Effect of the superposition of weak and strong collisions on unimolecular reaction rate

The effect of superimposed weak and strong collisions on unimolecular reaction rates is analyzed analytically. The reaction kinetics are described by a master equation with a two-scale collisional transition probability (CTP). Simple formulae for the low-pressure reaction rate are derived in terms of reaction rates for single-scale CTPs. The formulae accurately describe the crossover from the strong to the weak collision mechanism of reaction as the fraction of strong collisions decreases. Similar formulae are derived for a finite microcanonical reactivity.