In general terms, the energy that must be applied to an atomic nucleus, an atom, or a molecule to cause it to leave the ground state and go into an excited state is called the excitation energy. Similarly, a transition from one excited state to another can also have an associated excitation energy. For electronic transitions in atoms, the energy will be quantized according to the energy of the orbital of the excited state. In molecules, due to vibrational (vibronic) coupling between atoms, the electronic excitation energies take on a range of values corresponding to the different vibrational states.
Because nuclear binding forces are many orders of magnitude higher than atomic or molecular forces, nuclear excitation energies are much larger. Where an electronic excitation may require a few electron volts (eV), the excitation energy needed to put a nucleus in an excited state can be around a million eV (1 MeV).
Excitation energies can be measured with a variety of spectroscopic techniques. From these measurements, details of the structure and bonding of materials can be determined. This information can be used in chemical analysis, synthetic chemistry, and materials science, in general.