Experimental detection of rotational non-Boltzmann distribution in supersonic free molecular nitrogen flows

In the present study, we measure the rotational population in supersonic nitrogen-free jets using a resonantly enhanced multiphoton ionization (REMPI) method [Chem. Phys. Lett. 115, 492 (1985)], which is not influenced by secondary electrons, unlike the 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 P₀ ·D (where P₀ is the source pressure), depending inversely on the nozzle Knudsen number, at 15 Torr mm or lower. For P₀·D=15 Torr mm, the rotational temperature distribution along the centerline of the jet, measured by using only the linear portion of the Boltzmann plot lying at smaller rotational quantum numbers, coincides with Marrone's data [Phys. Fluids 10, 521 (1967)] measured by electron beam fluorescence, and the rotational relaxation rate Z_R calculated by the relaxation equation results in 1.3. However, the non-Boltzmann rotational distribution appears evidently in supersonic free molecular nitrogen flows for P₀ · D ≤15 Torr mm, in good agreement with the simulation results of the combination of the classical trajectory calculation and the direct simulation Monte Carlo method by Koura [Phys. Fluids 14, 1689 (2002)]. As a decrease in P₀ ·D, the deviation from the Boltzmann distribution and the partial freezing of the population arise more upstream.