Fluorescence
Fluorescence
Fluorescence is the process by which a substance absorbs electromagnetic radiation (visible or invisible light) from another source, then re-emits the radiation with a wavelength that is longer than the wavelength of the illuminating radiation. It can be observed in gases at low pressure and in certain liquids and solids, such as the ruby gemstone. Fluorescence is the principle that is the basis of the common fluorescent lamp used for lighting; it is also a useful laboratory diagnostic tool.
Fundamentals
Matter interacts with electromagnetic radiation (such as ultraviolet and visible light) through the processes of absorption and emission. The internal structure of atoms and molecules is such that absorption and emission of electromagnetic radiation can occur only between distinct energy levels. If the atom is in its lowest energy level or ground state, it must absorb the exact amount of energy required to reach one of its higher energy levels, called excited states. Likewise an atom that is in an excited state can only emit radiation whose energy is exactly equal to the difference in energies of the initial and final states. The energy of electromagnetic radiation is related to wavelength as follows; shorter wavelengths correspond to greater energies, longer wavelengths correspond to lower energies.
In addition to emitting radiation, atoms and molecules that are in excited states can give up energy in other ways. In a gas, they can transfer energy to their neighbors through collisions, which generate heat. In liquids and solids, where they attached to their neighbors to some extent, they can give up energy through vibrations. The observation of fluorescence in gases at low pressure comes about because there are too few neighboring atoms or molecules to take away energy by collisions before the radiation can be emitted. Similarly, the structure of certain liquids and solids permits them to exhibit strong fluorescence.
If the wavelength of the radiation that was absorbed by the fluorescent material is equal to that of its emitted radiation, the process is called resonance fluorescence. Usually though, the atom or molecule loses some of its energy to its surroundings, so that the emitted radiation will have a longer wavelength than the absorbed radiation. In this process, simply called fluorescence, Stoke’s law says that the emitted wavelength will be longer than the absorbed wavelength. There is a short delay between absorption and emission in fluorescence that can be a millionth of a second or less.
KEY TERMS
Angstrom —A unit of length equal to one ten-billionth of a meter.
Energy level —The internal energy state of an atom or molecule which is characterized by having only discrete, discontinuous values.
Excited state —Any energy level with an energy greater than that of the ground state.
Fluorescent efficiency —The ratio of the intensity of the fluorescent radiation to the intensity of the absorbed radiation.
Fluorescent lamp —A device that utilizes the phenomenon of fluorescence to produce light for illumination.
Ground state —The lowest energy level of an atom or molecule.
Metastable state —An energy level in which an atom or molecule can remain for a period longer than its other energy levels before returning to its ground state.
Phosphorescence —The persistent emission of radiation by a substance following the removal of the illuminating radiation.
Resonance fluorescence —Fluorescence in which the emitted radiation has the same wavelength as the absorbed radiation.
Stoke’s law —In fluorescence, the emitted wavelength is always longer than the absorbed wavelength.
Ultraviolet radiation —Radiation similar to visible light but of shorter wavelength, and thus higher energy.
Visible light —Electromagnetic radiation of wavelength between 4,000 and 8,000 angstroms.
Wavelength —The distance between two consecutive crests or troughs in a wave.
There are some solids, however, that continue to emit radiation for seconds or more after the incident radiation is turned off. In this case, the phenomenon is called phosphorescence.
As an example of fluorescence, consider the energy level diagram for the gemstone ruby in Figure 1.
Ruby is a crystalline solid composed of aluminum, oxygen, and a small amount of chromium, which is the atom responsible for its reddish color. If blue light strikes a ruby in its ground state, it is absorbed, raising the ruby to an excited state. After losing some of this energy to internal vibrations the ruby will settle into a metastable state—one in which it can remain longer than for most excited states (a few thousandths of a second). Then the ruby will spontaneously drop to its ground state emitting red radiation whose wavelength (longer than the blue radiation) measures 6,943 angstroms. The fluorescent efficiency of the ruby—the ratio of the intensity of fluorescent radiation to the intensity of the absorbed radiation—is very high. For this reason the ruby was the material used in building the first laser.
Applications
The most well-known application of fluorescence is the fluorescent lamp, which consists of a glass tube filled with a gas and lined with a fluorescent material. Electricity is made to flow through the gas, causing it to radiate. Often mercury vapor, which radiates in the violet and ultraviolet, is used. This radiation strikes the coating, causing it to fluoresce visible light. Because the fluorescence process is used, the fluorescent lamp is more efficient and generates less heat than an incandescent bulb.
