Spectroscopy is used as a tool for studying the structures of atoms and molecules. The large number of wavelengths emitted by these systems makes it possible to investigate their structures in detail, including the electron configurations of ground and various excited states.

Spectroscopy also provides a precise analytical method for finding the constituents in material having unknown chemical composition. In a typical spectroscopic analysis, a concentration of a few parts per million of a trace element in a material can be detected through its emission spectrum.

In astronomy the study of the spectral emission lines of distant galaxies led to the discovery that the universe is expanding rapidly and isotropically (independent of direction). The finding was based on the observation of a Doppler shift of spectral lines. The Doppler shift is an effect that occurs when a source of radiation such as a star moves relative to an observer. The frequency will be shifted in much the same way that an observer on a moving train hears a shift in the frequency of the pitch of a ringing bell at a railroad crossing. The pitch of the bell sounds higher if the train is approaching the crossing and lower if it is moving away. Similarly, light frequencies will be Doppler-shifted up or down depending on whether the light source is approaching or receding from the observer. During the 1920s, the American astronomer Edwin Hubble identified the diffuse elliptical and spiral objects that had been observed as galaxies. He went on to discover and measure a roughly linear relationship between the distance of these galaxies from Earth and their Doppler shift. In any direction one looks, the farther the galaxy appears, the faster it is receding from Earth.

Spectroscopic evidence that the universe was expanding was followed by the discovery in 1965 of a low level of isotropic microwave radiation by the American scientists Arno A. Penzias and Robert W. Wilson. The measured spectrum is identical to the radiation distribution expected from a blackbody, a surface that can absorb all the radiation incident on it. This radiation, which is currently at a temperature of 2.73 kelvin (K), is identified as a relic of the big bang that marks the birth of the universe and the beginning of its rapid expansion.

General methods of spectroscopy

Production and analysis of a spectrum usually require the following: (1) a source of light (or other electromagnetic radiation), (2) a disperser to separate the light into its component wavelengths, and (3) a detector to sense the presence of light after dispersion. The apparatus used to accept light, separate it into its component wavelengths, and detect the spectrum is called a spectrometer. Spectra can be obtained either in the form of emission spectra, which show one or more bright lines or bands on a dark background, or absorption spectra, which have a continuously bright background except for one or more dark lines.

Absorption spectroscopy measures the loss of electromagnetic energy after it illuminates the sample under study. For example, if a light source with a broad band of wavelengths is directed at a vapour of atoms, ions, or molecules, the particles will absorb those wavelengths that can excite them from one quantum state to another. As a result, the absorbed wavelengths will be missing from the original light spectrum after it has passed through the sample. Since most atoms and many molecules have unique and identifiable energy levels, a measurement of the missing absorption lines allows identification of the absorbing species. Absorption within a continuous band of wavelengths is also possible. This is particularly common when there is a high density of absorption lines that have been broadened by strong perturbations by surrounding atoms (e.g., collisions in a high-pressure gas or the effects of near neighbours in a solid or liquid).

In the laboratory environment, transparent chambers or containers with windows at both ends serve as absorption cells for the production of absorption spectra. Light with a continuous distribution of wavelength is passed through the cell. When a gas or vapour is introduced, the change in the transmitted spectrum gives the absorption spectrum of the gas. Often, absorption cells are enclosed in ovens because many materials of spectroscopic interest vaporize significantly only at high temperatures. In other cases, the sample to be studied need not be contained at all. For example, interstellar molecules can be detected by studying the absorption of the radiation from a background star.

The transmission properties of Earth’s atmosphere determine which parts of the electromagnetic spectrum of the Sun and other astronomical sources of radiation are able to penetrate the atmosphere. The absorption of ultraviolet and X-ray radiation by the upper atmosphere prevents this harmful portion of the electromagnetic spectrum from irradiating the inhabitants of Earth. The fact that water vapour, carbon dioxide, and other gases reflect infrared radiation is important in determining how much heat from Earth is radiated into space. This phenomenon is known as the greenhouse effect since it works in much the same way as the glass panes of a greenhouse; that is to say, energy in the form of visible light is allowed to pass through the glass, while heat in the form of infrared radiation is absorbed and reflected back by it, thus keeping the greenhouse warm. Similarly, the transmission characteristics of the atmosphere are important factors in determining the global temperature of Earth.

The second main type of spectroscopy, emission spectroscopy, uses some means to excite the sample of interest. After the atoms or molecules are excited, they will relax to lower energy levels, emitting radiation corresponding to the energy differences, ΔE = hν = hc/λ, between the various energy levels of the quantum system. In its use as an analytical tool, this fluorescence radiation is the complement of the missing wavelengths in absorption spectroscopy. Thus, the emission lines will have a characteristic “fingerprint” that can be associated with a unique atom, ion, or molecule. Early excitation methods included placing the sample in a flame or an electric-arc discharge. The atoms or molecules were excited by collisions with electrons, the broadband light in the excitation source, or collisions with energetic atoms. The analysis of the emission lines is done with the same types of spectrometer as used in absorption spectroscopy.