Vibrational relaxation in methyl hydrocarbons at high temperatures: Propane, isobutene, isobutane, neopentane, and toluene.

Vibrational relaxation has been seen in shock waves in propane, isobutene, isobutane, neopentane, and toluene dilute in krypton with the laser–schlieren technique. These experiments cover about 600–2200 K and post-shock pressures from 5 to 29 Torr. The process cannot be resolved in any for T<600 K, or in any for large molecule fraction. All the ultrasonic measurements of relaxation in these at room temperature show characteristic times in the 1–5 ns atm range, corresponding to fewer than five collisions, whereas the relaxation times in the shock waves range from 20 to 200 ns atm, with a clearly defined negative or “inverted” temperature dependence. It would seem the observed slowdown of relaxation with increasing T is simply a consequence of the much increased energy transfer required at high temperature in such large polyatomics when this is combined with a collision efficiency, here interpreted as 〈ΔE〉[sub down], already so large it cannot much increase. The simple method for the extraction of a 〈ΔE〉[sub down] from relaxation data offered here by consideration of the energy relaxation equation for E[sub vib]=0 appears to be original and should prove quite useful in connecting thermal relaxation data to values obtained from spectroscopy and master-equation analyses. Here it is found that the derived 〈ΔE〉[sub down] extrapolate well to room temperature ultrasonic measurements, showing a slight increase with temperature. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]