  A new two-dimensional axi-symmetric ultraviolet radiative transfer code is
presented, which is used to calculate photodissociation and ionization rates
for use in chemistry models of flaring circumstellar disks. Scattering and
absorption of photons from the central star and from the interstellar radiation
field are taken into account. The molecules are effectively photodissociated in
the surface layer of the disk, but can exist in the intermediate, moderately
warm layers. A comparison has been made with an approximate 2D ray-tracing
method and it was found that the latter underestimates the ultraviolet field
and thus the molecular photodissociation rates below the disk surface. The full
2D results show significantly higher abundances of radicals such as CN and C2H
than previous work, partly due to the fact that CO is dissociated to greater
depths. Results for different stellar radiation fields are also presented. The
CN/HCN ratio shows a strong dependence on the stellar spectrum, whereas other
ratios such as HCO+/CO show only little variation.
