Quantum tunneling through-space

Often molecular energy levels occur in closely spaced doublets having opposite parity. This is of particular interest when there are symmetrically equivalent minima, separated by a barrier, in the potential energy function of the electronic state under investigation. This happens in the PH molecule and such pairs of levels are called inversion doublets the splitting between such parity doublet levels depends on the extent of the quantum mechanical tunnelling through the barrier that separates the two minima. This is discussed further in section Al.4.4. 

---