Grüneisen approach for Universal Scaling of the Brillouin Shift in Gases

Abstract A Grüneisen relationship is defined for gases, following the formulation of the original microscopic Grüneisen ratio γ = (dlnω)/(dlnV ) for solids. In the case of gases acoustic excitations represent the modes at frequency ω to be considered. By comparing to measured Brillouin shifts in various gases (SF6, N2O, and CO2) and under various conditions of pressure and temperature, a specific value of the defined ratio γ0 = 0.064±0.004 is found to provide a universal description of the active modes in a gas. This finding of such universal gas law may find application in extrapolation of properties of ideal gases to regimes where those cannot be measured easily, like the acoustics and shocks at extremely high temperatures.