Spectroscopy is a powerful method of investigation of the structure of matter that is based on the analysis of the decomposition of light emitted by it in its fundamental wavelengths. The measurement and analysis (by means of a spectrometer) of a spectrum is called spectroscopy; it is the study of the interaction between matter and electromagnetic radiation and an essential tool for astronomers, chemists, and physicists.

With the discovery of the wave nature of light, the term spectrum was referred to as the intensity of light as a function of wavelength or frequency. More simply, spectroscopy is the science of measuring the intensity of light at different wavelengths. The graphical representations of these measurements are called spectra.

The decomposition of light into its component wavelengths, obtained by passing through a dispersive element such as a prism or a diffraction grating, is the spectrum, or the fingerprint of the light source, which unequivocally identifies the chemical nature and the physical conditions in which it is located. The spectroscopic research is therefore one of the strengths of modern astrophysics, thanks to which have been possible some fundamental discoveries of cosmology, such as the redshift of galaxies and the expansion of the universe.

Spectroscopy involves almost all areas of modern physics, from wave optics, to atomic theory.

By means of spectroscopy we can understand and study the chemical composition of the Sun, planets and stars, we can detect the presence of magnetic fields of sunspots, we can verify the mechanisms of formation of chemical radicals in the cometary crown; spectroscopy also provides us information on the presence of interstellar gases and allows us to analyze their constitution, measure the temperature and kinetics of the accretion rings of binary systems, classify supernovae, etc..

On a large scale spectroscopy allows us to classify galaxies, measure the speed of recession of quasars, detect, as we said the so-called “redshift” of celestial objects, measuring the shift of spectral lines in absorption or emission for Doppler and relativistic effects.

Most of the different kinds of spectroscopy, corresponding to the various regions of electromagnetic radiation, relate to particular kinds of energy-level transitions. Gamma-ray spectra arise from nuclear energy-level transitions; X-ray spectra from inner-electron transitions in atoms; ultraviolet and visible spectra from outer (bonding) electron transitions in molecules (or atoms); infrared spectra from molecular vibrations; and microwave spectra from molecular rotations.

Historical overview

With the discovery of the wave nature of light, the term “spectrum” came to refer to the intensity of light as a function of wavelength or frequency. More simply, spectroscopy is the science of measuring the intensity of light at different wavelengths. The graphical representations of these measurements are called spectra. Currently, the term spectrum has been generalized further, and refers to a flux or intensity of electromagnetic radiation or particles (atoms, molecules, or otherwise) as a function of their energy, wavelength, frequency, or mass.

Since the mid-nineteenth century spectroscopy assumed a fundamental importance for the analysis of the elements until it became a science in itself. Initially this science was eminently empirical and was thus obtained a large amount of information that needed a theoretical interpretation that could put order in the matter.

First J. Balmer, J. Rydberg and W. Ritz investigated the regularity of the spectra of the same element and the qualitative and quantitative relationships between various spectra, deducing empirical laws such as Balmer’s law for the lines emitted by hydrogen in the visible range.

Later N. Bohr, with the help of E. Rutherford atom model and quantum theory, formulated a theory for emission spectra, within the more general theory of atom. As spectroscopy techniques improved, spectra were much more complex than initially believed and many new phenomena were offered to experimental investigation.

If the spectrum is of emission and occurs in the visible band, the analysis of radiation with a spectrograph gives the slit of the instrument an image colored from red to violet that can be presented in the form of thin lines spaced from each other, when the radiation consists of a few components with wavelengths very different from each other, or in the form of layers variously extended and colored consisting of radiation with wavelengths very close to each other; it is from this image that we deduce the above scheme. In general, whatever the frequency range of interest and whatever the instrument used for the experimental investigation, we can make a first distinction between line spectra, band spectra and continuous spectra.

Absorption theory

To perform a spectrophotometric analysis we measure the extent of absorption of a light radiation with a sample placed in front of a radiation source. To interpret the phenomena that occur it is necessary to know the characteristics of light sources and the structure of matter. The absorption of radiation causes an increase in the internal energy of the absorbing substance. This implies an excitation of the component particles (electrons, atoms, molecules, etc.), which produces characteristic phenomena for each substance. According to quantum mechanics the energy of the particles constituting the matter is quantized, that is it can take only certain discrete values.

In normal conditions a particle is in the minimum energy state. When a radiation strikes a particle, if the energy of photons is equal to the difference between the energy of the excited state of the particle and that of a fundamental state, the radiation is absorbed and the particle goes from fundamental state to excited state.

Since to each molecular system is associated a characteristic distribution of energy levels (electronic, vibrational, rotational) the absorption of a given radiation is a characteristic property of that system and not of others. Quantum mechanics allows to explain why the absorption of a given radiation is specific for each substance and gives rise to a characteristic absorption spectrum. It also, through the development of selection rules, allows to establish which transitions are prohibited and which are allowed.