Infrared spectra

The infrared spectrum is part of the electromagnetic spectrum that lies beyond the red color that human eyes perceive as visible light. Every body emits some energy in the infrared or near infrared. Detection of infrared radiation by special instruments is used in numerous fields, including medicine, mapping, defense, communication, and astronomy.

Electromagnetic Spectrum

The small bands of infrared radiation seeping through the atmosphere were accidentally discovered in 1800 by German-born British astronomer William Herschel. When measuring the temperatures of the visible light spectra, he found a source of greater heat and wavelength radiation beyond the color red. However, it was not until 1881, when American physicist Samuel Pierpont Langley developed the bolometer, that the first in-depth studies of the infrared were possible. German physicist Max Planck's development of the quantum theory in 1900 and Einstein's discovery of photons in 1905 led to the development of quantum detectors, which further advanced the study of the infrared. These early detectors brought forth modern spectroscopes, spectrometers, and spectrophotometers.

The electromagnetic spectrum comprises visible light and six forms of invisible radiation: radio, microwave, infrared, ultraviolet, X, and gamma rays. All spectra travel at the speed of light in waves of energy bundles called photons and can be reflected, refracted, transmitted, absorbed, and emitted.

Infrared spectrometry encompasses the study of wavelengths in the electromagnetic spectrum that range between 0.7 micron in the near-infrared photographic region and 500 microns in the far-infrared rotation region. Wavelengths of the infrared spectra are most useful for detecting certain atoms and molecules visible in the infrared, such as hydrogen, the most abundant element in the universe. Infrared rays differ from the other components of the electromagnetic spectrum. Radio waves are propagated through the atmosphere and have wavelengths between 30,000 meters and 1 millimeter. Microwaves have wavelengths between 1 meter and 1 millimeter. X-rays have extremely short wavelengths, approximately 1 angstrom, and are generated by a sudden change in the velocity of an electrical charge. Gamma rays are similar to X-rays but are of a higher frequency and penetrating power. Ultraviolet rays are beyond violet in the spectrum and have wavelengths shorter than 400 angstroms.

Infrared Radiation

Infrared radiation is emitted in some amount by every macroscopic body in the universe that has a temperature above absolute zero (or −273 degrees Celsius). In each macroscopic body, molecules not only are moving in all directions but also are rotating, and at the same time the individual atoms within the molecule are vibrating with respect to one another. It is the interaction of molecules with radiation that is the essence of the study of infrared.

For a molecule to absorb radiation, it must have a vibrational or rotational frequency the same as that of the electromagnetic radiation. In addition, a change in the magnitude and/or direction of the dipole moment must take place. The dipole moment is a vector that is oriented from the center of gravity of the positive charges to that of the negative charges, and it is defined as the product of the size and the distance between these charges. Corresponding frequencies between radiation and molecules are possible because radiation has an electrical component, in addition to having a magnetic component. In contrast, a molecule has an electrical field. When the electrical field of the molecule is rotating or vibrating at the same frequency as is the incoming radiation, then it is possible for a transfer of energy to take place.

The second requirement for the study of infrared, the dipole moment change, must have something to couple the energy from the radiation to the molecule. If atoms differ in their electronegativity and they combine to form a molecule, the centers of the positive and negative electrical charges may not coincide, producing a permanent dipole moment. The energy to produce this work can come from the absorption of the incoming radiation by the molecule. A permanent dipole is also necessary for inducing rotation. Atoms rotate because the electric fields are not the same on each side, thereby allowing for a transfer of energy; when energy is transferred in this manner, a rotator will rotate faster under certain rotational frequencies, while a vibrator will not change its frequency but will increase its amplitude of vibration. Because vibrational frequencies are of the order of 10¹⁴ cycles per second and rotational frequencies are 10¹¹ cycles, they fall within the infrared region. Absorption bands for rotational spectra are quite sharp, but the bands for vibrational spectra tend to become broader because of the rotational levels associated with each vibrational level.

