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Isogenous is a term occasionally used in spectroscopy that denotes when two or more spectroscopic series of transition from a common state. The word was first used in this context in T.J. Stone & R.F. Barrow Laser excited fluorescence spectra of gaseous , Canadian Journal of Physics. Vol. 53, p. 1976, October 1975. In this case, an excited state of gaseous was pumped by an Argon ion laser; the excited state then relaxed to two closely separated low lying states giving two series of emission lines with a common origin.