Molecules in Interstellar Space
We begin with a history of the field of molecular line astronomy followed by a short account of molecular structure relevant to radio astronomy. (For more details consult the extensive treatments by Kroto or Townes & Schawlow listed in the references). In our presentation we go from the simpler to the more complex situations typical of the ISM, including radiative processes. As an example we analyze the excitation of carbon monoxide, CO. The population of this molecule is assumed to be in local thermodynamic equilibrium (LTE). This is a simple situation but has limited validity; more common are deviations from LTE. This treatment is extended to symmetric top molecules; an example is ammonia, NH3 . Then we treat asymmetric top molecules, using water vapor and formaldehyde as examples and as a last topic in molecular structure, we present a short account of hindered motions.