Page "Energy level" Paragraph 41

If an atom, ion, or molecule is at the lowest possible energy level, it and its electrons are said to be in the ground state.

If it is at a higher energy level, it is said to be excited, or any electrons that have higher energy than the ground state are excited.

Such a species can be excited to a higher energy level by absorbing a photon whose energy is equal to the energy difference between the levels.

Conversely, an excited species can go to a lower energy level by spontaneously emitting a photon equal to the energy difference.

A photon's energy is equal to Planck's constant ( h ) times its frequency ( ν ) and thus is proportional to its frequency, or inversely to its wavelength.

Correspondingly, many kinds of spectroscopy are based on detecting the frequency or wavelength of the emitted or absorbed photons to provide information on the material analyzed, including information on the energy levels and electronic structure of materials obtained by analyzing the spectrum.