Infrared Detectors

Detectors, either thermal or quantum, are commonly used to study the infrared. Each type uses a different property of electromagnetic radiation to convert the infrared to an electrical signal with an intensity equivalent to the amount of infrared striking the detector. A thermal detector measures heat-induced changes in a property of a material, usually electrical resistance. A quantum detector also measures change, although it uses a photon—not heat—to create successive events when it strikes a material. There are three types of quantum detectors, each of which uses a separation, or diffusion, of different types of electrons as a catalyst for an event. In brief, the photoconductive effect uses incidental radiation to increase electrical conductivity, the photovoltaic effect uses a special junction for diffusion that creates voltage from charge separation, and the photoelectromagnetic effect uses radiation falling on a semiconductor with a magnetic field. In addition to detectors, various other instruments are used in the study and application of the infrared, such as the radiometer, the comparator, the collimeter, and modulators, all of which perform unique and valuable tasks.

Spectrometers

A device basic to understanding how the infrared spectra works is the spectrometer. All spectrometers use certain elementary components. These include a source of radiation, a condensing source for focusing energy onto the monochromatic (pertaining to one color or one wavelength) slit, a monochromator to isolate a narrow spectral range, a radiation detector, and some form of amplifying system and output recorder. Single-beam spectrometers record energy versus wavelength, whereas a double-beam spectrometer measures the ratio between energy transmitted by the sample and energy incident on the sample, and plot transmittance or a related quantity as a function of wavelength or wavenumber. One micron is equal to one-millionth of a meter, and wavenumber is obtained by dividing 10,000 by the wavelength in microns.

Emission Spectrography

An application of the method can be illustrated by emission spectrography, which allows the determination of major, minor, and trace elements in many materials. Approximately seventy elements can be determined in rocks and other geologic materials. When a sample of material is correctly excited by an electric arc or a spark, each element in the sample emits light of a characteristic wavelength. The light enters the spectrograph via a narrow opening and falls on a diffraction grating, which is a band of equidistant parallel lines (from 10,000 to 30,000 or more lines per inch) ruled on a surface of glass or polished metal used for obtaining optical spectra. The grating separates the reflected light of each wavelength by a different angle. The dispersed light is focused and registered on a photographic plate in the form of lines of the spectrum.

The comparator-densitometer is used to measure the intensity, or darkness, of the spectrum lines registered on the spectrograph photographic plate. Using standard films or plates for each element, scientists change the spectrograph markings to indicate the percent concentration of each element. For a visual estimate, a special screen permits a comparison of the spectrum of the sample with the spectra of standards containing known element concentrations. A direct-reading emission spectrometer is one that is tied in with a computer in which are stored electronic signals from specific parts of the spectrum during the burn of a sample. The stored signals are emitted in sequence to an electronic system that measures the intensities of the spectral lines.

Thermal infrared analytical techniques are being employed to detect the mineralogical composition of the surface of Mars. The Mars Global Surveyor sent into orbit in 1999 employed a thermal emission spectrograph (TES) to determine the composition of the Mars surface. Much of the surface varies from basaltic to andesitic-basaltic rock compositions (e.g., similar to lava flows from volcanoes of the west coast of the United States) and soils derived from these rocks. TES detected two areas of hematite-rich material and failed to detect large surface exposure of carbonates. Mini-TES instruments aboard rovers, such as the 1997 Pathfinder Mission and later rovers, allowed a closeup analysis of rocks on the surface. The TES instruments offer a chance to identify mineralogy of the surface of Mars rather than elemental composition.

Use in Geochemical Research

Infrared is related to geophysics, geomorphology, structural geology, and exploration as well as to geochemistry. Specific areas in which the infrared spectra have been used in geochemical research include the study of the bonds between atoms in minerals and the gaining of unique information on features of the structure, including the family of minerals to which the specimen belongs, the mixture of isomorphic substituents, the distinction of molecular water from the constitutional hydroxyl, the degree of regularity in the structure, and the presence of both crystalline and noncrystalline impurities. For example, chalcedony, including flint, chert, and agate, has been shown by infrared spectroscopy and X-ray studies to contain hydroxyl in structural sites as well as in several types of nonstructural water that can be held by internal surfaces and pores. The content of the structural hydroxyl varies zonally in chalcedony fibers and in both natural and synthetic crystals of the same spectral type as chalcedony. The varieties of chalcedony, as well as rock crystal and amethyst formed at low temperatures and in association with chalcedony, together with crystals of synthetic quartz, show a distinctive infrared absorption spectrum in the region of 2.78-3.12 microns; natural quartz crystals formed at higher temperatures give a spectrum in this region. Structural hydroxyl is housed by different mechanisms in the two types of quartz. The fibrose nature of the low-temperature quartz may derive from the hydroxyl content and its effect on dislocations.

