The Excitation of the Vibrational Modes of Ethylene in Homo-molecular Collisions

Sound absorption and velocity were measured in ethylene at 500 kc/s and 700 kc/s in the ranges 0.07 to 2 atm and -10 to 80°C. All the vibrational modes relaxed out at approximately 1 Mc/s atm, but the assumption of two relaxation times, for the transfer of energy between translational and vibrational motion, fitted the measurements better than the assumption of a single relaxation time. Assuming that the average relaxation time found by fitting single relaxation curves to the measured points was approximately that of the lowest frequency mode, the value of the force constant β in the exponential intermolecular potential was found to be 6.71 x 10 8 cm -1 . This agrees well with the value 4.72 x 10 8 cm -1 calculated from the Lennard-Jones potential.