Energy level diagram of NH₃ showing the ground (v = 0; lower half) and the first excited vibrational (v₂ = 1; upper half) states. The truncated vertical axes on the left and right show the energy from the ground (E∕k; K) and the corresponding wavenumber (; cm ⁻¹ ), respectively;the horizontal axis shows the quantum number K. The integral value next to each level or inversion doublet shows the quantum number J; the letter “a” or “s” represents the symmetry of the level. Ground-state inversion doublets (K > 0) can only be resolved when the figure is zoomed in due to the small energy differences. Important transitions as discussed in this article are labelled: curved green and magenta arrows indicate the (sub)millimetre rotational line emission within the ground and excited states, respectively; blue triangles mark the radio metastable inversion line emission in the ground state; and upward red dotted arrows indicate the absorption in the MIR ν₂ band. These transitions are labelled by their rest frequencies in GHz (see Table 1), except the MIR transitions which are labelled by their transition names (see Table 2).