Unimolecular Chemical Kinetics in the Interstellar Environment: Competition of Infrared Radiation and Collision Activation Mechanisms

Abstract Unimolecular gas phase chemical reactions could be activated by both infrared (IR) radiation and intermolecular collision in the interstellar environment. Understanding the interplay and competition between the radiation and collision activation mechanisms is crucial for assessing accurate reaction rate constants with an appropriate model. In this work, guided by an extended version of the Lindemann theory, we show that the relative importance of the two mechanisms can be measured by a dimensionless number PR that is the ratio of the collision frequency to the radiation absorption rate of the molecule. The reaction kinetics is dominated by collision activation or radiation activation depending on whether PR is larger or smaller than a reference value PR *, which is determined to be PR * ≈ 10 based on magnitudes of molecular properties, and is verified by detailed calculations of a number of typical interstellar unimolecular reactions. The determination of the PR number requires only information on the environment and molecular properties rather than detailed simulations of chemical reactions; thus, the PR number can serve as an indicator for a priori evaluations before detailed calculations. This method of evaluating the relative importance of the two mechanisms is checked against master equation calculations of two interstellar reactions, the dissociation reaction of silicic acid around an asymptotic giant branch star and the methyl radical association in Titan’s atmosphere, and the validity is verified. The method can be used in the future to help determine the appropriate and effective modeling approach for chemical reactions in astrophysical environments.