Spectroscopic Study of Rotational Nonequilibrium in Supersonic Free Molecular Flows

In highly rarefied gas flows, there appear nonequilibrium phenomena not only between translational and rotational energy modes but also in rotational mode. To analyze these highly rarefied gas flows, we established the experimental system for 2R+2 N2‐REMPI and applied it to the measurement of rotational temperature in a supersonic free molecular flow of nitrogen. The data in the Boltzmann plot, obtained from the measured REMPI spectra, cannot be fitted by one line but approximately by two lines, revealing the non‐Boltzmann distribution of rotational energy in the ground state. In comparison with the Boltzmann distribution at the source condition, it is clarified that the rotational energy distribution at relatively high rotational level of J ⩾ 13 is unchanged during the flow expansion, while there are rotational transitions from levels in J ⩽ 12 to lower levels by the molecular collisions during the expansion.