Direct measurement of the acoustic decay times of hypersonic waves generated by SBS

The acoustic decay time of hypersound generated by stimulated Brillouin scattering (SBS) has been measured in several heavy gases (Xe, SF
6
, CCIF
3
, C
2
F
6
) over a range of pressures. A straightforward optical probing technique is used whereby the decay time can be measured directly with high precision. A theoretical formulation of the decay mechanisms in a polyatomic gas is presented, and it is shown both theoretically and experimentally that vibrational relaxation can become the major decay process at high gas density. The Brillouin gain coefficient and acoustic frequency are also experimentally determined or calculated for the gases investigated. Two Brillouin-active gases (CCIF
3
and C
2
F
6
) with high Brillouin gains, which have not been studied previously, have been characterized. A study of the gas C
2
F
6
near its critical point showed a large variation of the acoustic velocity which reaches a minimum value in the vicinity of its critical density.