Rotational relaxation in diatomic gas at high temperature observed with Brillouin scattering spectroscopy

It has recently become possible to observe Brillouin scattering spectra of gases at high temperatures as well as their rotational–translational (R–T) relaxation at room temperatures with optical beating Brillouin scattering spectroscopy. In this study, the sound velocity in nitrogen gas was measured at high temperature and a R–T relaxation in the frequency region of 100 MHz was observed. The obtained relaxation frequency value was found to be experimentally reasonable when compared to theoretical values. To our knowledge, optical beating Brillouin spectroscopy is the only method used to observe the total change in sound velocity in R–T relaxation at high temperature. Employing the spectroscopy allowed for a discussion of the temperature and frequency dependence on mechanical properties such as the specific ratio; that is, the internal energy degrees of freedom of the molecules.