Some atoms fail to absorb radiation even when the energy of the photons corresponds to the difference between two atomic levels. This phenomenon was discovered in 1961 by Ugo Fano of the University of Chicago, who explained it in terms of destructive interference: In the atoms that show this effect, there are two pathways leading to the same excited state, and their sum determines the probability for light to be absorbed. When these amplitudes interfere destructively, no photons are absorbed. One example is the transition from the ground state of helium to the 2s2p state, which lies in the continuum. The atom may be photoionized directly, or it may pass to the continuum via the discrete, autoionizing 2s2p state. As a result there is a window in the absorptionspectrum for helium.
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