Fluorescence And Phosphorescence

Type of physical science: Fluorescence and Phosphorescence, Light, Electromagnetism, Elementary particles, Particles, elementary, Atomic physics

Field of study: Nonrelativistic quantum mechanics

Fluorescence and phosphorescence are varieties of the process of spontaneous emission, whereby atoms and molecules can emit photons to move from high-energy to low-energy states without interacting with other photons.

Overview

Atoms and molecules interact with light in three ways. An atom or molecule can take up, or absorb, a photon or particle of light to move from a lower energy state to a higher energy state. It can, because of the interaction of the atom or molecule with other photons, itself give off a photon to move from a higher energy state to a lower energy state by a process called "stimulated emission." Finally, atoms or molecules can give off a photon to move from a high-energy to a low-energy state without interacting with other photons in the system. This last process, called "spontaneous emission," is divided into two types of processes, fluorescence and phosphorescence.

While fluorescence and phosphorescence both involve spontaneous emission of a photon, they differ in the specific process involved. Fluorescence is an "allowed" process--that is, a process that is predicted to occur using simple rules (called "selection rules") obtained from quantum mechanics. Phosphorescence is a "forbidden process" that, strictly speaking, would not be expected to occur based on quantum-mechanical selection rules. However, since the rules for allowed and forbidden processes are derived from simplified descriptions of systems, forbidden processes such as phosphorescence are usually found to take place, although with much lower likelihood than allowed processes such as fluorescence.

A simple example of a system involving both fluorescence and phosphorescence occurs in electronic transitions in organic molecules. Each electron in a molecule has its own individual electron spin. The molecule as a whole has a total electron spin that is equal to the sum of the individual electron spins. For most organic molecules in their ground, or lowest-energy, electronic state, the total electron spin is equal to zero. States with zero total electron spin are called "singlet states." The most common low-energy excited electronic states for molecules are singlet states or triplet states, which have a total electron spin of one in fundamental units.

The quantum-mechanical selection rule that applies to electron spin states that, for an allowed transition, the total electron spin must remain unchanged. Fluorescence corresponds to the case where this selection rule is obeyed, as occurs when a molecule in an excited singlet electronic state emits a photon to move to the ground electronic state. Phosphorescence corresponds to a process forbidden by the spin selection rule, such as occurs when a molecule in a triplet electronic state emits a photon and moves into a lower-energy singlet electronic state.

For most organic molecules, there are a pair of low-lying excited electronic states, one singlet state and one triplet state. Typically, the triplet state is found to be lower in energy than the singlet state. Because of this, phosphorescence usually appears at lower energies, and therefore longer wavelengths, than fluorescence. Since light absorption first occurs between low-lying vibrational and rotational states in the ground electronic state and vibrationally and rotationally excited states in the first excited singlet electronic state, absorption of light usually occurs at higher energies, and therefore lower wavelengths, than fluorescence or phosphorescence. An exception to this general rule occurs when molecules move into high-energy states by absorption of several photons of light. Multiphoton light absorption requires a high-intensity light source such as a laser.

The difference in the probability of an allowed or forbidden process taking place results in differences in the lifetime with which such processes occur. For electronic transitions, fluorescence usually occurs within a few hundred nanoseconds following creation of an excited electronic state, while phosphorescence occurs over a period of milliseconds or longer. There are, however, exceptions to this general rule. For example, fluorescence lifetimes of milliseconds to seconds are commonly observed for inorganic uranium compounds. Substances with unusually short fluorescence lifetimes also exist. Because of this, it is better to classify fluorescence and phosphorescence in terms of whether the light emission process is allowed or forbidden rather than by the observed lifetime of the process.

For simple cases, luminescence intensity following excitation is observed to decrease exponentially with time. Such processes, termed "first order," are characterized by a half-life, which is equal to the time it takes for the intensity of light emission to decrease to half of its initial value. However, more complex behavior for emission intensity is often observed. This is often a sign that the light-emitting states are created indirectly, as occurs in sensitized fluorescence.

Spontaneous emission of light can take place in all regions of the electromagnetic spectrum. For example, if a metal is bombarded with a high energy beam of electrons, it will emit X rays at energies higher than those for visible light. Spontaneous emission can also occur when molecules move from higher to lower vibrational energies within a particular electronic state. These transitions occur in the infrared region of the spectrum. Infrared fluorescence is more difficult to detect than fluorescence in the ultraviolet or visible regions of the spectrum. In part, this is because of the low energy of infrared photons. In addition, the rate of spontaneous light emission can be shown to depend on the third power of the frequency of the emitted light. As a consequence, infrared emission occurs at a slower rate, and therefore a lower intensity, than emission of visible or ultraviolet light.

