Supersonic expansion of molecular oxygen

Supersonic jets are gas dynamic quasi-universal structures showing a wealth of features combining laminar and turbulent flow, relaxation effects, shock waves, vortices, slip effects, and condensation, spanning a wide range of densities, temperatures, chemical species, and Kn numbers. In the supersonic expansion exists a zone between the nozzle and the shock wave, called the zone of silence. We apply the Raman spectroscopy in this zone to obtain the experimental number density and the population of the rotational levels. This method has a high special resolution ($<$5$\mu $m) and high-sensitivity spectroscopy ($<$photon/sec).