In this chapter we summarise briefly the notation of molecular spectroscopy, with examples of transitions used to identify molecular species in the interstellar medium. We also describe how radiation is transported in the interstellar medium, introducing ideas that will be needed in Chapters 8 and 9. Finally, we discuss the processes that determine level populations of molecules in the interstellar medium.

Molecular Spectroscopy

Whereas most atomic spectra are determined simply by transitions between individual electronic states, molecular spectra are more complex because molecules have additional degrees of freedom associated with vibration and rotation. Each electronic state of a molecule possesses a manifold of vibrational levels, and each of those vibrational levels has a ladder of rotational levels associated with it. Electronic transitions in molecules therefore occur between specific vibrational and specific rotational levels in each electronic state. The equivalent of a single atomic line corresponding to an electronic transition is – for a molecule – replaced by a set of many lines (see Figure 1.2 for part of the H2 electronic transition B–X, between the two lowest electronic states, showing the vibrational and rotational structure). These electronic transitions often lie in the ultraviolet spectrum.

Molecules also possess transitions that have no counterpart in atoms. Molecules may undergo transitions between specific rotational states of vibrational states, that is, ro-vibrational transitions. These transitions usually occur in the near-infrared range. Pure rotational transitions may also occur between rotational states of the same vibrational level. These tend to lie in the far-infrared, or in the millimetre to submillimetre range of the electromagnetic spectrum.