The vibrational energy levels are illustrated. These same energy levels can be represented more abstractly as shown in Figure 19. Energy level diagram. Resonance at the molecular level can be achieved by matching frequencies corresponding to the spacing between energy levels. A common device that uses this type of resonance is the microwave oven.
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.