Photochemistry, Plasma Chemistry, And Radiation Chemistry

Type of physical science: Chemistry

Field of study: Chemical processes

Photochemistry, plasma chemistry, and radiation chemistry form part of the general field of high-energy chemistry, which is concerned with the chemical changes induced in substances by the action of photons, electrons, and ionizing radiation. Although humankind has made wide application of high-energy chemistry to the manufacture of products as diverse as plastics and microelectronics, nature's intricate and life-sustaining use of photochemistry in plant photosynthesis and animal vision must be regarded as unsurpassed.

Overview

Chemical reactions initiated by collisions between molecules and photons, electrons and high-energy radiation fall within the general domain of high-energy chemistry, which is itself subdivided into the fields of photochemistry, plasma chemistry, and radiation chemistry.

The three fields are distinguished by different experimental techniques and by the specific radiation and particles employed. Although experiments involving gases are ordinarily the easiest to interpret, high-energy chemistry reactions can involve molecules in liquids and solids as well.

The basis for an appreciation of high-energy chemical phenomena lies, however, in an understanding of the electronic nature of the covalent bonds which bind the atoms together in many molecules. These electrons, which can be classified according to their energies, are called valence electrons. The electronvolt values of these energies, called energy levels, are discrete and can be arranged in order of lowest to highest, like the steps in a staircase. Unlike stair steps, however, the energy levels are ordinarily not uniformly spaced; moreover, no more than two electrons can populate each energy level. When the valence electrons are distributed so that all the lowest energy levels are filled, the molecule is said to be in its electronic ground state. In the staircase analogy, one may think of two electrons sitting on each stair step, starting at the lowest and continuing with each succeeding step up to the highest required to accommodate the last two electrons.

In high-energy chemistry, however, one must also consider the circumstance in which one of the electrons has been transferred to one of the upper, unoccupied stair steps. The electron configuration which results is called an electronic excited state of the molecule. The transition from ground state to excited state requires absorption of energy from some source.

In photochemistry, a lamp capable of producing a beam of ultraviolet light is ordinarily employed. Through absorption of a photon from the beam by a valence electron in a molecule, the molecule is converted from its ground state to one of its excited states. In photochemistry, the excited states of interest are those whose valence electron configurations are unable to support all the chemical bonds in the molecule. Consider, for example, the ammonia molecule, whose chemical formula, NH₃, indicates that each H atom is connected to the central N atom by a single bond. If the photon absorbed has energy in excess of 3.9 electronvolts, the excited state, denoted by NH₃*, in which two of the N-H bonds are unstable, can be produced. NH₃* decomposes into the radical, NH, and a molecule of hydrogen, denoted by H₂. Two NH radicals can unite in a collision, in which their unused valence electrons are reorganized to produce a hydrogen molecule, H₂, and a nitrogen molecule, N₂. These observations can be summarized with the series of chemical reaction equations:

NH₃ + energy → NH₃*, NH₃* → NH + H₂, NH + NH → N₂ + H₂ The sequence of equations 1 through 3 is said to form a reaction mechanism. It is meant to represent the various kinds of molecular collisions underlying the summarizing equation, 2NH₃ → 2N₂ + 3H₂, which expresses the overall conversion of NH₃ into N₂ and H₂. Equations 2 and 3 occur after the photon has been absorbed in equation 1, and since they do not directly require the presence of light, they are called "dark" reactions. By combined action of the reactions in the mechanism, ammonia has been decomposed by the absorption of light; it is said to have undergone photolysis, which comes from the Greek photo (light) and lysis (to split).

In plasma chemistry, an electric discharge (called a plasma) is started by injection of electrons from a tesla coil--for example, into a gas across which a voltage has been applied. An injected electron is accelerated by the voltage. Subsequently, the electron can give up energy to a molecule in a collision and transfer the molecule from its electronic ground state to one of its electronic excited states, such as NH₃*. The NH₃* may then decompose (according to equations 1 through 3) to form N₂ and H₂. A plasma electron having energy in excess of 10.2 electronvolts, however, can knock a valence electron entirely free of the highest occupied energy level of the NH₃ and convert it into the positive ion, NH₃+. Indeed, such ionizing collisions are necessary to keep the discharge operating. Starting with NH₃+, one can construct another mechanism for the conversion of NH₃ to N₂ and H₂:

