Neutral atomic carbon related to carbon monoxide. Also shown are model results with and without grain-assisted atomic-ion neutralization. Models with active grain-assisted atomic-ion neutralizationshown in solid gray and dashed orange curves have the parameters used in Fig. 3. The blue dotted curve with grain-assisted atomic-ion neutralization allows the ambient uv radiation to ionize carbon almost without attenuation. Models with inactive grain-assisted atomic-ion neutralizationare shown for n(H) = 32    cm^-3 and 512    cm^-3 at G₀ = 1. Sources of C I data are labelled as in Fig. 1.