Resonance fluorescence can be used as a laboratory technique for analyzing different phenomena such as the gas flow in a wind tunnel. Art forgeries can be detected by observing the fluorescence of a painting illuminated with ultraviolet light. Painting medium will fluoresce when first applied, then diminish as time passes. In this way paintings that are apparently old, but are really recent forgeries, can be discovered.
The molecule chlorophyll, which is found in all photosynthetic plants, fluoresces red in the presence of blue light. Research has shown that the amount of light emitted as fluorescence is a roughly 3 to 5 percent of the amount of photosynthesis occurring. This information has been used to estimate rates of photosynthesis in plants in various physiological conditions and it is very useful in ecological studies of lakes and oceans.
John Appel
Fluorescence
Fluorescence
Fluorescence is the process by which a substance absorbs electromagnetic radiation (visible or invisible light ) from another source, then re-emits the radiation with a wavelength that is longer than the wavelength of the illuminating radiation. It can be observed in gases at low pressure and in certain liquids and solids, such as the ruby gemstone. Fluorescence is the principle that is the basis of the common fluorescent lamp used for lighting; it is also a useful laboratory diagnostic tool.
Fundamentals
Matter interacts with electromagnetic radiation (such as ultraviolet and visible light) through the processes of absorption and emission . The internal structure of atoms and molecules is such that absorption and emission of electromagnetic radiation can occur only between distinct energy levels. If the atom is in its lowest energy level or ground state, it must absorb the exact
amount of energy required to reach one of its higher energy levels, called excited states. Likewise an atom that is in an excited state can only emit radiation whose energy is exactly equal to the difference in energies of the initial and final states. The energy of electromagnetic radiation is related to its wavelength as follows; shorter wavelengths correspond to greater energies, longer wavelengths correspond to lower energies.
In addition to emitting radiation, atoms and molecules that are in excited states can give up energy in other ways. In a gas, they can transfer energy to their neighbors through collisions which generate heat . In liquids and solids, where they attached to their neighbors to some extent, they can give up energy through vibrations. The observation of fluorescence in gases at low pressure comes about because there are too few neighboring atoms or molecules to take away energy by collisions before the radiation can be emitted. Similarly, the structure of certain liquids and solids permits them to exhibit strong fluorescence.
If the wavelength of the radiation that was absorbed by the fluorescent material is equal to that of its emitted radiation, the process is called resonance fluorescence. Usually though, the atom or molecule loses some of its energy to its surroundings, so that the emitted radiation will have a longer wavelength than the absorbed radiation. In this process, simply called fluorescence, Stoke's Law says that the emitted wavelength will be longer than the absorbed wavelength. There is a short delay between absorption and emission in fluorescence that can be a millionth of a second or less. There are some solids, however, that continue to emit radiation for seconds or more after the incident radiation is turned off. In this case, the phenomenon is called phosphorescence.
As an example of fluorescence, consider the energy level diagram for the gemstone ruby in Figure 1. Ruby is a crystalline solid composed of aluminum , oxygen , and a small amount of chromium, which is the atom responsible for its reddish color . If blue light strikes a ruby in its ground state, it is absorbed, raising the ruby to an excited state. After losing some of this energy to internal vibrations the ruby will settle into a metastable state—one in which it can remain longer than for most excited states (a few thousandths of a second). Then the ruby will spontaneously drop to its ground state emitting red radiation whose wavelength (longer than the blue radiation) measures 6,943 angstroms. The fluorescent efficiency of the ruby—the ratio of the intensity of fluorescent radiation to the intensity of the absorbed radiation—is very high. For this reason the ruby was the material used in building the first laser .
Applications
The most well-known application of fluorescence is the fluorescent lamp, which consists of a glass tube filled with a gas and lined with a fluorescent material. Electricity is made to flow through the gas, causing it to radiate. Often mercury vapor, which radiates in the violet and ultraviolet, is used. This radiation strikes the coating, causing it to fluoresce visible light. Because the fluorescence process is used, the fluorescent lamp is more efficient and generates less heat than an incandescent bulb.
Resonance fluorescence can be used as a laboratory technique for analyzing different phenomena such as the gas flow in a wind tunnel. Art forgeries can be detected by observing the fluorescence of a painting illuminated with ultraviolet light. Painting medium will fluoresce when first applied, then diminish as time passes. In this way paintings that are apparently old, but are really recent forgeries, can be discovered.
