A Practical Guide to RRKM Theory, its Simplified Multi-Well Version AWATAR and Master Equation Modelling of Radiative Processes

Abstract

Unimolecular reactions and rearrangements in the gas phase under ultra-high vacuum conditions are not correctly described with equilibrium thermodynamics, since the heat bath is missing, and radiative thermalization is slow for small systems. This is the realm of Rice Ramsperger Kassel Marcus theory, commonly referred to RRKM theory, where the unimolecular rate constant is determined by the internal energy E of the system and the energetically accessible quantum states of the minimum and transition structures. We recently expanded classical RRKM theory to allow for a description of dissociation of entities with multiple energetically low-lying minima on the potential energy surface, called AWATAR for All Wells And Transition structures Are Relevant. This approach is used for master equation modeling of dissociation caused by radiative processes, such as black-body infrared radiative dissociation (BIRD) or infrared multiple photon dissociation (IRMPD). In the present tutorial review, we start with examples of RRKM and AWATAR calculations. Radiative processes are modeled in real time as well as by converging the population of a molecular ensemble to a stationary state. Examples for the simulation of BIRD and IRMPD experiments are shown. The program AWATAR 1.0 which is used for the simulations is made available. All examples are annotated in the electronic supplementary information.