Application of REMPI to highly rarefied gas flows

In this study, the experimental system for 2R+2 N₂-REMPI (Resonantly Enhanced Multi-Photon Ionization) is established and applied to measurement of the rotational temperature in a supersonic free molecular nitrogen flow. In the 2R+2 N₂ REMPI technique, nitrogen molecules are ionized by two steps from the ground state to the resonance state by 2 photons and from the resonance state to the ionization state by 2 photons. The nitrogen ions are detected as a signal and its spectra depending on the wavelength of an irradiated laser beam are analyzed to measure the rotational temperature. The new Boltzmann plot method using spectral lines belonging to multiple branches is proposed to measure the rotational temperature more precisely, also showing the feasibility of the temperature measurement by the best fitting of the theoretical spectra to the experimental one. There appear the non-equilibrium phenomena in the highly rarefied gas flows, such as the translational and rotational non-equilibrium and the rotational non-equilibrium. The non-Boltzmann distribution (strong non-equilibrium) of the rotational mode in the ground state is found in the free molecular nitrogen flow using the REMPI technique