  The hydrogen ionization and dissociation front around an ultraviolet
radiation source should merge when the ratio of ionizing photon flux to gas
density is sufficiently low and the spectrum is sufficiently hard. This regime
is particularly relevant to the molecular knots that are commonly found in
evolved planetary nebulae, such as the Helix Nebula, where traditional models
of photodissociation regions have proved unable to explain the high observed
luminosity in H_2 lines. In this paper we present results for the structure and
steady-state dynamics of such advection-dominated merged fronts, calculated
using the Cloudy plasma/molecular physics code. We find that the principal
destruction processes for H_2 are photoionization by extreme ultraviolet
radiation and charge exchange reactions with protons, both of which form H_2^+,
which rapidly combines with free electrons to undergo dissociative
recombination. Advection moves the dissociation front to lower column densities
than in the static case, which vastly increases the heating in the partially
molecular gas due to photoionization of He^0, H_2, and H^0. This causes a
significant fraction of the incident bolometric flux to be re-radiated as
thermally excited infrared H_2 lines, with the lower excitation pure rotational
lines arising in 1000 K gas and higher excitation H_2 lines arising in 2000 K
gas, as is required to explain the H_2 spectrum of the Helix cometary knots.
