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A wavelength shifter is a photofluorescent material that absorbs higher frequency photons and emits lower frequency photons. The material absorbs one photon, and emits one or multiple lower-energy photons. The relaxation time of the excited molecule is usually in the order of nanoseconds.
These materials can be used to increase the efficiency of a photovoltaic cell (solar cell) by changing one "too-high" energy photon into multiple "just-right" energy photons. Wavelength shifters are often used in particle physics to collect scintillation light in particle detectors. This usually happens with acrylic slaps or Optical fibers doped with a wavelength shifter, in some cases also paints are used. Outside of science Wavelength shifter are sometimes used to achieve UV resistance of plastics instead of absorbers. Wavelength shifter are also used to shift UV light to the visible spectrum in Fluorescent lamps or LEDs, in most cases this is done with a Phosphor that can be considered a wavelength shifter with a long (ms) relaxation time.
Organic wavelength shifters usually contain one or more benzene-ring(s) (e.g. de:1,4-Bis(2-methylstyryl)benzol or p-Terphenyl) since the and bonds here are useful in the absorption/emission of the photon and the energy transport within the molecule. Modifications of the molecules allow in some cases the tuning of the acceptance and emission wavelength regime. The wavelength shift occurs due to the Franck–Condon principle, while excess energy is usually carried away in form of phonons.
Most organic wavelength shifters are planar molecules, causing a decrease in wavelength shifting efficiency when crystallized due to energy exchange between the molecules. Current research has also created 3 dimensional wavelength shifters that show the opposite effect since clustering together limits the energy that can be stored as rotational energy.
Wavelength shifter usually have many absorption and emission lines that are broad enough to create a absorption and emission spectrum. The separation between absorption and emission spectrum is defined by the so called Stokes shift.
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