There is always a delay between the moment the material has absorbed the higher energy photon and the moment the secondary lower energy photon is re-emitted. This delay is defined by the lifetime of excitation states, or simply by how long atoms or molecules are able to stay in excited high-energy conditions.
Delay time can vary many orders of magnitude for different materials. Based on practical observations, two types of Photoluminescence were historically established — Fluorescence and Phosphorescence.
Technically, delay time is the only difference between them. It is shorter for Fluorescence 10 to 10 -7 s and much longer for Phosphorescence up to a few hours and even days.
The effect is widely used in such everyday practical applications as industrial and residential lightning neon and fluorescent lamps as an analytical technique in science and as a quality and process control method in industry. In contrast to Fluorescence, it demonstrates itself as a glowing that lasts long after the excitation light is gone. This effect is generally used by the Department of Transportation to attract drivers' attention to road signs, in adertising campains to produce glowing stickers and promotion materials, as well as in numerous industries to notify people of potential hazards and dangers.
Electroluminescence is a Luminescence excited in gases and solids by applying an electromagnetic field. Molecules are excited upon creation of any form of electric discharge in material.
Electroluminescence of gases is used in discharge tubes. The electroluminescence effect, which readily occurs in semiconductors and light emitting diodes LEDs , is the most well-known application.
Natural blue diamond emits light when electrical current is passed through it. Triboluminescence occurs when a material is scratched, crushed, rubbed or stressed mechanically in any way. When a material is subjected to mechanical stress spatially separated, electrical charges are produced. Upon recombining these charges, a flash of light emerges as a result of electric discharge, ionizing the surrounding space.
Since electrical discharge is in the foundation of Triboluminescence, it can be classified as a part of Electroluminescence. Blue or red Triboluminescence can be observed when sawing a diamond during the cutting process. Another example includes sugar crystals, which produce tiny electrical sparks while crushing.
Crystalloluminescence is a type of Luminescence generated during crystallization, used to determine the critical size of the crystal nucleus. There is a theory that the light from crystalloluminescence emerges through the micro-fracture of growing crystallites. Separation of electrical charges may occur on the fracture facets on the surface of micro-fractures and their following recombination. This effectively classifies Crystalloluminescence as a type of Triboluminescence and a subtype of Electroluminescence.
Let us note that electrically charged micro-fractures may be developed due to multiple processes such as the movement of charged dislocations, piezoelectrification, etc. Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.
It is believed that when a bubble starts imploding, extremely high pressures inside the bubble cause the water to form ice-like structures. At the moment when the opposite sides of an imploding bubble collide, the very strong mechanical stress causes the ice to fracture.
However, the focus of this article is on photoluminescence which forms the basis of the powerful non-destructive spectroscopic technique, photoluminescence spectroscopy, that is used extensively in both academia and industry. Figure 2: Types of luminescence and their energy sources. Photoluminescence is the emission of light from a material following the absorption of light.
The word in itself is interesting in that it the combination of the Latin derived word luminescence and the Greek prefix, photo -, for light. Any luminescence that is induced by the absorption of photons is called photoluminescence. This could equally be light emission from an organic dye molecule in solution Figure 3a , or band-to-band recombination of electrons and holes following photoexcitation of a semiconductor Figure 3b.
Figure 3: Examples of photoluminescence. Describing any photon absorption induced light emission as photoluminescence is accurate; however, it is common practice, particularly by chemists, to further subdivide photoluminescence into fluorescence and phosphorescence.
There are various definitions of fluorescence and phosphorescence with the simplest being that fluorescence is prompt photoluminescence that occurs very shortly after photoexcitation of a substance, while phosphorescence is long-lived photoluminescence that continues long after the photoexcitation has ceased.
While this is a simple definition, it does not explain why such a difference in the time-scales of the photoluminescence occurs and some materials can fall into a grey area between the classic fluorescence and phosphorescence timescales.
A more thorough definition has to be based on the quantum mechanics of excited and ground states involved in the emission process. Using this approach fluorescence and phosphorescence can be defined as photoluminescence where the radiative transition does not require a change in spin multiplicity and photoluminescence where the radiative transition involves a change in spin multiplicity respectively. Fluorescence and phosphorescence are most commonly used to refer to photoluminescence from molecular systems.
Electrons in stable molecules always exist in pairs, as molecules with unpaired electrons are extremely reactive and unstable. Figure 4: Origin of the singlet and triplet states.
When a photon is absorbed by the molecule, one of the electrons is promoted to a higher energy level and the molecule is now in an excited state. When spins of all electrons in the system end up being paired, the system is said to be in a singlet state. When there is a set of electrons with unpaired spins, the system is said to be in a triplet state. The excited electron can then go back to the ground level by emitting a photon. When an electron is in an excited triplet state, if it emits a photon to go back to the ground state, the process is referred to as phosphorescence.
When an electron is in the excited singlet state, when it emits a photon to go back to the ground level, the process is referred to as fluorescence. Compared to phosphorescence, electrons spend much shorter times in their excited states in fluorescence. The process of fluorescence takes place via several stages. First, the excited electron falls to a lower vibrational energy state, in a process named relaxation.
Then, a photon is emitted as the electron falls to the ground state. After the photon emission, the electron again undergoes relaxation to fall to the lowest vibrational energy level at the ground state.
Note that during relaxation processes, the electrons lose energy but photons are not emitted. Consequently, the photons emitted during fluorescence carry less energy compared to the absorbed photon. As a result, the emission spectrum of a material undergoing fluorescence is shifted towards larger wavelengths compared to its absorption spectrum.
This shift in wavelengths is called the Stokes shift. In fluorescent lamps , ultraviolet waves are first produced by passing an electric current through a gas.
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