Experimental analysis of non-Boltzmann rotational energy distribution in supersonic free molecular flows

In the present study, we measure the rotational population in supersonic nitrogen free jets using a REMPI (resonantly enhanced multi-photon ionization) method, which is not influenced by secondary electrons, unlike an electron beam method. Nitrogen ions are detected as a REMPI signal and its spectra depending on the wavelength of an irradiated laser beam are analyzed to measure rotational temperature through the Boltzmann plot. Nitrogen gas expands into a vacuum chamber from a sonic nozzle with a D=0.50 mm diameter, setting P_oD (P₀: source pressure), depending inversely on the nozzle Knudsen number, at 15 Torr·mm or lower. For P₀D=15 Torr·mm, the rotational temperature distribution along the centerline of the jet, measured by Boltzmann plot, coincides with Marrone's data measured by electron beam fluorescence, and the rotational relaxation rate Z_R calculated by the relaxation equation results is 1.3. However, the non-Boltzmann rotational distribution appears evidently in the supersonic free molecular nitrogen flows for P₀D≤15 Torr·mm and as a decrease in the P₀D, the deviation from the Boltzmann distribution and the partial freezing of the population arise more upstream.