The energy of a molecule is determined by the translational, rotational, and vibrational components. Higher rotational states result in a series of peaks that are designated as P, Q and R bands.  The Q branch is typically a singlet, as the rotational contribution effectively is zero when there is no change in J. It was observed that trigonal pyramidal molecules like ammonia showed two Q branches.  The source of these branches is the inversion of the molecule; the phenomenon is called inversion doubling.

The ability of quantum mechanisms to explain the doubling of the ammonia spectrum in a sufficiently complex derivation without being tedious, presents students with a fascinating glimpse into the possibilities of quantum.