Fluorescence and phosphorescence are often categorized in terms of the means used to create the light-emitting states. The general term "luminescence" is used to describe any delayed light emission from a system. The most common way of creating a high-energy state in a system is by light absorption, a process called "photoluminescence." Excited states can also be created by heat (thermoluminescence), chemical reaction (chemiluminescence), biochemical reactions in living organisms (bioluminescence), electron bombardment (cathodoluminescence), or other processes. Light emission depends only on the properties of the system in which emission occurs and is independent of the specific process used to create the emitting states.

In addition to losing energy by emission of light, high-energy states can lose energy by nonradiative processes, that is, processes that do not involve emission of light. These processes are often called "relaxation" or "deactivation" processes. The most common way in which energy is lost nonradiatively is by collision of an excited atom or molecule with another molecule. In a collision, the excited atom or molecule can have its energy converted into vibrational, rotational, or electronic energy in the collision partner, or into translational energy, the kinetic energy caused by motion of an atom or molecule as a whole. Since these processes compete with fluorescence and phosphorescence in removing excess energy from atoms or molecules, light emission decreases when nonradiative processes occur with high efficiency. The decrease in light emission as a result of nonradiative removal of energy is called "quenching." Atoms or molecules that are unusually efficient in removing excess energy from excited species, such as molecular oxygen in the gas phase or halide ions in aqueous solution, are called "fluorescence quenchers."

Quenching of light emission from substances can also occur upon heating of a substance. This type of quenching is called "thermal quenching." While fluorescence is not usually affected by changes in temperature, phosphorescence intensity is often strongly temperature dependent. The dependence of the intensity of spontaneous emission on temperature is one way in which fluorescence and phosphorescence can be distinguished experimentally.

Closely related to the concept of quenching is the phenomenon of sensitized fluorescence. In sensitized fluorescence, one type of atom or molecule, called an "activator," is excited to high energy by an excitation process. Energy is then transferred from the activator to a second molecule, called the "sensitizer," usually by collision; the sensitizer then emits light. Since the wavelength of light emission depends on the choice of sensitizer, the same activator in combination with different sensitizers can be used to obtain light emission over a wide range of wavelengths.

In addition to light emission from gas molecules or molecules in solution, fluorescence and phosphorescence can also occur in solids. Solids that exhibit delayed emission of light are called "phosphors." Phosphors operating by sensitized fluorescence can be created by adding small amounts of activators and sensitizers to other chemical compounds. In cases where it is useful to prevent light emission from a solid, quenching molecules can be added to suppress fluorescence and phosphorescence.

For solids that are photoconductive, that is, whose conduction of electrical current changes upon exposure to light, phosphorescence can be generated by light excitation. Excitation creates free electrons or vacancies, called "holes," where electrons would normally be present. These free electrons and holes can migrate through the solid until they become trapped at defect sites in the crystal lattice. Over time, the electrons and holes will free themselves by thermal excitation. The combination of a free electron and hole results in emission of light. The light-emission properties of photoconductive solids can be carefully controlled by the addition of trace amounts of atoms to photoconductive materials.

Applications

Fluorescence and phosphorescence are efficient methods for generation of light. Not surprisingly, both methods have been used to devise light sources for a variety of applications. The fact that luminescence can be generated by a variety of excitation processes means that spontaneous light emission is often used as a means of converting other forms of energy into light.

Fluorescence between excited electronic states of atoms is a common method for generating visible light. When an electrical discharge passes through an inert gas or metal vapor, atoms are generated in excited electronic states. These states emit light, often with high efficiency, by fluorescence or, less commonly, phosphorescence. For example, the light in neon signs is generated by an electrical discharge passing through a tube containing neon or other noble gases. An electrical discharge through a sodium metal vapor produces the characteristic yellow color of a sodium lamp. Metal vapor lamps are often operated at high pressures, in part because one effect of increasing pressure is to spread the emitted light over a larger range of wavelengths than is obtained at low pressures.

The fluorescent lamp represents a second way of generating visible light. In this type of lamp, an electrical discharge passing through a low-pressure mercury vapor contained within a glass tube produces electronically excited mercury atoms. A majority of the light emitted by the excited atoms appears in the ultraviolet region of the spectrum. The walls of the tube are coated with a phosphor, which absorbs ultraviolet light from the mercury atoms and re-emits the light at visible wavelengths. By varying the phosphor used in the lamp, a variety of lamp colors can be obtained, as well as white-light fluorescence or ultraviolet light, as is generated in "black-light" fluorescent lamps. Fluorescent lamps are far more efficient than conventional tungsten light bulbs in producing visible light; in the generation of light by thermal heating, as is done in a tungsten lamp, a large fraction of the light produced appears in the infrared region of the spectrum. In addition, fluorescent lamps operate at lower temperatures than tungsten lamps and have far longer lifetimes.

