It is relatively common nowadays to see a plethora of glow-in-the-dark products across a vast spectrum of items. Although most regularly found in toys and watches, there seems to be no end to the types of item that can be modified to glow. So, how does this phenomenon occur?
Firstly, it is important to pin-point what is meant by the term ‘glow-in-the-dark’. In this instance, it is used to describe luminescence. Luminescence is the emission of light from a source that has not been heated; due to the lack of heat, it is a form of spontaneous emission. If heat is involved, then the phenomena is labelled incandescence and is not considered spontaneous. There are several different ways in which luminescence can occur, including through chemical reactions, electric currents and the compression of certain types of crystal.
The type of luminescence encountered most commonly in day-to-day products is phosphorescence. Phosphorescence is a specific type of photoluminescence that involves molecules called phosphors. Usually transition metals or rare earth elements, phosphors absorb energy from electromagnetic radiation and emit it as visible light over timescales between nanoseconds and hours. Funnily enough, phosphorus is not defined as a phosphor – it is chemiluminescent as opposed to photoluminescent.
Underpinning the ability for photoluminescent materials to emit light is quantum mechanics. When a photon collides with the phosphor and is absorbed, it excites an electron to a higher transition state. When the electron drops back down to its usual state, it releases a photon whose energy exactly corresponds to the size of the gap between the levels. This is what we perceive as the emitted ‘glow’. In the majority of photoluminescent events, this excitation and relaxation of electrons occurs over extremely short timescales, often in the region of nanoseconds. This is known as fluorescence. However, when phosphors (such as zinc sulphide and strontium aluminate) are involved, phosphorescence can emit light over much longer periods of time. This is the key difference between fluorescence and phosphorescence – the time taken to re-emit the energy.
So, why is it that phosphorescence can occur over such long timescales? Again, it’s all based on quantum mechanics. As you may know, ‘allowed transition states’ and ‘forbidden transition states’ are key within quantum mechanics. These, along with a rather expansive set of rules, govern the movement of electrons in their respective shells. In normal circumstances, when a ground state electron is excited to a higher energy level, it will transition into an exited singlet state. This means that it has paired up with another electron with an opposite spin (i.e. obeying the Pauli Exclusion Principle). The Pauli Exclusion Principle states that no two particles with a half integer spin (such as electrons) can occupy the same orbital at the same time (think of people on a bus – ideally, people aim to sit on an unoccupied pair of seats before going to sit next to a stranger).
However, in some cases (as in phosphorescence), the excited electron will transition into an excited triplet state. This is where things get a little complicated. In an excited triplet state, the excited electron is spinning parallel to the ground state. This is called intersystem crossing. As having the same spin is considered ‘forbidden’ within quantum mechanics (again, due to our friend Pauli), it means that the excited triplet state electron is in a bit of a pickle. The ‘forbidden’ transitions are less kinetically favourable than the ‘allowed’ transitions, meaning that it is more difficult for the electron to relax back to its normal ground state. This prolongs the relaxation progress, ensuring that the glow of released photons lasts for longer.
So, there it is. Objects glow in the dark due to the phenomena of phosphorescence – the process by which electrons are excited to ‘forbidden’ quantum spin states. As the electrons are allowed to relax, they emit a photon of light corresponding to the distance between the energy levels, producing the subtle glow which is now associated with watch hands and plastic stars. In conclusion, the entire glow-in-the-dark industry is reliant upon a small group of rebellious electrons not doing what they should. Pretty neat, huh?