H₃ + energy → NH₃⁺ + e^-, NH₃⁺ +NH₃ → NH₄⁺ + NH₂, NH₄⁺ + e^- → NH₃ + H, NH₂ + H → NH + H₂, NH + NH → N₂ + H₂ In equations 4 and 6, e^- denotes an electron with its negative charge. Although this mechanism again ends with equation 3, it includes reactions involving H atoms (equations 6 and 7); two different positive ions, NH₃⁺ and NH₄⁺ (equations 4, 5, and 6); and the radicals NH and NH₂ (equations 3, 5, and 7). Equations 3 through 7 also summarize the overall reaction, 2NH₃ → N₂ +3H₂. Although there is no single term describing the decomposition of a molecule in an electric discharge, the word "plasmolysis" might be appropriate.

In radiation chemistry, as in plasma chemistry, the basic energetic particle is the electron. The electrons, however, are produced in the gas by the action of ionizing radiation, which gets its name from the fact that it is energetic enough to create ions in any material absorbing it. Three kinds of radiation--which for historical reasons are denoted as α, β, and γ--are discussed here. The Greek designations date from the end of the nineteenth century, when ionizing radiation was first discovered and before the exact nature of the three types was understood. First noticed as the products of the decay of certain naturally occurring radioactive substances, they have in subsequent years also been produced artificially in nuclear particle accelerators. Most radiation chemistry experiments are carried out using one of these radiations. The energy expended in creating one electron-ion pair by absorption of ionizing radiation depends upon the gas but is always around 30 electronvolts. A single 4.5-million-electronvolt α particle, for example, which loses all of its energy crossing a gas, can produce about 150,000 electron-ion pairs. The 30 electronvolts of energy is shared between the positive ion and the electron in a statistical fashion. Electrons having at least 15 electronvolts are not uncommon in this distribution. Consequently, if the gas happens to be ammonia, there are ionization electrons having energy well in excess of that required in collisions with NH₃ molecules to initiate both reaction mechanisms summarized by equations 1 through 3 and by equations 3 through 7. Decomposition of a substance because of the effects of ionizing radiation is called radiolysis.

The foregoing discussion has stressed a few features that some chemists think are shared by photochemistry, plasma chemistry, and radiation chemistry. Photochemistry is the most clearly distinguished, because ordinary ultraviolet lamps cannot produce photons energetic enough to ionize most molecules; hence, mechanisms involving ions are not usually encountered.

Although not yet routine in photochemistry, the use of the light produced by electron accelerators called synchrotrons is an exception to this rule. Aside from a few simple comparisons and distinctions, it may be safely said that there is at present no general agreement among chemists on a set of basic concepts considered to underlie all of the branches of high-energy chemistry.

Ammonia is a case in point. Despite the simplicity of the schemes discussed above, there is no agreement on a single mechanism that is consistent with all the observations associated with its reactions. As an example, under certain experimental conditions, hydrazine, which has the formula, N₂H₄, appears in radiolyses, while its existence in electric discharges is still problematical. If hydrazine, or any of its possible radicals, plays a role in the high-energy chemistry of ammonia, the two simple mechanisms discussed above must obviously be extended.

Applications

Because of the earth's proximity to the sun, life on this planet has come to depend heavily upon photochemistry. Photosynthesis, upon which green plants depend, is a photochemical process whereby carbon dioxide and water are converted into oxygen and carbohydrates. The process relies upon a complicated set of dark reactions, which follow the initial absorption of a photon by a molecule of chlorophyll. In the animal kingdom, on the other hand, many species have developed organs for vision, which in most instances depend upon a photochemical process that begins with the absorption of a photon by a molecule of the visual pigment, rhodopsin. In photography, which is a human invention, an image is formed by the photochemical reduction of silver bromide to silver metal. In the processing of long chain molecules called polymers into useful plastics, additional mechanical strength can be added to the material by photochemical introduction of chemical bonds between the molecular chains.

This process is called cross-linking. In the microelectronics industry, where printed circuits are manufactured by the photolithography process, various patterns are etched into thin polymer coatings. This is done by shining light onto the polymer surface through the holes in a patterned screen, called a mask. The polymer film, called a photoresist, becomes cross-linked where it has absorbed light. When the film is washed with a suitable solvent, the unexposed areas dissolve, leaving intact the pattern formed by the cross-linked areas.

By comparison with photochemistry, nature has made little use of plasma chemistry, primarily because natural electric discharges, except for lightning, occur infrequently on Earth.

