Fluorescence , Phosphorescence and photoluminescence

luminescence, fluorescence and example of fluorescence, phosphorescence , Jablonski diagram, Photoluminescence.
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1. FLUROSCENCE ,PHOSPHERSCENCE, PHOTOLUMINISCENCE Submitted by : Preeti Choudhary (17/MAP/016)

2. LUMINISCENCE A hot body that emits radiation solely because of its high temperature is said to exhibit incandescence. All other forms of light emission are called luminescence . When luminescence occurs, the system loses energy and if the emission is to be continuous, some form of energy must be supplied from elsewhere. When the external energy supply is by means of the absorption of infrared, visible or ultraviolet light, the emitted light is called photoluminescence and this is the process that takes place in any fluorimetric analysis. Luminescent materials

3. FLUROSCENCE The emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. The emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can only be seen when exposed to UV light.

4. FLUORESCENC E

5. For example Commonly employed sources in fluorescence spectrometry have spectral outputs either as a continuum of energy over a wide range or as a series of discrete lines. An example of the first type is the tungsten-halogen lamp and of the latter, a mercury lamp. It is advantageous to employ a source whose output is a continuum and the most commonly employed type is the xenon arc. Fluorescent minerals emit visible light when exposed to ultraviolet light

6. PHOSPHORESCENCE The indirect process of conversion from the excited state produced by absorption of e with an even number of electrons, to an excited tri nergy , the singlet state, to a triplet state, is known as intersystem crossing. Direct transition from the ground state, usually a singlet state, for a molecule plet state is theoretically forbidden, which means that the reverse transition from triplet to ground state will be difficult. Thus, while the transition from an excited singlet state, for example, S1, to the ground state with the emission of fluorescence can take place easily and within 10-9 - 10- 6 seconds, the transition from an excited triplet state to the ground state with the emission of phosphorescence requires at least 10-4 seconds and may take as long as 10 2 seconds.

7. Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation. Commonly seen examples of phosphorescent materials are the glow-in- the-dark toys, paint, and clock dials that glow for some time after being charged with a bright light such as in any normal reading or room light. Typically the glowing then slowly fades out within minutes to up to a few hours in a dark room. Phosphorescent bird figure

8. Jablonski Diagram

9. The triplet state of a molecule has a lower energy than its associated singlet state so that transitions back to the ground state are accompanied with the emission of light of lower energy than from the singlet state. Therefore, we would typically expect phosphorescence to occur at longer wavelengths than fluorescence.

10. Photoluminescence (abbreviated as PL ) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photo excitation (excitation by photons). Time periods between absorption and emission may vary: ranging from short femtosecond for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphorescent processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours. PHOTOLUMINISCENCE

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