Rate of CH[sub 4] dissociation over 2800-4300 K: The low-pressure-limit rate constant

The rate of dissociation of CH[sub 4] 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% CH[sub 4]-Kr, 2% CH[sub 4]-Ar, and 0.5% CH[sub 4]-Kr, with moderate pressures of 230-660 Torr. Density gradient profiles are model-extrapolated some 0.5[mu]s 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 k[sub o] having the known bond energy as the barrier. This model also predicts rates very close to a summary of previous work below 2500K. A modified-Arrhenius fit of these RRKM calculations of the low-pressure-limit rate constant should be accurate ([+-]50%) over 1600-4000 K. 37 refs., 7 figs., 6 tabs.