Temperature dependence of vibrational relaxation times in cyclopropane, ethylene and sulphur hexafluoride

Sound absorption and velocity measurements were made in cyclopropane, ethylene and sulphur hexafluoride at 400 and 700 kHz in the frequency/pressure range 0·2-30 MHz atm −1 and in the temperature range -70 to 150°C. Relaxation times for the transfer of vibrational energy were deduced and compared with previous measurements. These results are discussed in terms of the theories of molecular energy transfer. Evidence for double relaxation processes is found in the case of ethylene and sulphur hexafluoride, but the measurements in cyclopropane fit a single relaxation process within the estimated experimental error.