Full Reaction Pathway Dynamics for Atmospheric Decomposition Reactions: The Photodissociation of H $_{2}$ COO

Abstract

Branching ratios for fragmentation channels of important meta- and unstable species are essential for a molecular-level characterization of atmospheric chemistry. Here, the molecular product channels for the decomposition dynamics of the smallest Criegee intermediate, H $_{2}$ COO, are quantitatively investigated. Using a high-quality, full-dimensional machine learned potential energy surface (CASPT2/aug-cc-pVTZ), the translational, rotational, and vibrational energy distributions of the CO $_{2}$ +H $_{2}$ , H $_{2}$ O+CO, and HCO+OH fragmentation channels were analyzed to elucidate partitioning of the available energy. The CO $_{2}$ + H $_{2}$ product forms through two different pathways that bifurcate after formation of the OCH $_{2}$ O intermediate. Along the direct pathway, CO $_{2}$ is preferentially vibrationally excited with H $_{2}$ in its vibrational ground state, whereas for the indirect pathway going through formic acid, H $_{2}$ can populate levels with $v > 0$ . For all product channels passing through energized formic acid, the lifetime distributions are described by stretched exponentials with $β$ ranging from 1.1 to 1.7. This is a clear signature of non-RRKM effects and suggests that the explicit molecular dynamics needs to be followed for a quantitative and realistic description of the photodissociation dynamics.

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