  The dissociation and subsequent ionisation of molecular material in a
detached, spherically symmetric stellar wind is modelled as a star evolves from
the post-asymptotic giant branch phase to a planetary nebula (i.e. from a
stellar effective temperature of Teff=6000K to Teff=33000K). The model is
compared to the observed mass and distribution of ionised and atomic hydrogen
in the planetary nebula IC418. It is found that the mass of circumnebular
atomic hydrogen observed in IC418 can easily be produced by the dissociation of
originally molecular material by the ultraviolet radiation of the central star.
However, the observed spatial distribution of the gas is not accurately
recreated by the current model.
