For ammonia, NH, the symmetric bending vibration is observed as two branches near 930 cm and 965 cm. This so-called inversion doubling arises because the symmetric bending vibration is actually a large-amplitude motion known as inversion, in which the nitrogen atom passes through the plane of the three hydrogen atoms, similar to the inversion of an umbrella. The potential energy curve for such a vibration has a double minimum for the two pyramidal geometries, so that the vibrational energy levels occur in pairs which correspond to combinations of the vibrational states in the two potential minima. The two v = 1 states combine to form a symmetric state (1) at 932.5 cm above the ground (0) state and an antisymmetric state (1) at 968.3 cm.
The vibrational ground state (v = 0) is also doubled although the energy difference is much smaller, and the transition between the two levels can be measured directly in the microwave region, at ca. 24 Ghz (0.8 cm). This transition is historically significant and was used in the ammonia maser, the fore-runner of the laser.