Excitation of emission lines by fluorescence and recombination in IC 418

We compare calculated intensities of lines of C ii, N i, N ii, O i and O ii with a published deep spectroscopic survey of IC 418. Our calculations use a self-consistent nebular model and a synthetic spectrum of the central star atmosphere to take into account line excitation by continuum fluorescence and electron recombination. We found that the N ii spectrum of the s, p and most d states is excited by fluorescence due to the low-excitation conditions of the nebula. Many C ii and O ii lines have significant amount of excitation by fluorescence. Recombination excites all the lines from the f and g states and most O ii lines. In the neutral-ionized boundary, the N i quartet and O i triplet dipole-allowed lines are excited by fluorescence, while the quintet O i lines are excited by recombination. Electron excitation produces the forbidden optical lines of O i, and continuum fluorescence enhances the N i forbidden line intensities. Lines excited by fluorescence of light below the Lyman limit thus suggest a new diagnostic to explore the inner boundary of the photodissociation region of the nebula.