  Far-infrared [C ii] 158 micron, [O i] 145 micron and [O i] 63 micron fine
structure emission line fluxes were measured from archival Infrared Space
Observatory Long Wavelength Spectrometer spectra of 46 extragalactic sources,
with 28 sources providing detections in all three lines. For 12 of the sources,
the contribution to the [C ii] 158 micron line flux from H ii regions could be
estimated from their detected [N ii] 122 micron line fluxes. The measured [C
ii]/[O i] and [O i] 63/145 micron line flux ratios were compared with those
from a grid of PDR models previously computed using the UCL PDR code.
Persistent offsets between the observed and modelled line ratios could be
partly attributed to the effects of [O i] 63 micron self-absorption. Using the
SMMOL code, we calculated model [O i] line profiles and found that the strength
of the [O i] 63 micron line was reduced by 20-80%, depending on the PDR
parameters. We conclude that high PDR densities and radiation field strengths,
coupled with the effects of [O i] 63 micron self-absorption, are likely to
provide the best match to the observed line flux ratios.
