Carbon cluster coagulation and fragmentation kinetics in shocked hydrocarbons

Molecular dynamics simulations of the dissociation of methane, ethylene, and benzene at 50 GPa show that thermally activated carbon–carbon bond breaking produces a stationary n−5/2 distribution of short-lived n-atom carbon chains. Chain fragmentation rates follow an Arrhenius law in temperature, and the fraction of carbon existing as small chains increases with increasing temperature, ultimately approaching 1 at some critical temperature. Mean-field coagulation theory with added fragmentation terms yields the stationary n−5/2 distribution and shows that the rate of carbon coagulation to bulk material slows down with increasing temperature, approaching zero at the critical temperature.