Photoluminescence

Type of physical science: Chemistry

Field of study: Chemical processes

Emission of light by an atom or molecule following absorption of electromagnetic radiation is known as photoluminescence. Photoluminescent materials are used in devices such as light bulbs and lasers and also form the basis for a variety of chemical measurements.

Overview

Luminescence is a general term for the process that occurs when light is given off by a substance. This covers a wide variety of phenomena, from neon signs to the light of fireflies. One way to distinguish between different types of luminescence is by the manner in which the particles (atoms or molecules) gain energy that is later emitted as light. If the energy increase comes from a chemical reaction, the process is called chemiluminescence. Chemiluminescence that occurs in a living system is known as bioluminescence. Some types of luminescence occur after the particles gain or absorb energy from an outside source. When this energy is in the form of ultraviolet, visible, or infrared radiation, the emission process is called photoluminescence.

Photoluminescence can be categorized as either fluorescence or phosphorescence. The easiest way to classify photoluminescence is by how rapidly the emission occurs. If the energy is emitted very quickly (less than one-millionth of a second, or 1 microsecond), it is usually termed "fluorescence." Processes that take longer than 1 microsecond are usually phosphorescent processes. The reason for this difference in emission times will be discussed later.

To explain photoluminescence, it is necessary to describe what happens when an atom or molecule gains energy. This process is known as absorption. Atoms consist of electrons, protons, and neutrons. The protons and neutrons are grouped together in a very small region of the atom called the nucleus. Moving around the nucleus are the electrons. The nucleus has a positive charge as a result of the positively charged protons (neutrons have no charge), while the electrons are negatively charged. To move an electron farther away from the nucleus requires energy because of the attraction between the opposite charges. Each electron has its own energy, which depends on the attractive force from the nucleus and the influence of other electrons that are present. There are only a few energy values possible for an electron in a given atom. All other energies are impossible. This concept is known as quantization and is a basis of quantum mechanics, which is the mathematical field that describes the behavior of molecules, atoms, and subatomic particles. For the energy of an electron to change, the electron must absorb a specific amount of energy, exactly equal to the difference in energy between the higher energy state and the lower energy state. When the electron returns to the lower energy state, it emits exactly the same amount of energy as was absorbed. When the electrons are arranged with the lowest possible energies, an atom is said to be in the ground electronic state. After at least one electron has absorbed energy and is in a higher energy state, the atom is in an excited electronic state.

Because the number of excited states is limited, atoms can absorb only a few, specific energies.

Molecules consist of groups of atoms that are chemically bonded together. Bonds are one or more pairs of electrons that are shared by two atoms. In a molecule, the atoms can vibrate or rotate relative to one another. Changes in these vibrations and rotations require changes in energy. For every possible electronic energy state of a molecule, the molecule can undergo several different vibrations and rotations that slightly change its energy. Instead of one value for every electronic state, there is a range of energies associated with every change in electron energy. Therefore, unlike atoms, molecules absorb ranges of energies instead of a few specific values.

One way to describe the energy of electromagnetic radiation (which includes ultraviolet, visible, and infrared radiation) is to specify the wavelength. The wavelength and the energy are inversely proportional. Therefore, a high energy will cor- respond to a short wavelength, and a low energy to a long wavelength. The wavelength is usually measured in nanometers. One nanometer is equal to 1 billionth of a meter. Visible light falls in the wavelength range of 350 to 800 nanometers. Ultraviolet radiation has wavelengths less than 350 nanometers, and infrared radiation has wavelengths greater than 800 nanometers.

The vast majority of molecules have an even number of electrons. These electrons usually exist in pairs, with the members of each pair spinning in opposite directions. Such electrons are called spin-paired. Molecules that have all of their electrons spin-paired are said to be in a singlet state. Usually, the lowest energy state of a molecule is a singlet state. During the absorption process, the electron that is gaining energy may or may not undergo a change in spin.

