On the Separation of Timescales in Chemically Activated Reactions

Chemically activated reactions are important in describing the composition of reactive gases including flames, planetary atmospheres, and the interstellar medium (ISM). In a chemically activated reaction, two reactants combine to populate a vibrationally excited well that can undergo unimolecular transformations (isomerization, dissociation) or be thermalized through collisions with the bath gas. Once a well has been thermalized, it may still have sufficient energy to undergo further unimolecular reaction, in a purely thermal process. If the timescale for the thermally activated process is sufficiently short, such that it approaches that of the chemically activated reaction, the two concurrent processes become inseparable and the value of the phenomenological rate coefficient is no longer obvious. Here, we introduce the thermal decay (TD) procedure to determine phenomenological rate coefficients for chemically activated reactions proceeding on timescales approaching those of thermal reaction, principally for use in stochastic master equation simulations of multiple-well multiple-channel unimolecular reaction processes. By fitting the thermal decay of the initially activated well to a first-order kinetic model, the would-be thermal yield can be eliminated so as to arrive at the chemically activated component in a reliable and objective fashion. This technique is demonstrated here for the reaction of 1,3,6-heptatriyne with H using the MultiWell code and a 16-well 33-channel C7H5 reaction model. A computer program implementing the TD method and for postprocessing of MultiWell output data, PPM, is provided.