Chemical mechanism used in our computation of H ₂ formation. PAH represents any normally hydrogenated PAH (e.g., coronene C ₂₄H ₁₂, ovalene C ₃₂H ₁₄); PAH _−H represents the dehydrogenated species where one peripheral hydrogen is missing, while PAH _+H represents a PAH with an additional hydrogen atom (e.g., C ₃₂H ₁₅ for ovalene). The + superscript denotes the corresponding cation. Anion chemistry is not included for simplicity. The arrows represent the various processes that transform one PAH into another. Four main processes are considered: (1) ionization with rate constant k _ion; (2) recombination between ions and electrons with rate constant k _rec and branching ratio a and (1− a) toward H ₂ and H formation, respectively; (3) photodissociation with H atom ejection ( k _diss) or H ₂ ejection ( k' _diss); (4) chemistry with H and H ₂ addition through radiative association ( k _H and k _H2).