If there is no change in spin, the electron is still spin-paired, and the molecule remains in a singlet state. If the electron does change spin, so that both members of the pair are spinning in the same direction, the molecule is in a triplet state. The changing of electron spin is known as intersystem crossing. Intersystem crossing has a low probability of occurring, although the chances of intersystem crossing occurring are higher when both singlet and triplet states have very similar vibrations.

Once in an excited state, the molecules and atoms do not remain there. Instead, they relax to lower energy states by one of several processes. Some of these processes do not involve emission of radiation. In these processes, the energy is lost during collisions with other particles as thermal energy. These processes are called radiationless transitions (because no electromagnetic radiation is emitted), and compete with fluorescence and phosphorescence. Only a relatively small number of molecules have a combination of features that favor fluorescence or phosphorescence over the radiationless transitions.

Fluorescence is the emission of energy that accompanies the transition of an electron from an excited singlet state to a lower-energy singlet state. Since there is no intersystem crossing, the emission starts to occur virtually immediately. The fluorescence lifetime is a measure of how long a particle remains in the excited state. These lifetimes range from about a microsecond down to a few picoseconds (one-trillionth of a second). In atoms, the wavelength of energy emitted is exactly the same as what was absorbed. This is called resonance fluorescence.

For molecules, some of the energy that was absorbed is lost as vibrational energy during a collision with another molecule. This loss is known as vibrational relaxation. Because of vibrational relaxation, molecular fluorescence occurs at slightly longer wavelengths than molecular absorption. Like molecular absorption, molecular fluorescence occurs over a range of wavelengths. Emission that occurs after intersystem crossing is called phosphorescence. During phosphorescence, the electron usually changes spin again, ending up back in a singlet state. The changing of electron spins has a low probability of occurrence. Therefore, fewer compounds exhibit phosphorescence than fluorescence. Following intersystem crossing, the electron is in a triplet state. The electron may stay in this state for as little as 1 ten-thousandth of a second, but usually no longer than about twenty seconds. Therefore, phosphorescence occurs at later times following absorption than fluorescence. Phosphorescence even continues after the energy needed for absorption is no longer present, while under the same circumstances, fluorescence ceases immediately. For this reason, phosphorescence is said to have an afterglow, while fluorescence does not.

Frequently, conditions must be carefully controlled in order to see phosphorescence. In most cases, the compound must be kept at very low temperatures and in a rigid environment.

Phosphorescence, like fluorescence, competes with radiationless methods of losing energy. One of these methods is loss of energy through collisions. Because of the long lifetime of the triplet state, this collisional loss of energy is more likely to happen than in fluorescence. The collisions are minimized at low temperatures and when the molecule is kept in a rigid environment.

Applications

Photoluminescence has a wide variety of applications in everyday life as well as in the scientific community. One of the more familiar applications is in fluorescent lamps. The fluorescent lamp consists of a glass outer casing coated on the inside with a photoluminescent material. Contained inside the glass casing are electrodes and gaseous mercury atoms. As a current passes through the lamp, the gas-phase mercury atoms are excited to upper energy states by absorbing energy from collisions with electrons. These mercury atoms then emit light as they return to the ground state. This emitted light is in the ultraviolet region. The material that coats the inside of the glass envelope must absorb this ultraviolet light and reemit visible light.

Another function of the fluorescent coating is to emit light that closely resembles white light (white light is a mixture of all wavelengths of light in the visible region). This is accomplished by using a mixture of substances that fluoresce at a variety of wavelengths, which, when combined, approximate white light.

Another application of photoluminescence, used in everyday life and scientific research, is the laser. The term "laser" is an acronym for "light amplification by stimulated emission of radiation." In order for a laser to function, there have to be more particles in excited states than in the lower energy states. This condition is known as a population inversion. When one particle emits radiation as it relaxes, this radiation strikes another excited particle and causes it to emit radiation in the same direction and having the same wavelength: This is known as stimulated emission. Substances that can be used as lasers have to meet the same requirements as those molecules that undergo photoluminescence. Sometimes, the population inversion is achieved by absorption of electromagnetic radiation, but in other cases, the absorbed energy is electrical energy or thermal energy. Lasers are used in medicine as a surgical tool, in bar-code scanners like those used in supermarkets, for welding and cutting in industrial sites, in entertainment devices such as compact disc players and laser disc players, as well as in scientific laboratories for research.