Another useful application of luminescence is in the conversion of high-energy particles or photons into visible light. In scintillation, the impact of a particle or photon with a phosphor causes ionization to occur. Recombination of the electron produced by the ionization process with the ionized atom results in the production of a pulse of light. Scintillation screens consisting of a sheet of glass or quartz coated with an appropriate phosphor were used in many early experiments on radioactivity, with the light pulses detected directly by eye. In a scintillation counter, the strength of radioactivity is determined by measuring the intensity of light generated by the scintillation process. For a wide range of values, it is found that the intensity of radioactivity is proportional to the intensity of light generated by scintillation.

In a cathode-ray tube, collisional energy, in this case from a beam of electrons, is also converted into light. Electrons generated at the cathode of the tube pass through one or more focusing electrodes, which deflect the beam. When the electrons strike the phosphor that coats the walls of the display screen on the opposite end of the tube, light is generated. Simple cathode-ray tubes are used to display electrical signals in oscilloscopes; they are also used to generate images in the monochrome video display unit of a computer or the picture tube of a black-and-white television set.

The picture tube used in most color televisions and computer monitors is a more sophisticated version of the simple cathode-ray tube. The display screen in a color television contains three different types of phosphors, which fluoresce at wavelengths corresponding to the three primary colors, red, blue, and green. Separate electron beams are used to excite fluorescence from each phosphor. Images of any color can be generated on the television screen from the appropriate combination of primary colors. The lifetime for fluorescence from the phosphors used in a color-television tube must be carefully adjusted to give the illusion of a continuous picture display on the television screen.

The ability to produce phosphors with long phosphorescence lifetimes also has a number of practical applications. Paints and dyes with phosphorescence lifetimes of several hours can be used in compasses, watches, and other items to make it possible to read these objects in the dark. Phosphors with long lifetimes can also be used as a means of storing signals in oscilloscopes and other electronic devices.

A final application of luminescence is in the operation of lasers. While lasers operate by stimulated emission, it can be shown by a simple argument that there is a direct relationship between the rate constants for stimulated and spontaneous emission of light. The triggering event in most lasers is emission of a photon by spontaneous emission, followed by amplification of the emitted photon by stimulated emission. The fact that stimulated emission represents coherent production of light is responsible for most of the important properties of laser light, including its high intensity, narrow wavelength range for emission, low beam divergence, and coherence. Lasers have thousands of applications in research, communication, medicine, electronics, and other areas.

Context

While the earliest reports of fluorescence date back to the sixteenth century, it was not until George Stokes began his investigations of light emission in quinine solutions in 1852 that luminescence was systematically studied. The word "fluorescence" itself is derived from the name of fluorspar, a mineral found to exhibit fluorescence. During the next fifty years, many minerals and organic compounds were found to have fluorescent properties. Several applications of fluorescence were also developed during this period, including the first fluorescent lamp, built by Alexandre Becquerel in 1867, and the use of fluorescent solutions or phosphors to detect ultraviolet light. Many early experimental studies of radioactivity also used fluorescent materials as radiation detectors.

In 1917, Albert Einstein used the concept of quantization along with the principles of classical thermodynamics to derive a relationship between the rate constants for light absorption, stimulated emission, and spontaneous emission. He showed that spontaneous emission rates are proportional to the frequency of the emitted light raised to the third power. In conjunction with his previous demonstration that the energy of a photon of light is proportional to its frequency, Einstein's result implied that light emission for high-energy states is generally more rapid than emission for low-energy states.

With the development of the field of quantum mechanics in the late 1920's, the distinction between fluorescence and phosphorescence was given a sound theoretical basis. Unlike earlier definitions based on the time or temperature dependence of light emission, fluorescence and phosphorescence were distinguished in quantum theory by whether the observed light emission corresponded to an allowed or forbidden process. The description of molecular systems in terms of rotational, vibrational, and electronic energy levels made it possible to understand more clearly the relative energies with which light absorption, fluorescence, and phosphorescence take place in molecules, as well as the competition between radiative and nonradiative energy relaxation.

