When considering fluorescence, energy must be considered at a molecular level. When molecules form, two or more atoms form an association where the energy of the molecule is lower than that of the constituent atoms when they were separate. The total energy of the molecule is the sum of the energies holding the nuclei together and the energy of the chemical bonds holding the molecule together. It is the vibrational and electronic energy of the molecule that contributes to fluorescence. Molecules, like atoms, will generally be in their ground state.
Molecules can move to a greater energy state only when energy is added to their system.
One of the ways a molecule can gain energy is by absorbing light. If a molecule absorbs light, the energy of the light must equal the energy required to put the molecule in one of the higher energy states. When a molecule reaches an excited state, it does not stay there for very long but quickly returns to a lower energy state either by emitting light or by colliding with another atomic particle. When an molecule emits light, the energy of that light is equal to the energy difference between the quantum levels that molecules has moved between. Fluorescent penetrant materials usually contain several dye compounds that are especially suited for the production of fluorescence.