Fluorescence is a sensitive technique in the field of analytical chemistry. Since it is possible to detect very small amounts of light with modern instruments, extremely low concentrations of fluorescing molecules can be measured. In fact, if only one molecule out of a billion undergoes fluorescence, it can be detected by these instruments. The amount of fluorescence is directly proportional to the number of molecules emitting light. In general, luminescence methods are about ten to one thousand times more sensitive than other methods, such as absorption measurements.

Specific applications of these techniques can be found in many different areas. For example, photoluminescence is used to detect tiny quantities of drugs in blood and urine. Some substances that have been detected using photoluminescence include: codeine, aspirin, caffeine, and cocaine. Several carcinogens can also be detected using these techniques. One method used for detecting cancerous tissue involves staining biological tissue with a fluorescent compound.

The wavelengths of light that are emitted and the quantity of emitted light are different for cancerous and for normal tissue. By monitoring the conditions that can cause these changes in the fluorescing molecules, it is possible to obtain useful information about possible causes of cancer. Other areas that use fluorescence to determine concentrations of trace amounts of molecules include pharmaceuticals and clinical samples. Phosphorescence is not as widely used as fluorescence because extremely cold temperatures are required.

Photoluminescence can also be used to transfer energy from one molecule to another.

Molecules that abosrb over one range of wavelengths usually emit at slightly longer wavelengths.

If a nearby molecule absorbs light of the same wavelengths as a luminescent molecule emits, the energy is transferred from the emitting molecule to the absorbing molecule. This type of energy transfer is useful in several different situations. For example, it is one of the ways that chlorophyll molecules transfer needed energy to the site where photosynthesis occurs. It can also be used in photography, to make better use of the entire range of visible light. Molecules that absorb different wavelengths are attached to the chemicals used in photographic development in order to sensitize these chemicals to wavelengths of light that are ordinarily wasted. The efficiency of certain types of semiconductors can be increased using the same technique, so that more of the solar spectrum can be converted into usable energy.

Context

It is interesting to note that the phenomenon of photoluminescence was known long before the theory behind the process was understood. In fact, fluorescence was used in the experiments that were crucial in explaining the structure of the atom. Antoine-Henri Becquerel, a French scientist, working with a fluorescent uranium mineral performed experiments that led to the discovery of radioactivity in 1896. In 1897, a British physicist named Sir Joseph John Thomson discovered some of the properties of the electron with the aid of a cathode-ray tube.

The cathode-ray tube is a device very similar in operation to the modern-day television set. It consists of two electrodes surrounded by a glass case. The electrodes are connected to a high-voltage power source, and all the air inside the glass tube is removed by a vacuum pump.

Part of the glass is coated with a fluorescent material. When an electron comes in contact with this material, the energy of the electron is absorbed, and the material fluoresces.

In 1910, one of Thomson's former students, Ernest Rutherford, carried out another experiment in which fluorescence served as a means of detecting particles. In this experiment, positively charged radioactive α particles were directed through a thin piece of gold foil. The foil was surrounded by a screen of material, which would fluoresce when one of the radioactive particles would strike it. While passing through the gold foil, the positively charged particles would be attracted by negatively charged electrons and repelled by positively charged protons.

Rutherford and his coworkers expected the protons and electrons to be randomly scattered throughout the atom. This would have led to only minor deflections of the radioactive particle from its original path. Therefore, they expected to observe only small angles of deflection. This is what was observed most of the time; but they also found that some particles were scattered at much larger angles. This result was completely unexpected. Rutherford's surprise at these results is evidenced by his words: "It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you." It was this experiment that led Rutherford to postulate that all the positive charge and the vast majority of the mass of the atom were found in a very small region of the atom. This region was later termed the "nucleus." When the positively charged α particle came close to the gold nucleus, with its large positive charge, the two like charges repelled each other, causing large deflections of the α particles.

