How does phosphorescence work

The main difference is the time in which it takes to do so. In fluorescence, the emission is basically immediate and therefore generally only visible, if the light source is continuously on such as UV lights ; while phosphorescent material can store the absorbed light energy for some time and release light later, resulting in an afterglow that persists after the light has been switched off. Depending on the material, this afterglow can last anywhere from a few seconds to hours. Picture a scene at the night club: teeth, eyes and fabric glowing under the black light are fluorescent, the emergency exit sign is phosphorescent and the glow sticks are chemiluminescent.

The light produced by the glow stick is a result of two chemicals that were mixed when a small capsule in the stick was broken and mixed by shaking the stick. Be prepared to dive into your remnant knowledge of physical chemistry. To understand the principles behind why molecules absorb and emit light, we need to first look at electrons and understand their ground and excited states and electron spin, concepts that are still not fully understood by modern quantum mechanics.

Photoexcitation A pre-requisite for photoluminescence, regardless of whether it is fluorescence or phosphorescence, is the ability of a molecule to absorb light radiation leading to electronic excitation. A molecule-bound electron absorbs a photon and therefore its energy and becomes excited.

The resulting electronically excited states are intrinsically unstable and electrons will relax back to their ground state by several combinations of mechanical steps, dissipating energy in different ways in the process. Vibrational relaxation is extremely rapid 12 to 10 sec and leads to dissipation of the energy within one excited state through vibrational energy, which is quickly dissipated as heat to neighboring molecules. As the energy is not dissipated by the emission of light, vibrational relaxation is a non-radiative transition.

Internal Conversion is another non-radiative transition, which is iso-energetic and also rapid 10 to 10 sec. No energy is dissipated during the transition. An electron can fully dissipate the initially absorbed energy through vibrational relaxation and internal conversion alone.

In this case the relaxation process will be entirely non-radiative and the molecule will neither fluoresce nor phosphoresce and all absorbed energy will be dissipated through heat.

The probability at which radiative events occur versus fully non-radiative relaxation defines the quantum yield of a fluorophore and therefore how bright it will shine. An electron gets excited by absorpting a photon of a certain wavelength. It relaxes to vibrationless levels of the lowest excited state S1 through a series of non-radiative transitions vibrational relaxation and internal conversion.

Further relaxation to ground state S0 by fluorescence results in emission of a photon of lower energy and longer wavelength than the exciting photon. Fluorescence One radiative mechanism by which excited electrons may relax is a light-emitting transition from the lowest excited state S 1 to ground state S 0 in a fast 10 -9 to 10 -6 sec process called fluorescence. The energy difference is dissipated by emitting a photon.

Due to the electron having shed some of the original excitation energy by vibrational relaxation, the emitted photon will be of lower energy and thus of longer wavelength. The resulting emitted wavelength is independent of the excitation wavelength, as usually excited molecules decay to the lowest vibrational level of the lowest excited state by non-radiative processes before fluorescence emission takes place. Depending on the molecule, non-radiative decay might be responsible for dissipating a smaller or bigger portion of the excitation energy, resulting in molecule-specific shifts between the excitation wavelengths and the wavelengths being emitted.

This phenomenon is termed Stokes shift. As frequently the same electronic transitions are involved in excitation and emission of a fluorescent molecule, the excitation and emission spectra often resemble reflections of each other, which is referred to as the mirror image rule of fluorescence.

The probability by which excitation and emission events occur at different wavelengths depicted by arrow width define the fluorescence spectra of a molecule. Phosphorescence To understand the difference between fluorescence and phosphorescence, we need to take a little detour into electron spin. Spin is a fundamental, unvarying property of the electron and a form of angular momentum that defines behavior in an electromagnetic field.

Two electrons in a single orbital will always have antiparallel spin at singlet ground state S 0. Upon promotion of one electron into excited state, the electron maintains its spin orientation and a singlet excited state S 1 is formed, where the both spin orientations remain paired as antiparallel.

