Representation of "broad" falloff curves for dissociation and recombination reactions.

Expressions for representing the pressure dependence of unimolecular dissociation and the reverse recombination reactions are compared. Situations are considered where the broadening of the corresponding falloff curves is particularly pronounced, i.e. where broadening factors at the center of the falloff curves, Fcent, are very small and falloff curves correspondingly become very “broad”. Such situations arise when unimolecular reactions of molecules with large numbers of low-frequency modes and high-temperature situations are considered. Recombination reactions of polyatomic species in atmospheric chemistry and dissociation reactions of large molecules in combustion are the fields of application of the present results. While previously proposed expressions for asymmetric broadening factors showed artifacts when Fcent decreased to values below about 0.4, alternative functions are described which avoid these problems for situations where Fcent decreases to values far below 0.1