Laboratory experiments with electric discharges meant to duplicate the effect of lightning in the primordial atmosphere, however, have produced amino acids. These substances are the precursors to proteins, which are among the molecules basic to life. An important industrial use of electric discharges is the deposition of material from a plasma to form a thin film on the surface of an object which requires a special coating. Of particular interest is the use of electric discharges in hydrocarbon vapors (compounds containing carbon and hydrogen, which are isolated from petroleum) to deposit thin films consisting of microscopic contiguous grains of diamond. Although these grains have no value as gems, the diamond films stick to most any surface and create an extremely hard, transparent coating. Such coatings enhance the wear-resistance of objects, which for economic or engineering reasons must be made from softer materials.

One of the most dramatic applications of radiation chemistry to industry has been the production of heat-shrinkable plastic tubing. Exposure to heavy doses of ionizing radiation imparts a "memory" to some plastics. After irradiation, the plastic is stretched, but it can be made to resume its original dimensions when heated. An electrical wire may be given an insulating coating by slipping it through a tube made of this plastic, which upon heating shrinks and forms a tight fit conforming to the shape and dimension of the wire. Before any nuclear reactors could be designed, the radiation chemistry of coolants coming in direct contact with the core had to be understood first. Water and carbon dioxide proved to have the right combination of both nuclear and chemical properties for this purpose. Radiation chemistry has also found application in the area of food sterilization. Although the destruction of microbes in food by ionizing radiation is a biological phenomenon, the kill mechanism is probably chemical in origin.

Many of the chemical and physical details associated with photochemistry, plasma chemistry, and radiation chemistry still remain obscure. Solution of these challenging problems not only will lead to new applications but also will determine to what extent the term "high-energy chemistry" is retained as a generic description of the three.

Context

References to photochemistry appeared in the middle of the eighteenth century in the earliest chemical (as opposed to alchemical) literature. C. J. D. von Grotthus (1785-1822) in 1817 and John William Draper (1811-1882) in 1841 made the elementary observation that only light which is absorbed is capable of inducing photochemical change. This is sometimes called the First Law of Photochemistry. After Max Planck (1858-1947) introduced the theory of light emission and absorption based upon photons, Johannes Stark (1874-1957) and Albert Einstein (1879-1955) proposed that one activated molecule (such as NH₃*) is produced by the absorption of one photon. This idea is sometimes termed the Second Law of Photochemistry. The use of lasers as light sources, however, has shown that the Second Law is not absolute. Lasers produce such intense beams of photons that a molecule can absorb and be activated by two or more photons before it has had time to disappear through dark reactions. Despite the wide application of lasers, some chemists are interested in using the sun as a photochemical light source. By combining the action of sunlight and a small applied voltage (the joint action is called photoelectrochemistry), these chemists are attempting to release hydrogen (H₂) and oxygen (O₂) gas from liquid water (H₂O). If ever successful on an industrial scale, this process should produce low-cost H₂, which when burned in air can serve as a fuel for both heating and transportation.

Some of the earliest careful studies in plasma chemistry were performed between 1860 and 1900 by Marcelin Berthelot (1827-1907). A century after these beginnings, plasmas are being used extensively to etch patterns in printed circuits in an application that could not have been anticipated by Berthelot. Productivity in the microelectronics industry, which relies heavily on both plasma and photochemical techniques, is advancing so rapidly that by the twenty-first century, printed circuits consisting of as many as a billion separate components should be possible.

The advent of radiation chemistry occurred later and depended upon the discovery of X rays by Wilhelm Rontgen (1845-1923) in 1895 and natural radioactivity by Henri Becquerel (1852-1908) in 1896. The earliest observation of a radiation chemical effect was, in fact, attributable to Rontgen, who in announcing his discovery reported the ability of X rays to make images on photographic film. Subsequently, Pierre Curie (1859-1906) and Marie Curie (1867-1934) observed the power of ionizing radiation to color glass and porcelain and to produce ozone from oxygen. Sir William Ramsay (1852-1916) and Frederick Soddy (1877-1956), as well as William Henry Bragg (1862-1942) and Samuel C. Lind (1879-1965), made some of the first quantitative studies leading to the idea that the number of ions formed by radiation passing through as gas was related to the number of molecules decomposed. By the end of World War II, the health effects of ionizing radiation had been clearly documented, and some radiation chemists aspired to provide a chemical explanation for the phenomena observed in radiation biology.