Use in

The widest applications for the infrared spectra are in remote sensing, which is the process of detecting chemical and physical properties of an area by measuring its reflected and emitted radiation. Remote sensing has been a great aid to geologists in their study of the earth. Thermal infrared scanning has been used to monitor and update mine waste embankment data and to locate faults and fracture zones. Landsat thematic mapper and airborne thermal infrared multispectral scanner data have been used to do surface rock mapping in Nevada. Advanced visible and infrared imaging spectrometers and other remotely sensed data have been used to locate water-producing zones beneath the surface in parts of the Great Plains region. Infrared reflectance surveys have been used to locate an extinct hot spring system in the Idaho batholith. Infrared surveys have also included quantitative measurement of thermal radiation from localized heat flow in Long Valley, California, as well as surveys of the lava dome on Mount St. Helens, Washington. Reflectance variations related to petrographic texture and impurities of carbonate rocks have been analyzed by visible and near-infrared spectra.

In addition, infrared surveys and photography of volcanic zones around Mauna Loa and Kilauea were used to obtain information impossible to gather from the ground. The effects of the 1977 earthquake in Nicaragua were surveyed by infrared photography in an attempt to find access and evacuation routes as well as safe areas for temporary camps; in addition, the photographs provided geomorphological information for future study. Infrared surveying has been used to study California's San Andreas fault. In one section near the Indio Hills, the fault trace is not topographically distinguishable but can be located by a margin of vegetation on the northeast side of the fault. Infrared sensing verified its location by imaging a band of alluvium kept cool by the water dammed up by the displaced rock.

Additional Applications

On a large scale national defense system infrared is used in secret communications, night reconnaissance, missile guidance, and gun sitings and tracking. Weather and pollution control programs use infrared to detect levels of radiation and chemicals. In medicine, infrared is used to find hot areas on the surface of the human body that may indicate possible areas of disease. Studies have revealed positive applications in the detection of, for example, breast cancer, skin burns, frostbite, tumors, abscesses, and appendicitis. As an analyzer, infrared can pinpoint damage in semiconductors that results from overheating. It can also be used to study electrical circuit performance while in operation, detect underrated components in regular circuits, and predict component failure or shortened lifetime: These applications are only a few that demonstrate infrared's analytical ability. In space technology, infrared can be used in telescopes for locating new cosmic bodies, observing star formation, studying planet temperatures, and determining the chemical and/or physical nature of distant sources of infrared radiation. Other major uses of infrared include detecting forest infiltrations and spotting welding defects; other fields that utilize infrared include photography and organic chemistry.

Miscellaneous uses of infrared are as widely varied as the major applications already discussed. Infrared photography can reveal original charcoal sketches under oil paintings. Ecologists can study the thermodynamic world on the planet and observe the animals and plants as they adapt to the volatile thermal balance. They can also track schools of fish by mapping “warm” areas on the water's surface. Criminologists and police can use night vision to survey high crime areas in the dark without the use of a spotlight. Warm air masses associated with turbulence can be detected and avoided, resulting in smoother, safer airplane flights. Infrared studies are contributing to improved telecommunications as a result of the introduction of fiber optics, which uses glass as opposed to copper and other scarce metals. Other uses of the infrared are highly technical and are found in many fields, including geology, biology, agriculture, engineering, and defense.

Principal Terms

angstrom: a unit of wavelength of light equal to one ten-billionth of a meter

bolometer: a detector of radiant energy that works by determining the change in resistance of an electrical conductor due to temperature change

macroscopic: large enough to be observed by the naked eye

photon: a quantum of radiant energy

spectrograph: an instrument for dispersing radiation (as electromagnetic radiation) into a spectrum and photographing or mapping the spectrum

spectrometer: an instrument used in determining the index of refraction; a spectroscope fitted for measurements of the observed spectra

spectrophotometer: a photometer for measuring the relative intensities of the light in different parts of a spectrum

spectroscopy: the subdiscipline of physics that deals with the theory and interpretation of the interactions of matter and radiation (as electromagnetic radiation)

spectrum: an array of the components of an emission or wave separated and arranged in the order of some varying characteristic (such as wavelength, mass, or energy)

spectrum analysis: the determination of the constitution of bodies and substances by means of the spectra they produce

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