How does phosphorescence occur

Unlike the relatively swift reactions in a common fluorescent tube, phosphorescent materials used for these materials absorb the energy and "store" it for a longer time as the subatomic reactions required to re-emit the light occur less often. Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of 10 nanoseconds. However, for light to be absorbed and emitted at these fast time scales, the energy of the photons involved i.

In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity see term symbol , usually a triplet state. As a result, the energy can become trapped in the triplet state with only quantum mechanically "forbidden" transitions available to return to the lower energy state.

These transistions, although "forbidden", will still occur but are kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states.

If the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called "glow-in-the-dark" materials. Some examples of "glow-in-the-dark" materials do not glow because they are phosphorescent. For example, " glow sticks " glow due to a chemiluminescent process which is commonly mistaken for phosphorescence.

In chemi-luminescence, an excited state is created via a chemical reaction. The excited state will then transfer to a "dye" molecule, also known as a sensitizer, or fluorophor , and subsequently fluoresce back to the ground state. Common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate. Use of zinc sulfide for safety related products dates back to the s. However, the development of strontium oxide aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category.

Strontium oxide aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.

Sometimes fluorescent dyes are added to phosphorescent materials to change the color of the light. Fluorescent materials absorb energy and immediately release light. Phosphorescent objects glow more brightly under a black light than in the dark because they may contain fluorescent dyes and because some phosphorescent transitions occur quickly.

In fluorescence, a surface absorbs and re-emits a photon almost instantly about 10 nanoseconds. This type of photoluminescence is fast because the energy of the absorbed photons matches energy states and allowed transitions of the material.

Phosphorescence lasts much longer milliseconds up to days because the absorbed electron crosses into an excited state with higher spin multiplicity. Quantum mechanics allows for forbidden transitions, but they are not kinetically favorable, so they take longer to occur. A Jablonski diagram is commonly used to display the difference between fluorescence and phosphorescence. It received light from the Sun and then like the Moon gave out light in the darkness.

The stone was impure barite, although other minerals also display phosphorescence. Other phosphorescent gems include some diamonds known to Indian king Bhoja as early as , rediscovered by Albertus Magnus and again rediscovered by Robert Boyle and white topaz.

The Chinese, in particular, valued a type of fluorite called chlorophane that would display luminescence from body heat, exposure to light, or being rubbed. The energy is emitted as electromagnetic radiation or photons. The emitted light has a longer wavelength and a lower energy than the absorbed light because a part of the energy has already been released in a non-radiative decay process [10].

This is the reason that an emission in the visible spectrum can be achieved by excitation with non-visible UV-radiation. This shift towards a longer wavelength is called Stokes shift [11]. Both fluorescence and phosphorescence are spontaneous emissions of electromagnetic radiation. The difference is that the glow of fluorescence stops right after the source of excitatory radiation is switched off, whereas for phosphorescence, an afterglow with durations of fractions of a second up to hours can occur [6,7].

To compare the photo-physical processes behind both phenomena, there are some facts about electrons that are helpful for understanding: Electrons are particles that have a so-called spin and a spin quantum number. This number is a property that we actually cannot imagine or describe easily.

It is often compared with a spinning top, either spinning in a clockwise or anti-clockwise direction. However, this description is neither mathematically nor physically quite correct. In the Jablonski diagram for fluorescence see Fig. Within those states, there are several energy levels. The higher the level is, the more energy an electron possesses when being in that level. In the case of singlet states, the electrons have antiparallel spins.

The electrons are lifted from the ground state S 0 , for example, to an energy level of the second excited state S 2 , when excited by electromagnetic radiation. After excitation stops, the electrons only stay in that excited state for a short period of time ca.

In doing so, energy initially can be released to the surroundings by vibrational relaxation. That means thermal energy is released by the motion of the atom or molecule until the lowest level of the second excited state is reached. The bigger gap between the second and first excited state is overcome by internal conversion.

That describes an electronic transition between two states while the spin of electrons is maintained.

Now, the electrons can relax further due to more vibrational relaxation until they reach the lowest energy level of the S 1 state. Theoretically, the electrons could relax even further in a non-radiative way until they eventually reach the ground state again. However, it can be the case that the last amount of energy is too large to be released to the surroundings because the surrounding molecules cannot absorb this much energy.

Then, fluorescence occurs, which leads to an emission of photons possessing a certain wavelength. The emission lasts only until the electrons are back in the ground state. Since during all those transitions the electron spin is kept the same, they are described as spin-allowed [6,7,10]. For phosphorescence, things are a bit different see Fig. There are again an S 0 ground state and the two excited states, S 1 and S 2. Additionally, there is an excited triplet T 1 state which lies energetically between the S 0 and S 1 state.

The electrons again have antiparallel spins in the ground state. Excitation happens in the same way as in fluorescence, namely through electromagnetic radiation. The release of energy through vibrational relaxation and internal conversion while maintaining the same spin is the same here, as well, but only until the S 1 state is reached.

Alongside the singlet states, a triplet state exists and so-called intersystem crossing ISC can occur since the T 1 state is energetically more favorable than the S 1 state. This crossing, like internal conversion, is an electronic transition between two excited states. But contrary to internal conversion, ISC is associated with a spin reversal from singlet to triplet. This ISC process is described as "spin-forbidden". It is not completely impossible — due to a phenomenon called "spin-orbit coupling" — however, it is rather unlikely [7].