New developments following the end of World War II gave additional importance to the study of luminescence. Following the invention of the transistor and related semiconductor devices in the 1950's, new photoconductive solids were created. This work resulted in the development of novel methods for conversion of electrical energy into light, such as the process that takes place in light-emitting diodes. The invention of the first laser in 1960 gave new importance to experimental studies of fluorescence and phosphorescence. Because of the close relationship between spontaneous and stimulated light emission, detailed knowledge of the rates and mechanisms of light emission in substances was critical in the search for new and more useful lasers. Lasers themselves became a powerful tool in studying fluorescence, resulting in new experimental techniques such as laser-induced fluorescence and generation of high-energy states by multiphoton light absorption.

The growth in the understanding of the light-emitting properties of substances over the past few centuries has resulted in a large number of practical applications of luminescence and has played an important role in the development of many areas of science, including quantum theory, electronics, and solid-state physics. As further discoveries in these and other fields are made, it seems likely that new applications of the luminescent properties of materials will continue to be found.

Principal terms

FLUORESCENCE: The spontaneous emission of light by an excited state of an atom, molecule, or solid by an allowed process, usually occurring over a relatively short period of time

LUMINESCENCE: The general term for the emission of light by high-energy states of atoms, molecules, or other substances, independent of the means of excitation

PHOSPHOR: A solid material exhibiting delayed emission of light following excitation

PHOSPHORESCENCE: The spontaneous emission of light by an excited state of an atom, molecule, or solid by a forbidden process, usually occurring over a relatively long period of time

PHOTON: A particle of light

SELECTION RULE: A rule obtained from the quantum-mechanical description of a system, making it possible to predict whether a specific process is allowed or forbidden to occur

SENSITIZED FLUORESCENCE: Fluorescence obtained indirectly by excitation of activator molecules followed by energy transfer to light-emitting sensitizer molecules

SPONTANEOUS EMISSION: The emission of light from a substance in the absence of interaction with other photons

STIMULATED EMISSION: The emission of light from a substance as a result of interaction with other photons

Bibliography

Barltrop, J. A., and J. D. Coyle. Principles of Photochemistry. New York: John Wiley and Sons, 1975. A basic introduction to molecular photochemistry, with a focus on processes involving organic molecules. The third chapter gives an extensive discussion of fluorescence, phosphorescence, and nonradiative processes. Other sections of the book discuss quenching of fluorescence, types of electronic transitions in molecules, and the application of photochemistry in studies of mechanisms of chemical reactions.

Calvert, Jack G., and James N. Pitts, Jr. Photochemistry. New York: John Wiley and Sons, 1966. Perhaps the most comprehensive single book on photochemistry. Individual sections of the book cover the interaction of light with atoms, small molecules, and polyatomic molecules, with numerous examples of specific systems. There is also an extensive discussion of experimental techniques in photochemistry.

Mitchell, Allen C. G., and Mark W. Zemansky. Resonance Radiation and Excited Atoms. New York: Cambridge University Press, 1971. A reprint of the original 1934 edition of this classic volume on the interaction of light with atoms. Includes excellent discussions of experimental techniques for studying atomic fluorescence and sensitized fluorescence in atomic systems.

Okabe, Hideo. Photochemistry of Small Molecules. New York: John Wiley and Sons, 1978. An excellent survey covering the interaction of light with atoms and small molecules. Individual chapters cover the basic quantum theory of atoms and molecules, photochemistry, experimental techniques, and sensitized fluorescence, followed by a detailed discussion of light interaction in a variety of small molecules. Some material requires a strong background in physics or chemistry for complete understanding.

Rendell, David. Fluorescence and Phosphorescence Spectroscopy. New York: John Wiley and Sons, 1987. A textbook on fluorescence and phosphorescence, with an emphasis on the use of these techniques in analytical chemistry. Individual chapters provide a general introduction to luminescence spectroscopy, experimental techniques, quantitative analysis by fluorescence, and applications of fluorescence spectroscopy to a variety of analytical problems.

Rohatgi-Mukherjee, K. K. Fundamentals of Photochemistry. New York: John Wiley and Sons, 1978. A solid introduction to photochemical processes in organic and inorganic molecules. Several chapters discuss fluorescence, phosphorescence, quenching of fluorescence, and the use of fluorescence measurements to understand the properties of molecules. The last chapter gives a general discussion of experimental techniques used to study fluorescence, phosphorescence, and chemiluminescence.

Walker, S., and H. Straw. Spectroscopy. Vol. 2. New York: Macmillan, 1962. The second volume of a two-volume series on various types of spectroscopies. Provides an extensive discussion of fluorescence spectroscopy; the latter part of the volume also discusses instrumentation used in fluorescence measurements.