All relaxation events in fluorescence are spin neutral and the spin orientation of the electron is maintained at all times. However, this is different for phosphorescence. Fast 10 to 10 -6 sec Intersystem crossing from singlet exited state S 1 to an energetically favorable triplet excited state T 1 leads to inversion of the electron spin. Triplet excited states are characterized by parallel spin of both electrons and are metastable. Relaxation occurs via phosphorescence , which results in another flip of the electron spin and the emittance of a photon.

Additionally, more energy is dissipated by non-radiative processes during phosphorescent relaxation than in fluorescence, therefore the energy difference between the absorbed and emitted photon is bigger and the wavelength shift more pronounced. Thus, phosphorescence is characterized by a bigger Stokes shift than fluorescence. It causes a rather weak emission of photons because the electron spin has to be reversed again. The energy is trapped in this state for a while and can only be released slowly [6].

After all energy has been released, the electrons are back in the ground state [6,7,10]. The spin-allowed and -forbidden processes serve as explanations for an immediately ceasing glow of fluorescence and for the afterglow of phosphorescence. Phosphorescence usually occurs only with "heavier" molecules since the spin has to be reversed with the help of spin-orbit-coupling.

Whether electromagnetic radiation is emitted at all, and with which wavelength, depends on how much energy can be released beforehand by non-radiative decay [6,7]. It also depends on the properties of so-called quenchers that are surrounding molecules and are able to take up larger amounts of energy.

All processes that can lead to an inhibition of radiative decays can cause fluorescence quenching. Examples are non-radiative decay processes, but also the destruction of the fluorescent molecule [10].

The quantum efficiency describes the efficiency of the process and is defined as the ratio of absorbed and emitted photons [13]. This property is different for each substance. Even though this text focuses on photoluminescence, the photo-physical processes are the same for all types of luminescence [4].

In addition to products like glow sticks, fluorescence and phosphorescence are used in many other ways. Further examples are guideposts leading to an emergency exit that need no electric supply but glow at night due to phosphorescence. Even plants can be made fluorescent: Spinach can be modified with the help of nanotechnology so that it can detect traces of explosive substances in the groundwater.

The leaves contain carbon nanotubes to which nitroaromatics can bond. If they do, a fluorescent signal is released by the plant and can be detected with infrared cameras [14]. The video demonstrates different types of luminescence. On the left-hand side, it shows the fluorescence of the dye curcumin, which is contained in the spice turmeric, under UV light [5,15]. Curcumin is dissolved in alcohol to make the fluorescence visible. The plastic spider and the compound in the small tube are examples for phosphorescence.

Strontium aluminate, which is contained in the tube, is initially excited by UV radiation and eventually emits green light. The cause for this is a doping with elements like europium, which makes the compound usable as a luminescent pigment [15]. Bending the glow stick at the right-hand side initiates a chemical reaction between hydrogen peroxide and a dye and phenyl oxalate.

Chemiluminescence can be observed. Video 1. Fluorescence, phosphorescence, and chemiluminescence in comparison. Arnold et al. ISBN: Wiechoczek, Wenn Mineralien selber leuchten — Phosphoreszenz, Fluoreszenz und Lumineszenz in German , chemieunterricht. Wiechoczek, Chemie mit Curry in German , chemieunterricht. Atkins, J. DOI: Please note that to comment on an article you must be registered and logged in. Registration is for free, you may already be registered to receive, e. When you register on this website, please ensure you view our terms and conditions.

All comments are subject to moderation. 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. Interest in the nature of phosphorescence and other types of luminescence eventually led to the discovery of radioactivity in In addition to natural minerals, phosphorescence is produced by chemical compounds.

The best-known of these is zinc sulfide, which has been used in glow-in-the-dark stars and other products since the s. Zinc sulfide usually emits a green phosphorescence, although phosphors may be added to change the color of light. Phosphors absorb the light emitted by phosphorescence and then release it as another color. Today, doped strontium aluminate is the phosphorescent compound of choice. It glows ten time brighter than zinc sulfide and stores its energy much longer.