Photochemistry

The principal applications of photochemistry (including photography) are in the initiation of reactions by light that can pass through glass or quartz windows. Such light has a wavelength of not less than about 185 nanometres. Light of shorter wavelength is also effective, but the windows required (sapphire, lithium fluoride, or extraordinarily thin aluminum) and the associated mechanical difficulties seriously limit application of photochemical methods in the range from 185 nanometres down to a conceivable lower limit of about 85 nanometres. Photochemical techniques are particularly applicable when a specific initial process (the breakage of a particular bond in a molecule of a particular substance, for example) is required. For such purposes, high-intensity ultraviolet lamps are generally employed, the window is either glass or quartz, and the initiation reaction is limited to the relatively thin layers in which the light is absorbed. The processes include the photochlorination of aromatic compounds (such as benzene, toluene, and xylene), sulfhydration of olefins, production of cyclohexanone oxime, photopolymerization (principally in surface-curing processes), sulfoxidation, and vitamin-D synthesis. Tunable lasers provide a potential means of initiating photochemical processes of practical interest, one such example being the separation of isotopes.

High-energy radiation

The large-scale use of such ionizing radiation for modifying and synthesizing materials, known as radiation processing, represents a minor yet significant technology. It involves irradiating materials either with a beam of electrons produced by a high-voltage particle accelerator or with gamma rays emitted by the radioisotope cobalt-16 or, in a few cases, cesium-137. The electrons are generally accelerated to an energy range of 0.15–10 MeV. (By comparison, the electron energy in a typical television set is only 0.025 MeV.) The gamma rays given off by cobalt-16 have an energy of 1.25 MeV, while those emitted by cesium-137 have approximately half that amount.

Exposure to such electrons and gamma rays does not induce radioactivity in the materials irradiated, and so the technique can be used in the manufacture or processing of many kinds of consumer and industrial products. Moreover, radiation processing has several major advantages over conventional technologies. It consumes far less energy than thermally and chemically initiated processes and at the same time causes less environmental pollution. Paints and certain other coatings, for example, can be cured at room temperature with one-tenth of the energy required in heat curing. Radiation preservation of food involves substantially less energy expenditure than that associated with either refrigeration or canning. A radiation source that releases 1 kilowatt of gamma energy (roughly equivalent to the electrical requirements of a toaster) can irradiate 10 tons of potatoes per hour; the exposure to a small dose of ionizing radiation inhibits sprouting and thereby delays spoilage.

Because of such advantages, radiation processing has found increasingly wider application. It has proved particularly valuable in the processing of plastics. Chemical reactions induced by electron-beam irradiation permit the cross-linking of polymers that make up the foamed plastic used for sound and thermal insulation. A large fraction of the wire and cable employed in high-temperature applications and much of the wiring in telecommunications equipment are covered by insulation cross-linked by electron irradiation. The heat-shrinkable polyethylene packaging for hams and turkeys and various other poultry products is manufactured by the same process. The coating of certain audio and video recording tape is cured by exposure to electron beams, as is the rubber in a large percentage of automobile tires. Sterilization of disposable medical supplies, such as syringes, blood transfusion kits, and hospital gowns, is usually done with gamma rays. Other potential applications of radiation processing include the treatment of a wide assortment of food products so as to reduce the amount of chemical preservatives employed, the treatment of sewage for pathogen reduction, and the precipitation of sulfur dioxide and nitrogen oxide (the primary source materials for acid rain) from the stack gases of electric power plants and smelting facilities that burn fossil fuels.

Radiation source technology has developed to a point where reliable, safe, and inexpensive sources are readily available. When electron accelerators are used, radioactivity is not involved in any aspect of the process and there is no conceivable hazard to the surrounding community. In processing facilities that use gamma radiation, the source is encapsulated in a double layer of stainless steel to prevent the escape of radioactivity to the environment. Other safeguards minimize the possibility of accidental exposure of either the plant personnel or the population at large.

Lasers

As noted above, lasers have become a valuable tool in medicine. They also have important uses in a number of other areas, as, for example, communications. Laser light can carry voice messages and digitally encoded information and can do so in large amounts because of its high frequency. Except in satellite-to-satellite communications, laser beams are transmitted via optical fibres. The speed with which the focal spot of a narrow laser beam can be controlled makes it suitable for a variety of applications in information processing—e.g., use in optical scanners, optical disc storage systems, and certain types of computer printers.

A highly intense laser beam can instantly vaporize the surface of a target. When laser pulses are concentrated on frozen deuterium-tritium pellets, they can initiate nuclear fusion (see nuclear fusion). High-powered lasers can be used as space weapons to destroy reconnaissance and communications satellites and perhaps even ballistic missiles. These same capabilities have led to the use of lasers in research as well as in surgery. The laser microprobe is used for microanalysis of surface composition. Laser beams have been found to have a selective effect on cellular components, or organelles: those components that absorb light of the wavelength of the beam are destroyed, whereas transparent parts of the cells remain unaffected. Organelles such as mitochondria, which are responsible for cell respiration, or chloroplasts, which are involved in plant-cell photosynthesis, can be separately studied in this manner.

An intense beam of laser light can be used for small-scale cutting, scribing, and welding in certain industrial processes. Laser “pens” capable of producing such high-intensity light beams have proved useful in the assembly of various electronic components, such as computer memory and logic units consisting of integrated arrays of microcircuit elements.

The use of special dyes can alter laser action. The availability of high-pulse-intensity laser beams is also revolutionizing microscopy. It is possible to photograph microaction in a small fraction of a second and to use holography for image synthesis.