The molecules which generate astrophysical masers form a small subset of those so far discovered in space. At the end of 2003, the total number of molecular species detected was 129, not including isotopic substitutions, with the largest containing 13 atoms (Lequeux, 2005). There is no indication that we have reached anything approaching an exhaustive list of astrophysical molecules. Indeed, there has been a recent detection of the fullerenes C60 and C70 (Cami et al., 2010). Of those known in space, only 11 molecules (and the hydrogen atom) are known to give rise to masers, and again I have not included isotopic substitutions in this count. The maser molecules tend to be small, ranging from hydrogen – the simplest atom – to methanol, the most complex maser molecule, which has six atoms. Despite their simplicity, the maser molecules have a surprising variety of structures, the basics of which are explained below.

Molecular spectroscopy

Broadly speaking, molecular transitions can be placed into the following energy hierarchy. Transitions of the highest energy result from re-arranging the electron wavefunction of the molecule. Such transitions between electronic states are analogous to those in individual atoms, and have typical wavelengths that range from the near infrared, through the visible, and into the ultraviolet region of the electromagnetic spectrum. Electronic transitions are rarely involved in the pumping of astrophysical masers, though there are examples of schemes which do employ them.