Revisiting falloff curves of thermal unimolecular reactions

摘要

Master equations for thermal unimolecular reactions and the reverse thermal recombination reactions are solved for a series of model reaction systems and evaluated with respect to broadening factors. It is shown that weak collision center broadening factors Fc _en _(tw)c can approximately be related to the collision efficiencies _c through a relation Fc _en _(tw)c ≈ max {_c ~(0.14), 0.64(0.03)}. In addition, it is investigated to what extent weak collision falloff curves in general can be expressed by the limiting low and high pressure rate coefficients together with central broadening factors F _(cent) only. It is shown that there cannot be one best analytical expression for broadening factors F(x) as a function of the reduced pressure scale x k _0k _∞. Instead, modelled falloff curves of various reaction systems, for given k _0, k _∞, and F _(cent), fall into a band of about 10 width in F(x). A series of analytical expressions for F(x), from simple symmetric to more elaborate asymmetric broadening factors, are compared and shown to reproduce the band of modelled broadening factors with satisfactory accuracy.