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[57.08] Low-Temperature V-T Rates for the Self-Relaxation of Methane from 300 to 90 K

B.P. Michael (NRC/NASA-GSFC), J.E. Allen, Jr. (NASA/GSFC)

Vibrational-to-translational (V-T) transfer rates for light hydrocarbons at low temperatures are important parameters in thermal-structure models of the upper atmospheres of the outer planets and their satellites. However, the required data are either simply not available or do not extend to the low temperatures found in those systems. Because methane is such an important constituent in outer planet atmospheres, we have initiated a program to measure the temperature dependence of (V-T) rates for its relaxation by appropriate collision partners. To properly treat that data and extract the desired rate, it is necessary to know the self-relaxation rate for methane over the same temperature range. Therefore, we have determined the self-relaxation rate for methane from 300 to 90 K in approximately 10 K increments using a photoacoustic technique. Our measurements extend the available experimental data of Perrin and Jolicard (1986) from 140 to 90 K, i.e., by ~50 K.

Perrin, M.Y. and Jolicard, G. 1986, {\em Chem Phys. Lett.} 127, 118.

{\em Support from NASA's Planetary Atmospheres Program through the National Research Council is grafefully acknowledged.}