Spectroscopy

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Applications of Spectrum Analysis

The two main uses of spectrum analysis are in chemistry and astrophysics.

A

Chemical Analysis

The spectrum of a particular element is absolutely characteristic of that element. Different elements, however, sometimes give rise to lines that are quite close together, leading to the possibility of error or misinterpretation. The Fraunhofer C line at 430.8 nm, for example, is caused by two different lines, one formed by calcium with a wavelength of 430.7749 nm and the other formed by iron with a wavelength of 430.7914 nm. With an ordinary spectroscope, distinguishing between these two would be difficult. The other lines of calcium, however, are very different from those of the other lines of iron. Thus, the comparison of the entire spectrum of an element with a known spectrum simplifies its identification. When the spectrum of an unknown substance is excited by flame, as in the flame test, or by an arc, spark, or other suitable method, a quick analysis with a spectrograph is usually sufficient to determine the presence or absence of any particular element. Absorption spectra are frequently useful in identifying chemical compounds.

Spectra beyond the ultraviolet region, of X rays and gamma rays, are detected by suitable ionization detectors. Gamma-ray spectra are useful in neutron-activation analysis. In this technique, a specimen is irradiated with neutrons in a nuclear reactor and becomes radioactive, emitting gamma rays. The spectra of these gamma rays serve to identify minute quantities of certain chemical elements in the specimen. Along with more conventional types of spectroscopy, this technique is valuable in crime detection.

Raman spectroscopy, discovered in 1928 by Indian physicist Sir Chandrasekhara Venkata Raman, has had widespread recent application in theoretical chemistry. Raman spectra are formed when, under certain conditions, light in the visible or ultraviolet region is first absorbed, then is reemited at a lower frequency after causing molecules to rotate or vibrate. See Raman Effect.

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Two magnetic methods of spectroscopy at the radio-frequency region of the spectrum, longer than the infrared band, are valuable in providing chemical information on molecules and showing their detailed structure. These methods are nuclear-magnetic resonance (nmr) and electron-paramagnetic resonance (epr), the latter also being called electron-spin resonance (esr). These methods depend on the fact that electrons and protons spin like little tops. To align the spins, the specimen is placed in a magnetic field. Electrons or protons in the specimen “flip” over, reversing their spin axes, when the proper amount of radio-frequency power is supplied.

B

Astrophysical Applications

The distance at which a spectroscope may be placed from the source of light is unlimited. Thus, spectroscopic analysis of the light of the Sun permits an accurate chemical analysis of the constituents of the Sun (see Spectroheliograph). The Fraunhofer lines were discovered and named early in the 19th century after their discovery as absorption lines in the spectrum of the Sun; a secondary discovery was that these same lines could be produced on Earth. The element helium was discovered on the Sun and named many years before its presence on Earth was detected. More recently, spectroscopic study of the Sun has given strong indirect evidence for the existence of a negative hydrogen ion. Thus, spectroscopic study of the stars has provided scientists with valuable theoretical knowledge, and is continuing to do so because the stars provide laboratories in which conditions unattainable on Earth are maintained, such as extremely high temperatures and extremely high and low pressures. Certain lines, for example, found in the spectra of nebulas were long thought to be due to an element, tentatively called nebulium, undiscovered on Earth. Scientists now know that these lines are produced by common elements under exceedingly high vacuum conditions. Late in 1969, for example, the Lunar and Planetary Laboratory at the University of Arizona announced that the spectral analysis of the rings surrounding the planet Saturn showed them to be largely formed of ammonia ice. Scientists also utilized spectroscopy to analyze the composition of the planet Jupiter and its atmosphere after fragments of Comet Shoemaker-Levy 9 crashed into the planet in July 1994. The collisions brought heated interior gases to the surface of the planet's atmosphere, where telescopes recorded them in detail.

A shift in the position of the spectrum lines occurs when the source of the radiation is moving toward or away from the observer. This shift in wavelength, known as the Doppler effect discussed above, provides a fairly accurate value for the relative speed of any source of radiation. In general, if all the lines in the spectrum of a star are shifted toward the red, that star is moving away from Earth, and the velocity of recession can be calculated from the amount of the shift. Conversely, if the star is moving toward Earth, the spectrum is shifted toward the violet. The Doppler shifts observed in the spectra of exterior galaxies indicate that the universe is expanding. See Cosmology.

The spectra of a few distant stars periodically split up; the doublets then combine into single lines again. This phenomenon is due to the presence of two stars called double stars, or spectroscopic binaries, that are revolving about each other so close together that a telescope cannot resolve them. When one of the stars is moving toward Earth and the other away, all the lines from one star are shifted toward the violet and all the lines of the other star are shifted toward the red. When both stars are moving transverse to the line of sight from Earth, the lines from the two stars coincide. See Star.

All the molecules of a gas are in constant motion, so that at any instant some are moving toward a spectroscope and some away from it (see Gases). The wavelengths of some of the photons are smaller, and those of others larger, than if all the atoms were at rest. Because of this variability of wavelength, each spectrum line is broadened slightly. If the temperature is raised, the average speed of the molecules is increased, and the lines are still broader. Thus, measurement of the width of certain spectrum lines gives an indication of the temperature of the source, such as the Sun. In many cases, the interior of a source is at a higher temperature than the exterior. An emission spectrum of broad lines will then arise from the interior, and an absorption spectrum will be produced in the exterior; the exterior, however, being cooler, produces narrower lines, and the result for each line is a bright region with a dark center. This phenomenon is called self-reversal.

Related to the Doppler effect is the Mössbauer effect, the discovery of which was announced in 1958 by German physicist Rudolf Ludwig Mössbauer. In a Mössbauer-effect experiment, the recoil-free emission and absorption of gamma rays from one nucleus to another is measured. For absorption to occur, the energy spectrum of gamma rays from the emitter must nearly match the spectrum of possible energies of excitation in the absorber. The slightest change in the motion of the absorber relative to the emitter causes the apparent energy of gamma rays “seen” by the absorber to change. By moving the source or the absorber, scientists may sort out the energies of the gamma rays with high precision. This information is valuable in studies of the electronic and magnetic fields at the nuclei of a solid. The effect also provides an accurate picture of relative motion for use in applications such as the docking of space vehicles. See Space Exploration.

High-resolution spectroscopy is employed in nuclear physics to study the influence of nuclear size and shape on outer atomic structure. Also, when a light source is placed in magnetic or electric fields, spectral lines are often split or widened, thus revealing important information about the atomic structure of the source, or about the fields, not otherwise available. Dutch physicist Pieter Zeeman discovered in 1896 that, when a ray of light from a source placed in a magnetic field is examined spectroscopically, the spectral line is widened, or even doubled. This phenomenon was named Zeeman effect. The so-called Stark effect was named after German physicist Johannes Stark, who succeeded in splitting spectral lines into several components with a strong electric field in 1913.

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