Although spurred by rapid developments in biochemistry, progress in this direction has, nevertheless, been measured because of the surprisingly wide variety of radiation chemical effects exhibited by water, upon which all life depends.

Principal terms

ALPHA PARTICLE: a composite particle identical to the nucleus of the ordinary helium atom and consisting of two protons and two neutrons

BETA PARTICLE: an electron emitted by a radioactive nucleus

COVALENT BOND: a chemical bond consisting most commonly of one or more pairs of electrons shared between two atoms participating in the bond

ELECTRONVOLT: the amount of energy gained by an electron when it is accelerated by only 1 volt

GAMMA RAY: a photon having energy in excess of about 100,000 electronvolts

MOLECULAR FORMULA: an abbreviation for a molecule; for example, NH₃ stands for the ammonia molecule and indicates that three hydrogen atoms (H) are bonded to a central nitrogen atom (N); or the symbol, NH₃+, which represents an ammonia molecule with one electron removed (that is, a positive ion)

NUCLEUS: the part of an atom that consists of neutrons and protons, the positively charged atomic core around which the atomic electrons revolve

PHOTON: the elementary particle (also called quantum) of light, X-rays, and γ rays, which accounts for their absorption by matter

RADICAL: a molecular fragment such as NH, which has unused valence electrons

TESLA COIL: an electrical device, invented by Nikola Tesla (1856-1943), which is used to produce a weak current of high-energy electrons

Bibliography

Ausloos, Pierre, ed. FUNDAMENTAL PROCESSES IN RADIATION CHEMISTRY. New York: Interscience, 1968. This work provides a broad view of radiation chemistry and consists of ten chapters, each written by an authority in the field.

Calvert, Jack, and James N. Pitts, Jr. PHOTOCHEMISTRY. New York: John Wiley, 1967. This treatise covers all aspects of photochemistry, including light absorption, molecular bonding and structure, dark reactions, and reaction mechanisms.

Chapman, Brian. GLOW DISCHARGE PROCESSES. New York: John Wiley, 1980. This book covers, at a relatively elementary level, the physics of gases, electron-molecule collisions, the various types of discharges, and the application of plasmas to the treatment of surfaces.

Hart, Edwin J., and Michael Anbar. THE HYDRATED ELECTRON. New York: Wiley-Interscience, 1970. The fact that water could dissolve electrons came as a surprise to many radiation chemists. The chemical properties of the hydrated electron and the dramatic story of its discovery are summarized in this volume.

Holihan, John R., and Alexis T. Bell, eds. TECHNIQUES AND APPLICATIONS OF PLASMA CHEMISTRY. New York: John Wiley, 1974. This volume is a compilation of chapters by experts in the field. The ten chapters and the appendix describe the fundamentals of plasma chemistry, the design of electric circuits for producing discharges, and most of the principal applications.

Lind, Samuel C. RADIATION CHEMISTRY OF GASES. New York: Reinhold, 1961. The author, who was one of the earliest workers in the field, summarizes in this classic volume the main results leading from the discovery of radioactivity to 1961.

Manos, Dennis M., and Daniel L. Flamm. PLASMA ETCHING: AN INTRODUCTION. New York: Academic Press, 1989. Providing coverage of a rapidly evolving field, this book reviews both the fundamentals of plasma chemistry and the latest applications to the microelectronics industry.

Okabe, Hideo. PHOTOCHEMISTRY OF SMALL MOLECULES. New York: Wiley-Interscience, 1978. Since the number of possible steps in a photochemical reaction mechanism increases rapidly with the size of the molecule absorbing the light, this volume is restricted in its coverage to molecules consisting of no more than five atoms.

Spinks, J. W. T., and R. J. Woods. AN INTRODUCTION TO RADIATION CHEMISTRY. 3d ed. New York: Wiley-Interscience, 1990. In its third edition, this is the standard textbook in the field.

Turro, N. J. MODERN MOLECULAR PHOTOCHEMISTRY. Palo Alto, Calif.: Benjamin/Cummings, 1978. It is difficult to find a photochemistry book that is more clearly written than this one.

Quantum Mechanics of Chemical Bonding

Chemical Reaction and Collisions

Radiation: Interactions with Matter

Radioactive Nuclear Decay and Nuclear Excited States