Measurement of Hypersound Speed in Gases by Stimulated Brillouin Scattering*

The frequency shifts associated with the 180° stimulated Brillouin scattering (SBS) from N2, CH4, CO2, and SF6 at moderate pressures were determined with an error of ±0.0004 cm−1. The apparatus and the ruby-laser system designed for this experiment are described. The theory of SBS is reviewed with special consideration of the frequency shift associated with the stimulated process. Experimental and theoretical reasons for the differences observed between the SBS hypersonic speeds and ultrasonic speeds are discussed. Available spontaneous Brillouin-scattering results are compared. Especially at lower pressures, the SBS measurements may yield hypersonic speeds that are lower than those at ultrasonic frequencies.