Variation of the gas-phase abundances of fundamental species in carbon chemistry, normalised by their respective peak abundances as a function of visual extinction, Av. The cloud parameters for the different models are as follows: nH = 102 cm−3, G0 = 1 (top-left), nH = 102 cm−3, G0 = 1 × 104, nH = 103 cm−3, G0 = 1, and nH= 103 cm−3, G0 = 1 × 104 (bottom-right). All the models presented here use a constant cosmic-ray ionisation rate of ζH = 2.2 × 10−16 s−1. The pink and grey shaded regions in each panel highlight the transition layer between H–H2 and C+ –C, respectively.
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