  We report the first spectroscopic mapping of an atomic carbon line in an
infrared dark cloud (IRDC).
  By observing the spatial distribution of the [CI] emission in an IRDC,
comparing it with the 13CO emission and the known distribution of internal
heating sources, we can quantify the role of internal and external UV
irradiation in the production of atomic carbon.
  We used the 2x4 pixel SMART receiver of the KOSMA observatory on Gornergrat
to map the [CI] 3P1 - 3P0 line in the IRDC G48.65-0.29 and compared the
resulting spectra with data from the BU-FCRAO 13CO 1-0 Galactic Ring Survey.
  The [CI]/13CO effective beam temperature ratio falls at about 0.3 with local
deviations by less than a factor two. All velocity components seen in 13CO are
also detected in[CI]. We find, however, significant differences in the
morphology of the brightest regions seen in the two tracers. While 13CO
basically follows the column density distribution derived from the
near-infrared (NIR) extinction and the submm continuum, the [CI] emission peaks
at the locations of the two known NIR point sources. We find CI/CO abundance
ratios between 0.07 and 0.13, matching the lower end of the range previously
measured in star-forming regions.
  Evaluating the relative importance of the irradiation by embedded sources and
by the Galactic interstellar UV field, we find that in G48.65-0.29 most [CI]
emission can be attributed to externally illuminated surfaces. Embedded sources
have a significant impact on the overall abundance distribution of atomic
carbon as soon as they are found in an evolved state with noticeable NIR flux.
