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An atomic or molecular system with an excess of negative charge. Negative ions, also called anions, are formed in attachment processes in which an additional electron is captured by an atom or molecule. Negative ions were first reported in the early days of mass spectrometry. It was soon learned that even a small concentration of such weakly bound, negatively charged systems had an appreciable effect on the electrical conductivity of gaseous discharges. Negative ions now play a major role in a number of areas of physics and chemistry involving weakly ionized gases and plasmas. Applications include accelerator technology, injection heating of thermonuclear plasmas, material processing, and the development of tailor-made gaseous dielectrics. In nature, negative ions are known to be present in tenuous plasmas such as those found in astrophysical and aeronomical environments. The absorption of radiation by negative hydrogen ions in the solar photosphere, for example, determines the Sun's spectral distribution. See Ion sources