Effect of Temperature on the Rotational and Vibrational Relaxation Times of Some Hydrocarbons

Ultrasonic velocity and absorption measurements in methane, ethylene, ethane, and propane are reported. The temperature range is from room temperature to 800°C, and the frequency/pressure range encompasses both the vibrational and the apparent rotational relaxation. In methane and ethylene the vibrational relaxation time of the lowest frequency mode varies with exp (T−1/3), while the apparent rotational relaxation time increases with temperature. Evidence is presented that the vibrational and rotational degrees of freedom are excited independently, even when the time constants are within a factor of 15 of each other.