Thomson's and Rutherford's studies were crucial to the development of the modern picture of the atom. Both experiments would not have been possible without photoluminescent materials. The theory behind photoluminescence, however, could not be explained until the details of atomic and molecular structure were known. This illustrates that sometimes a process can be used even though the theory behind it is not understood.

Fluorescence and phosphorescence have long served as sensitive detection techniques.

The time over which the emission occurs varies with the molecule's environment. Monitoring the amount of fluorescence, the wavelength of emitted light, and the lifetime of emission are all techniques that can be used to characterize chemical systems. These measurements have been used in such diverse fields as solid-state physics, forensic chemistry, and biomedical research.

Principal terms

ABSORPTION: the process by which atoms or molecules gain energy during interaction with external radiation

ELECTROMAGNETIC RADIATION: a type of energy that is transmitted by oscillations of electric and magnetic fields

EXCITED STATE: the condition of an atom or molecule after it has absorbed external radiation

GROUND STATE: the condition of an atom or molecule in which all the electrons have their lowest possible energies

INTERSYSTEM CROSSING: a process in which the spin of an excited electron is reversed

LASER: a device that produces a narrow beam of intense, single-wavelength light

LUMINESCENCE: a general term for the process by which substances emit electromagnetic radiation

STIMULATED EMISSION: the process responsible for emission in lasers in which all the excited molecules emit radiation of identical energy and in the same direction

WAVELENGTH: one of the wave properties of electromagnetic radiation, sometimes used to differentiate between forms of radiation because it is inversely proportional to energy

Bibliography

Bawden, Arthur T. MATTER AND ENERGY. New York: Henry Holt, 1957. An introductory physical science textbook that is written on a very understandable level. Includes sections on the wave properties of light and the interactions between matter and electromagnetic radiation. There are several pages devoted to fluorescence and phosphorescence, as well as a thorough description of the operation of fluorescent lamps.

DeLuca, John A. "Introduction to Luminescence in Inorganic Solids." JOURNAL OF CHEMICAL EDUCATION 57 (1980): 541-545. Discussion of luminescence in certain types of solids. Contains excellent diagrams that illustrate energy transfer from one molecule to another through luminescence. Includes a section that discusses some of the commercial applications for molecules that undergo photoluminescence. The use of mathematical equations is kept to a minimum.

Dyke, Thomas R., and J. S. Muenter. JOURNAL OF CHEMICAL EDUCATION 52 (1975): 251-258. Includes a thorough section on the theory behind fluorescence and phosphorescence. The authors use diagrams to explain topics such as intersystem crossing and its role in phosphorescence. Experiments with several types of molecules that undergo phosphorescence are described. These experiments are designed to be performed in an undergraduate chemistry laboratory.

Hercules, David M., ed. FLUORESCENCE AND PHOSPHORESCENCE ANALYSIS. New York: Interscience, 1966. This book consists of eight chapters, each of which discusses a different aspect of photoluminescence. Each chapter is written by a scientist who is working in the area of photoluminescence. The first four chapters focus on theory and instrumentation, while the latter four chapters examine various applications of fluorescence and phosphorescence pertaining to scientific research interests.

Skoog, Douglas A. PRINCIPLES OF INSTRUMENTAL ANALYSIS. 3d ed. Philadelphia: Saunders College Publishing, 1985. This textbook is written for students enrolled in an undergraduate chemistry course. In the chapter entitled "Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectroscopy," there is an excellent description of the theory behind photoluminescence. The chapter also includes sections on the instrumentation used for photoluminescence measurements, and on applications of these measurements.

Electrons and Atoms

Photochemistry, Plasma Chemistry, and Radiation Chemistry