Visible light from thermal radiation begins emitting at minimum temperatures of a few hundred degrees K, while Luminescence can be observed at any temperature. That is why Luminescence is sometimes labeled a cold light.
The second condition, in which Luminescence should last for a time exceeding the period of electromagnetic oscilation , distinguishes it from reflected and stray light. In Luminescence there are intermediate processes between absorption and emission duration which exceeds the period of a single electromagnetic oscilation. As a result, Luminescence looses correlation between phases of absorbed and emitted light, in contrast to reflected and stray light, in which the phase correlation can always be observed.
Luminescent analysis is performed using the intrinsic Luminescence of materials under observation, or special markers luminophors are added when the material itself does not demonstrate luminescent properties. Luminescence applications are so numerous and diverse that contemporary reviews and books are unnable to accommodate them all. Photoluminescence, a Luminescence stimulated by light absorption in UV-Vis-NIR spectral region, represents any process in which material absorbs electromagnetic energy at a certain wavelength and then emits part of it at a different usually longer wavelength.
Therefore, only a part of the absorbed energy is transformed into luminiscent light. The rest of it ends up as molecular vibrations, or simply as a heat. Photoluminescence is the most popular type of Luminescence because a large selection of reliable and inexpensive excitation sources are available and also because the effect can often be observed with the naked eye.
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. 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. The ultraviolet rays then cause fluorescence in a coating applied onto the inside of the light bulb. Fluorescent lamps light up due to the effects of fluorescence. Luminescence refers to any mechanism where photons are generated, without an input of heat. Fluorescence refers to a specific type of luminescence where the energy to produce the photon comes from the absorption of a photon with higher energy.
An excited singlet state is produced in the intermediate stages. However, spin-orbit coupling relaxes this restriction and a radiative transition from the T 1 to the S 1 becomes possible.
It should also be noted that the emission from some materials does not always neatly fall into one category or the other. An example of this is thermally activated delayed fluorescence TADF. This gives rise to a delayed S 1 to S 0 transition which results in photoluminescence at a timescale between fluorescence and phosphorescence, known as delayed fluorescence.
Chemists and biologists, who primarily study molecular systems, favour the use of fluorescence and phosphorescence since in these highly localised molecular systems there are distinct singlet and triplet states.
In contrast; physicists predominately study semiconducting materials where the electrons are highly delocalised and the concept of singlet and triplet frequently ceases to be relevant. This is one of the reasons why physicists tend to use the broader term photoluminescence to describe light emission.
Whatever you decide to call it; photoluminescence, fluorescence and phosphorescence can provide a wealth of information on the properties of molecules and materials; ranging from the determination of charge carrier lifetime in solar cells to measuring the solvation dynamics around micelles in living cells.
To measure fluorescence and phosphorescence a spectrometer is required and Edinburgh Instruments offers a range of single photon counting photoluminescence spectrometers to measure photoluminescence spectra, lifetimes, anisotropy and quantum yields of your samples. To find out how we can help you with your research requirements, please contact us.
If you have enjoyed reading this article about fluorescence and phosphorescence , and would like to be the first to see all the latest news, applications, and product information from Edinburgh Instruments, then sign-up to our infrequent newsletter via the red sign-up button below, and follow us on social media. What is Luminescence? What is Photoluminescence? What is the difference between fluorescence and phosphorescence? Products for Luminescence, Photoluminescence, Fluorescence, and Phosphorescence To measure fluorescence and phosphorescence a spectrometer is required and Edinburgh Instruments offers a range of single photon counting photoluminescence spectrometers to measure photoluminescence spectra, lifetimes, anisotropy and quantum yields of your samples.
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