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Expulsion of an activated helium atom ([He.sup.*]) can lead to two conditions, depending on whether the electrons within this species are antiparallel (parahelium) or parallel (orthohelium).
Likewise, if the ground state electron is 'spin up', the excited electron must be 'spin down' to enable the transition, He* [right arrow] He + h v, again producing the identical He I lines from singlet state parahelium.