Branch of analysis devoted to identifying elements and compounds and elucidating atomic and molecular structure by measuring the radiant energy absorbed or emitted by a substance at characteristic wavelengths of the electromagnetic spectrum (including gamma ray, X-ray, ultraviolet, visible light, infrared, microwave, and radio-frequency radiation) on excitation by an external energy source. The instruments used are spectroscopes (for direct visual observation) or spectrographs (for recording spectra). Experiments involve a light source, a prism or grating to form the spectrum, detectors (visual, photoelectric, radiometric, or photographic) for observing or recording its details, devices for measuring wavelengths and intensities, and interpretation of the measured quantities to identify chemicals or give clues to the structure of atoms and molecules. Helium, cesium, and rubidium were discovered in the mid-19th century by spectroscopy of the Sun's spectrum. Specialized techniques include Raman spectroscopy (see Chandrasekhara Venkata Raman), nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), dynamic reflectance spectroscopy, microwave and gamma ray spectroscopy, and electron spin resonance (ESR). Spectroscopy now also includes the study of particles (e.g., electrons, ions) that have been sorted or otherwise differentiated into a spectrum as a function of some property (such as energy or mass). Seealso mass spectrometry; spectrometer; spectrophotometry.
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