Stark Effect
In the presence of a static external electric field the 2J+1 degeneracy of each rotational state is partly removed, an instance of a Stark effect. For example in linear molecules each energy level is split into J+1 components. The extent of splitting depends on the squares of the electric field strength and of the dipole moment of the molecule. In principle this provides a means to determine the value of the molecular dipole moment with high precision. Examples include carbonyl sulfide, OCS, with μ = 0.71521 ± 0.00020 Debye. However, because the splitting depends on μ2, the orientation of the dipole must be deduced from quantum mechanical considerations.
Read more about this topic: Rotational Spectroscopy
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