  Infrared vibration-rotation lines can be valuable probes of interstellar and
circumstellar molecules, especially symmetric molecules, which have no pure
rotational transitions. But most such observations have been interpreted with
an isothermal absorbing slab model, which leaves out important radiative
transfer and molecular excitation effects. A more realistic non-LTE and
non-isothermal radiative transfer model has been constructed. The results of
this model are in much better agreement with the observations, including cases
where lines in one branch of a vibration-rotation band are in absorption and
another in emission. In general, conclusions based on the isothermal absorbing
slab model can be very misleading, but the assumption of LTE may not lead to
such large errors, particularly if the radiation field temperature is close to
the gas temperature.
