As we have seen, electrons in atoms, ions and molecules exist in discrete orbitals, which describe (amongst other things) an energy level. The energy level of an electron tells us how easily we can remove it from its orbital – electrons in orbitals close to an atomic nucleus tend to be very stable and thus need a lot of energy to remove, whereas electrons farther away are less stable and require less energy.
Completely removing an electron from an atom is called ionisation, as we end up with an negatively-charged electron and a positively-charged ion (charged atom or molecule). The energy required to do this is called the ionisation energy.
If we want to ionise an atom, we need an energy source such as heat or light (this energy is carried by photons, represented in the diagrams as hν). If the energy supplied is equal to or greater than the ionisation energy, ionisation will occur (any excess is used to increase the electron’s kinetic energy). If the energy supplied is less than the ionisation energy, ionisation cannot occur – but electrons are excited into higher energy levels, often called energy states.
Under normal conditions, the electrons surrounding an atom are said to be in ground state – they occupy the orbitals closest to the nucleus, which are the most stable. Higher energy states exist, but they are only occupied by electrons when there is some energy input (such as heat or light). Excited states are therefore unstable – without constant energy input to maintain the higher energy level, electrons will quickly relax back to ground state (this is sometimes called decay; note that this is not the same as radioactive decay) . Again, the energy has to go somewhere and the atom must emit a photon with energy equal to the difference between the two levels. This process is known as luminescence.
This behaviour is widely taken advantage of in chemistry (to study energy levels and detect certain elements), but also finds its way into common products such as UV fluorescent paints. Paint molecules are designed so that they absorb ultraviolet (UV) light, which excites electrons into higher molecular orbitals. Some of the energy is lost through molecular vibrations (as heat), so when the electrons relax back to ground state the photon emitted has a little less energy. This puts the frequency down into the visible range, so the paint appears to be glowing – it is absorbing invisible UV radiation, but emitting visible radiation.