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Molecular rotation changes a molecule's orientation. Its rotational kinetic energy depends on the arrangement and masses of its chemical atoms. A rigid rotor freezes internal distances to isolate this motion; rotational spectroscopy tests the resulting energy eigenvalues through transitions between quantum states. The molecular beam electric resonance experiment provides a complementary probe of the coupling to an electric field.

Rotational spectroscopy probes transitions between molecular rotation quantum states. In the simple linear rigid rotor, line spacing determines a moment of inertia, while an isotope shift of a linear rotor tests the mass dependence. More general molecular geometry requires several principal moments of inertia and more complicated assignments. The combination makes spectra sensitive fingerprints of both composition and shape.