The beginnings of spectroscopy

Kirchhoff and Bunsen began publishing accounts of their epoch-making experiments in 1859. The experiments consisted of vaporizing various chemical compounds using a gas burner (known to high school chemistry students everywhere as the Bunsen burner), and analyzing the light the compounds gave off. In the case of sodium chloride, for example, they would deposit a little pellet of salt on a platinum wire loop, and then hold the loop in the gas flame. As the salt compound glowed in the flame, the experimenters pointed a spectroscope at the flame to capture the substance's spectrum.

At the heart of Kirchhoff and Bunsen's spectroscope lay a prism. A prism will spread or disperse a ray of sunlight into a rainbow. The prism bends the path of incoming light of different wavelengths by different amounts, so that each color (or wavelength) composing the apparently white beam of sunlight emerges from the prism at its own angle, forming a spectrum.

When Kirchhoff and Bunsen analyzed the light that the heated compound or metal gave off by dispersing it with prisms, they noted that instead of a continuous spectrum or rainbow of colors, a few isolated bright lines appeared, of various colors. What excited them was to find that every element seemed to produce its own characteristic lines: sodium, for example, produces a pair of bright, closely-spaced lines in the yellow part of the spectrum, as well as fainter lines elsewhere. Magnesium produces a distinctive bright triplet of lines in the blue/green part of the spectrum, as well as some strong lines in the green and yellow.

Kirchhoff and Bunsen could not yet explain why or how these lines occurred, but the fact that each element corresponded to a unique set of lines suggested an entirely new way to perform chemical analysis. And because the method was so sensitive — they noted that very small amounts of sodium contamination of a sample would produce the characteristic yellow doublet, for example, in addition to the lines of the intended sample—they predicted that spectrum analysis would enable chemists to find rare elements that traditional chemical analysis had missed.

A second facet of Kirchhoff and Bunsen's experiment, and one that their contemporaries found particularly hard to understand, was to show that the bright colored lines they saw when they heated their samples could be, in a sense, reversed, in another kind of spectrum. When a continuous spectrum of light passed through a sodium flame, for example, the resulting ''absorption spectrum'' showed the rainbow of colors, with a pair of dark lines in the yellow, exactly where the sodium doublet would be. Thus there are three types of spectrum: the continuous spectrum, the emission or bright-line spectrum, and the absorption spectrum. The continuous spectrum arises from any incandescent solid or liquid, and reveals no clues about chemical composition. The emission spectrum of colored lines against a dark background arises from hot vaporized substances, and reveals the unique signature of any chemical element in the pattern of lines. The absorption spectrum of dark lines in a ''rainbow'' of color arises when a viewer sees a continuous spectrum of light illuminating a vapor from behind, and (as we now understand) the vapor selectively absorbs the light at certain wavelengths instead of emitting at those wavelengths, as in the emission spectrum.

Kirchhoff and Bunsen concluded one of their seminal papers on spectrum analysis, in 1860, with some comments on Fraun-hofer's lines and on the potential applications of spectrum analysis in astronomy. Fraunhofer, the lens maker whose firm had built the ''Great Refractors'' for Wilhelm Struve and the heliometer for Bessel, had noted in his experiments with glass prisms that unexplained dark lines appeared in the solar spectrum (figure 6.3). The effect is not readily apparent when sunlight passes through only one prism, because the dark lines are very fine, but when sunlight passes through a series of prisms and the light is dispersed as widely as possible, hundreds of unequally spaced dark lines appear. Fraunhofer labeled the more prominent of the dark lines A through H, running from the red end of the spectrum toward the blue. Kirchhoff and Bunsen suggested a connection between Fraunhofer's lines and their observations on absorption spectra.

Spectrum analysis, Kirchhoff and Bunsen said, offers a simple means for detecting even small traces of elements in laboratory samples, and more; it ''opens to chemical research a hitherto completely closed region extending far beyond the limits of the Earth and even of the solar system. Since in this

Figure 6.3 Fraunhofer's solar spectrum. The optical instrument maker, Joseph Fraunhofer, dispersed sunlight through prisms and detected the presence of hundreds of fine dark lines amid the colors of the rainbow. Top panel: The intensity of light from the Sun is strongest in the yellow part of the spectrum, as shown by the peak of the curve. Bottom panel: Fraunhofer labeled the more prominent lines with letters of the alphabet. The lines were later shown to relate to the presence of various elements in the Sun. For example, the C and F lines are due to hydrogen, A and B are due to oxygen, and D is due to sodium. (Credit: Adapted from the original by Layne Lundstrom.)

Figure 6.3 Fraunhofer's solar spectrum. The optical instrument maker, Joseph Fraunhofer, dispersed sunlight through prisms and detected the presence of hundreds of fine dark lines amid the colors of the rainbow. Top panel: The intensity of light from the Sun is strongest in the yellow part of the spectrum, as shown by the peak of the curve. Bottom panel: Fraunhofer labeled the more prominent lines with letters of the alphabet. The lines were later shown to relate to the presence of various elements in the Sun. For example, the C and F lines are due to hydrogen, A and B are due to oxygen, and D is due to sodium. (Credit: Adapted from the original by Layne Lundstrom.)

analytical method it is sufficient to see the glowing gas to be analyzed, it can easily be applied to the atmosphere of the sun and the bright stars.''11

The only difference between the familiar laboratory spectra and the spectra of the Sun, they noted, was that the solar spectral lines appeared ''reversed'' as dark lines. Today we know that the Fraunhofer solar spectrum is an absorption spectrum because the temperature of the Sun decreases with height. (The temperature increases in the extremely tenuous outer layer called the corona, but that rise does not affect the Fraunhofer spectrum.) The deeper layers act as a heater, producing a continuum of emission, and the cooler, more elevated layers act as a ''vapor,'' selectively blocking some of the light from below. Kirchhoff and Bunsen thought of the Sun as having a surface and an atmosphere, so they wrote that ''the spectrum of the sun with its dark lines is just a reversal of the spectrum which the atmosphere of the sun would show by itself.''12

Kirchhoff and Bunsen concluded with a kind of call to arms to laboratory chemists: ''The chemical analysis of the sun's atmosphere requires only the search for those substances that produce the bright lines [in the laboratory] that coincide with the dark lines of the spectrum.''13 The problem turned out to be much more complex than they made it sound, due to the large number of lines and the special conditions of temperature and pressure prevailing in the Sun's atmosphere; by 1895, some 14000 lines were known in the Fraunhofer spectrum, but in 1924, more than 6000 of these lines had yet to be identified with their corresponding elements.14 However, as scientists learned over the course of the next century, the investigations Kirchhoff and Bunsen urged held just as much promise as they thought. In effect, they paved the way to an understanding that some philosophers of science thought was impossible: a knowledge of the chemical composition of the stars.15

The scientific societies in England, which were already in a state of excitement over Charles Darwin's 1859 publication of the Origin of Species, eagerly took up discussions of Kirchhoff and Bunsen's papers. Warren De La Rue, a prominent astro-photographer who was, like Huggins, a member of both the Microscopical and Royal Astronomical Societies, wrote in 1861, ''The physicist and the chemist have brought before us a means of analysis so wonderfully exact that, as Dr. Faraday recently said, if we were to go to the sun, and to bring away some portions of it and analyze them in our laboratories, we could not examine them more accurately than we can by this new mode of spectrum analysis.''16

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