temperatures and densities of the ground states of the ma- jority species (N 2 ,O 2 and NO) that were obtained by laser techniques. As for the NO(X 2 Π ) ground state, vibration and rotation of the excited state are found out of equilib- rium. The NO(A 2 Σ + ) excited state is found to be populated by an energy transfer from the metastable N 2 (A 3 Σ + u ). The steady state of the plasma allows using this property to de- rive N 2 (A 3 Σ + u ) densities and N 2 electronic excitation tem- peratures. Close to the wall, a production of excited NO by a catalytic process is also considered involving N 2 (A 3 Σ + u ) as source of adsorbed atoms. The present results confirm that the kinetic temperature cannot be compared to the rotational temperature derived from optical emission spectroscopy in such plasma conditions. 1 Introduction The re-entry of a space vehicle in the atmosphere of Earth benefits from a 50-year experience, from the first manned D. Studer · P. Boubert ( ) · P. Vervisch CORIA—UMR 6614 CNRS, Université et INSA de Rouen, 76801 Saint-Etienne du Rouvray Cedex, France e-mail: boubert@coria.fr orbital missions to the space shuttle flights, and in particu- lar from the Apollo missions coming back from the Moon. Reusable materials were developed that ensure a safe re- entry for velocity smaller than 8 km/s. For larger velocities, ablative materials as those used for the solar system explo- ration missions are preferred. The projected sample return missions and eventually manned missions to Mars will re- quire large entry velocities (up to 15 km/s) in the Martian atmosphere as well as in the Earth atmosphere in order to limit the journey duration. Such high velocities challenge the actual knowledge in non-equilibrium plasma physico- chemistry especially concerning non-equilibrium radiative fluxes, radiative ablation products and catalycity of materi- als. The existing physical and chemical models met some success in predicting ground state densities including in non-equilibrium conditions, but failed to deal with excited species [1, 2]. Vibrational specific [3, 4] or electronic spe- cific [5] collisional-radiative models that are in develop- ment are expected to bring the needed answers. Neverthe- less, some test cases are necessary to validate such mod- els, and the purpose is to fully characterize some plasmas in interaction with thermal protection system (TPS) mate- rials [6]. This means measuring ground state densities as well as excited states’ densities and determining the en- ergy distribution in internal modes or temperatures when they are suitable. Temperatures are indeed one of the easi- est data to obtain especially through optical diagnostic tech- niques. Optical emission spectroscopy (OES) is a powerful tool to get those temperatures when equilibrium is ensured between ground state energy modes and excited state en- ergy modes [7–9]. However, some assumptions about equi- librium between energy storage modes should be made very carefully in some non-equilibrium plasmas. Nevertheless, OES is often used as a temperature diagnostic method in in-