Microwave ovens contain a microwave generator called a klystron. The klystron vacuum tube was extensively used in radar during World War II and subsequent communications applications. The frequencies corresponding to the spacing between the energy levels in the OH bonds of a water molecule, DE , are in the range of frequencies easily generated by a tunable microwave generator. Resonance is used to identify molecules and atoms by noting the frequencies at which it occurs. Spectroscopy is a method for doing this by matching the frequency of an external source with the natural frequencies of atoms or molecules one wants to study.
Vibrational spectroscopy measures characteristic vibrations between atoms within a molecule.
Because the vibrations are specific to different atoms, bonds, and environments they can be used to study how molecules are put together. For example, one can distinguish between a single, double, or triple carbon-carbon bond by the specific frequencies at which these bonds will vibrate. Unlike the electron spin and its magnetic moment, however, there is no simple ratio between the magnetic moment of a nucleus and its spin. There are large variations. The characteristic frequencies investigated in NMR spectroscopy are those of spinning nuclei in an applied magnetic field.