  We explore the effects of photodissociation at the stages of post-asymptotic
giant branch stars to find a mechanism able to produce multi-polar shapes. We
perform two-dimensional gasdynamical simulations to model the effects of
photodissociation in proto-planetary nebulae. We find that post-asymptotic
giant branch stars with 7,000 K or hotter are able to photodissociate a large
amount of the circumstellar gas. We compute several solutions for nebulae with
low-velocity multi-lobes. We find that the early expansion of a dissociation
front is crucial to understand the number of lobes in proto-planetary nebulae.
A dynamical instability appears when cooling is included in the swept-up
molecular shell. This instability is similar to the one found in
photoionization fronts, and it is associated with the thin-shell Vishniac
instability. The dissociation front exacerbates the growth of the thin-shell
instability, creating a fast fragmentation in shells expanding into media with
power-law density distributions such as r^-2.
