  A detailed modeling of simultaneous UV-photochemical and thermochemical
processes in exoplanet atmosphere-like conditions is essential for the analysis
and interpretation of a vast amount of current and future spectral data from
exoplanets. However, a detailed reaction kinetic model that incorporates both
UV photochemistry and thermal chemistry is challenging due to the massive size
of the chemical system as well as to the lack of understanding of
photochemistry compared to thermal-only chemistry. Here, we utilize an
automatic chemical reaction mechanism generator to build a high-fidelity
thermochemical reaction kinetic model later then incorporated with
UV-photochemistry enhanced by metastable triplet-state carbon monoxide (a3Pi).
Our model results show that two different photochemical reactions driven by
Lyman-a photons (i.e. H2 + CO(a3Pi) -> H + HCO and CO(X1Sig+) + CO(a3Pi) ->
C(3P) + CO2) can enhance thermal chemistry resulting in significant increases
in the formation of CH4, H2O, and CO2 in H2-dominated systems with trace
amounts of CO, which qualitatively matches with the observations from previous
experimental studies. Our model also suggests that at temperatures above 2000
K, thermal chemistry becomes the dominant process. Finally, the chemistry
simulated up to 2500 K does not produce any larger species such as C3 species,
benzene or larger (i.e. PAHs). This might indicate that the photochemistry of
C2 species such as C2H2 might play a key role in the formation of organic
aerosols observed in the previous experimental study.
