Spectroscopy

I

Introduction

Spectroscopy, in physics and physical chemistry, the study of spectra. The basis of spectroscopy is that each chemical element has its own characteristic spectrum. This fact was recognized in 1859 by the German scientists Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. They developed the prism spectroscope in its modern form and applied it to chemical analysis. This instrument, one of two principal spectroscope types, consists of a slit for admitting light from an external source, a group of lenses, a prism, and an eyepiece. Light that is to be analysed passes through a collimating lens, which makes the light rays parallel, and the prism; then the image of the slit is focused at the eyepiece. One actually sees a series of images of the slit, each a different colour, because the light has been separated into its component colours by the prism. The German scientists were the first to recognize that characteristic colours of light, comprising the spectrum, are emitted and absorbed by each element.

II

Spectrograph

In a spectrograph, the eyepiece is replaced by a camera. Colour photography is not necessary to identify the images of the slit, known as the spectrum lines; their wavelengths can be calculated from their positions on the film. Spectrographs are useful throughout the ultraviolet and visible regions of the spectrum, and as far as 1200 nm (0.000048 in) in the infrared region. Spectroscopy in the extreme ultraviolet and infrared regions is similar to that in the visible region, except that glass does not transmit such radiations; lenses and prisms are made of quartz, fluorite, sylvine, or rock salt. Concave mirrors can also be substituted for lenses. Special photographic emulsions are used. The ultraviolet spectrum may be investigated by these means to wavelengths of less than 60 nm (0.0000024 in); infrared spectra may be investigated by special means to regions beyond 0.01 cm (0.004 in).

III

Spectrophotometer

The spectrophotometer is widely used for measuring the intensity of a particular spectrum in comparison to the intensity of light from a standard source. The concentration of the substance that emits or absorbs the spectrum can be determined from this comparison. The spectrophotometers are also useful for studying spectra in the nonvisible areas because their detecting elements are bolometers or photoelectric cells. The former are particularly applicable to infrared spectrum analysis, and the latter to ultraviolet spectrum analysis.

IV

Diffraction Grating

The second type of spectroscope in common use is the diffraction-grating spectroscope, first used in the early 1800s by the German physicist Joseph von Fraunhofer. Light is dispersed by means of a diffraction grating rather than a prism. A diffraction grating consists of a metal or glass mirror surface on which a large number of parallel lines are ruled by a diamond. A good grating has a very high dispersive power, thus permitting the display of much greater detail in spectra. The lines of the diffraction grating may be inscribed on a concave mirror, so that the grating also serves to focus the light and renders lenses unnecessary. In such a spectroscope, the light need not pass through any transparent substance, and these instruments have been used through the entire ultraviolet region and into what is commonly considered the X-ray region. Gratings may be adapted to spectrographs and spectrophotometers in the same manner as prisms.