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We use the Barnard 68 dark globule as a test case for a spherically symmetricPDR model exposed to low-UV radiation fields. With a roughly sphericalmorphology and an accurately determined density profile, Barnard 68 is idealfor this purpose. The processes governing the energy balance in the cloudsurface are studied in detail. We compare the spherically symmetric PDR modelby Stoerzer, Stutzki & Sternberg (1996) to observations of the three lowestrotational transitions of 12CO, 13CO J = 2-1 and J = 3-2 as well as the [CI]3P_1-3P_0 fine structure transition. We study the role of Polycyclic AromaticHydrocarbons (PAHs) in the chemical network of the PDR model and consider theimpact of depletion as well as of a variation of the external FUV field. Wefind it difficult to simultaneously model the observed 12CO and 13CO emission.The 12CO and [CI] emission can be explained by a PDR model with a external FUVfield of 1-0.75 chi_0, but this model fails to reproduce the observed 13CO by afactor of ~2. Adding PAHs to the chemical network increases the [CI] emissionby 50% in our model but makes [CII] very faint. CO depletion only slightlyreduces the 12CO and 13CO line intensity (by <10% and <20%, respectively).Predictions for the [CII] 2P_3/2-2P_1/2, [CI] 3P_2-3P_1 and 12CO J= 5-4 and 4-3transitions are presented. This allows a test of our model with futureobservations (APEX, NANTEN2, HERSCHEL, SOFIA).

Photon Dominated Region Modeling of Barnard 68