Tru Physics

Fluorescence

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Fluorescence is a type of photoluminescence in which a substance absorbs light at a certain wavelength and then emits light at a longer wavelength, sometimes within the visible spectrum. This phenomenon is used in a wide range of applications, including biomedical imaging, chemical analysis, and materials science. One of the primary differentiating factors between fluorescence and other forms of photoluminescence (like phosphorescence) is the time scale involved. Excited electrons generally return to the ground state very quickly, within nanoseconds.

A fluorescent protein is responsible for the blue light emitted by this particular jellyfish.
A fluorescent protein is responsible for the blue light emitted by this particular jellyfish.

Fluorescence occurs when a molecule or atom absorbs energy from a photon. An excited electron in “kicked” into a higher energy level. The excited state is usually short-lived, and the molecule or atom quickly returns to its ground state by releasing the excess energy as a photon. This emitted light has a longer wavelength than the absorbed light, and is typically in the visible or ultraviolet range. This is an example of Stokes shift.

The fluorescence quantum yield (QY) is one way of measuring the efficiency of this process. Very simply, the QY can be defined as a simple ratio:

\Phi=\dfrac{N(\text{photons emitted})}{N(\text{photons absorbed})}

However, a slightly more complicated definition which handles non-radiative decay more explicitly is written as:

\Phi=\dfrac{k_f}{\sum k_i}

where k_f is the rate at which the radiation is spontaneously emitted and \sum k_i is the sum of all rates of excited state decay. Specifically, \sum k_i includes non-radiative rates of decay.

Efficiencies are generally in the range of 10% to 100% for fluorescent materials. This is much higher than the efficiency range of phosphorescent materials.

One of the key advantages of fluorescence is that it is highly sensitive, allowing for the detection of very small amounts of a substance. This makes it a valuable tool in many fields, including biology and medicine, where it is used to visualize and track specific molecules or cells.

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Comments

2 responses to “Fluorescence”

  1. Phosphorescence – Tru Physics

    […] a substance emits light after being exposed to light energy (electromagnetic radiation). Unlike fluorescence, which is usually short-lived, phosphorescence can continue to emit light for a period of time […]

    Reply
  2. X-Ray Fluorescence – Tru Physics

    […] When a material is exposed to an incident beam of X-rays, the atoms in the material can absorb this energy and become excited. This causes the ejection of an electron from an inner orbital, creating a vacancy. To fill this vacancy, an electron from an outer orbital drops down, releasing energy in the form of an X-ray photon. This process is known as fluorescence. […]

    Reply

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