  Using the upGREAT instrument on SOFIA, we have imaged [C II] 157.74 and [O I]
63.18 micron line emission from a bright photodissociation region (PDR)
associated with an ionized ``bubble'' located in the Nessie Nebula, a
filamentary infrared dark cloud. A comparison with ATCA data reveals a classic
PDR structure, with a uniform progression from ionized gas, to photodissociated
gas, and on to molecular gas from the bubble's interior to its exterior. [O I]
line emission from the bubble's PDR reveals self-absorption features. Toward a
FIR-bright protostar, both [O I] and [C II] show an absorption feature at a
velocity of $-18$ km/s, the same velocity as an unrelated foreground molecular
cloud. Since the gas density in typical molecular clouds is well below the [O
I] and [C II] critical densities, the excitation temperatures for both lines
are low (~20 K). The Meudon models demonstrate that the surface of a molecular
cloud, externally illuminated by a standard G_0 = 1 interstellar radiation
field, can produce absorption features in both transitions. Thus, the commonly
observed [O I] and [C II] self-absorption and absorption features plausibly
arise from the subthermally excited, externally illuminated, photodissociated
envelopes of molecular clouds. The luminous young stellar object
AGAL337.916-00.477, located precisely where the expanding bubble strikes the
Nessie filament, is associated with two shock tracers: NH3 (3,3) maser emission
and SiO 2-1 emission, indicating interaction between the bubble and the
filament. The interaction of the expanding bubble with its parental dense
filament has triggered star formation.