John Appel
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Angstrom
—A unit of length equal to one ten-billionth of a meter.
- Energy level
—The internal energy state of an atom or molecule which is characterized by having only discrete, discontinuous values.
- Excited state
—Any energy level with an energy greater than that of the ground state.
- Fluorescent efficiency
—The ratio of the intensity of the fluorescent radiation to the intensity of the absorbed radiation.
- Fluorescent lamp
—A device that utilizes the phenomenon of fluorescence to produce light for illumination.
- Ground state
—The lowest energy level of an atom or molecule.
- Metastable state
—An energy level in which an atom or molecule can remain for a period longer than its other energy levels before returning to its ground state.
- Phosphorescence
—The persistent emission of radiation by a substance following the removal of the illuminating radiation.
- Resonance fluorescence
—Fluorescence in which the emitted radiation has the same wavelength as the absorbed radiation.
- Stoke's law
—In fluorescence, the emitted wavelength is always longer than the absorbed wavelength.
- Ultraviolet radiation
—Radiation similar to visible light but of shorter wavelength, and thus higher energy.
- Visible light
—Electromagnetic radiation of wavelength between 4,000 and 8,000 angstroms.
- Wavelength
—The distance between two consecutive crests or troughs in a wave.
Fluorescence
Fluorescence
Fluorescence is an optical phenomenon wherein a material emits light in response to some external stimulus. Normally, the fluorescent light that is emitted is of a specific color or group of colors that is released when the material is bombarded with light in some other part of the color spectrum.
Certain minerals have a characteristic fluorescence pattern when hit with white light or ultraviolet light. Fluorite and calcite are two examples of fluorescent minerals. There are also many organic dye molecules with useful fluorescent properties. These molecules absorb light energy from external sources, and this energy causes some excitation of the electron orbitals in a process called pi-bonding. When the excited pi-bonds relax back to a lower energy state, photons of a specific wavelength are emitted in the process, giving rise to the fluorescent light. These organic dyes can be characterized by the wavelengths of light that they absorb (excitation wavelengths), and the wavelengths of light that they emit (emission wavelengths). The excitation and emission wavelengths are properties of each dye that are highly specific and reliable. Organic dyes tend to degrade over time as they are bombarded with light in a process called photodegradation. During photodegradation, the excited pi-bonds can break, rather than relaxing into their lower energy state. Organic fluorescent dyes have been in use for many years.
More recently, it has been discovered that very small particles of certain semiconductor materials also fluoresce, and the color of the fluorescence is dependent only on the size of the particles. These materials are referred to as quantum dots. Quantum dots absorb energy from a range of wavelengths, but the energy is not taken into pi-bonds. Rather, the fluorescence results from quantum mechanical interactions within the material. Smaller particles emit light on the blue end of the spectrum, whereas larger particles emit light on the red end of the spectrum. Because light energy is not absorbed into fragile pi-bonds, the rate of photodegradation is much lower for quantum dots compared with organic dyes, and thus the fluorescent signals are brighter and more durable.
There are a number ways in which fluorescence plays into forensic investigations. Biological materials sometimes have a characteristic fluorescent property that facilitates quick identification under UV examination. Semen stains, for example, may be identified by their characteristic fluorescence under ultraviolet light examination. Fluorescence of other biological samples can be brought about by chemical treatment to make their detection easier. Fingerprints can be treated with fluorescent powders to permit identification and detection even of relatively faint (latent) or degraded prints. Likewise, application of highly fluorescent materials, such as spy dust, permits tagging and tracking of suspects or agents across fairly wide areas by following the path of dispersal of the fluorescent agent as they drag and redistribute an unseen powder with their shoes or clothing. Certain chemicals, such as explosives and nerve agents, can sometimes be traced in the environment from their characteristic fluorescent spectral patterns. Examination of microscopic fibers for fluorescence can produce evidence linking suspects to crime scenes or other physical locations.
Fluorescence is a tool that allows evidence that would normally be invisible to come to light. It is a source of evidence only found with careful examination by those who are aware of its latent powers.
see also Chemical and biological detection technologies; Confocal microscopy; Microscopes; Semen and sperm.
fluorescence
fluorescence
fluo·res·cence / floŏ(ə)ˈresəns; flôrˈesəns/ • n. the visible or invisible radiation emitted by certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light. ∎ the property of absorbing light of short wavelength and emitting light of longer wavelength.
fluorescence
—fluorescent adj.