RATE OF CH4 DISSOCIATION OVER 2800-4300-K - THE LOW-PRESSURE-LIMIT RATE-CONSTANT

The rate of dissociation of CH4 dilute in both Kr and Ar has been measured in incident shock waves with the laser-schlieren technique. The measurements cover the very high temperature range 2800-4400 K in 2% CH4-Kr, 2% CH4-Ar, and 0.5% CH4-Kr, with moderate pressures of 230-660 Torr. Density gradient profiles are model-extrapolated some 0.5 mus to t = 0 and rate constants evaluated at the vibrationally-relaxed, chemically frozen condition. These rate constants form a quite consistent set (RMS deviations +/- 7%) with no discernible difference between mixtures. Their temperature variation shows definite Arrhenius curvature, but this is in full accord with an adjusted RRKM model of k0 having the known bond energy as the barrier. This model also predicts rates very close to a summary of previous work below 2500 K. A modified-Arrhenius fit of these RRKM calculations of the low-pressure-limit rate constant log k0 (cm3/mol s)) = 47.279 - 8.106 log T - 25660/T should be accurate (+/- 50%) over 1600-4000 K.