Rotational distribution of N2 in Ar shock wave

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Abstract
The rotational distribution of N2 in the Ar shock wave is quantitatively studied using the rotational transition cross sections of the exponential gap and power laws. The relaxation of the rotational distribution in the shock wave is described by the master equation, where the velocity distributions are taken as the local Maxwell distribution at a low Mach number and the Mott–Smith bimodal distribution at a high Mach number. The rotational distribution indicates a near Boltzmann distribution at a low Mach number. At a high Mach number a merging pattern of the Boltzmann distributions at the lower and higher rotational levels corresponding to temperatures before and after the shock wave, respectively, is obtained at the fore part of the shock wave, where the rapid increase in the populations of the higher levels is caused by the accumulation of the fluxes from the lower levels. The merging pattern is well established by the existence of hot molecules in the Mott–Smith bimodal velocity distribution. The rotational temperature profile precedes the density profile both at low and high Mach numbers. The fact that the pattern of the rotational distribution in the N2–Ar system is in qualitative agreement with the measured pattern in the pure N2 system may indicate that the rotation–rotation transition does not play an important role in the pattern of the rotational distribution of N2 in the shock wave.