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The production, measurement, and analysis of spectra; an essential tool of astronomers, chemists, and physicists. All spectra and spectral lines arise from transitions between discrete energy states of matter, as a result of which photons of corresponding energy (and hence characteristic frequency or wavelength) are absorbed or emitted. From the energy levels thus determined, atomic and molecular may be studied. Moreover, by using the observed spectra as "fingerprints," spectroscopy may be a sensitive tool of chemical analysis.

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.

There are several more specialized kinds of spectroscopy. Raman spectroscopy (see Raman spectroscope), based on the effect discovered by C. V. Raman, scans the scattered light from an intense monochromatic beam. Some of the scattered light is at lower (and higher) frequencies than the incident light, corresponding to vibration/rotation transitions. The technique thus supplements infrared spectroscopy. Mössbauer spectroscopy, based on the Mössbauer effect, gives information on the electronic or chemical environments of nuclei; as does nuclear magnetic resonance spectroscopy, based on transitions between nuclear spin states in a strong magnetic field. Electron spin resonance spectroscopy is similarly based on electron spin transitions when there is an unpaired electron in an orbital, and so is used to study free radicals. The instrument used is a spectroscope, called a spectrograph if the spectrum is recorded photographically all at once, or a spectrometer if it is scanned by wavelength and calibrated from the instrument.

The advent of spectroscopy, in the 1860s, marked the birth of astrophysics as an observational science.

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