paraheliumn.
/ˌparəˈhiːlɪəm/, U.S. /ˌpɛrəˈhiliəm/ The form of helium whose spectrum does not exhibit the fine structure of triplets present in the spectrum of orthohelium, owing to the spins of the two orbital electrons being antiparallel. Cf. orthohelium n.
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parahelium1896
orthohelium1922
1896 C. Runge & F. Paschen in Astrophysical Jrnl. 3 18 We come to the conclusion that clèveite gas consists of two elements, one corresponding to the second and third series (single lines), and the other corresponding to the fifth and sixth series (double lines..). If this is true, the name Helium should be given only to the second element... The first element Professor Stoney has proposed to call Parhelium.
1922 A. D. Udden tr. N. Bohr Theory of Spectra iii. iii. 86 Helium was at first assumed to be a mixture of two different gases, ‘orthohelium’ and ‘parhelium’, but now we know that the two spectra simply mean that the binding of the second electron can occur in two different ways.
1927 Proc. Royal Soc. A. 115 484 The coupling forces between the electronic spins and the orbits which give rise to the very weak transitions between ortho- and para-helium.
1974 G. Reece tr. F. Hund Hist. Quantum Theory iv. 60 Runge and Paschen were able to add series for O, S, Se and He in 1895–7. They discovered the two spectral systems of orthohelium and parahelium in the process.
1994 Physical Rev. D. 49 3234 It is quite interesting that the theoretical predictions of energy levels of parahelium in the S state are lower than the energy levels of orthohelium.
This entry has been updated (OED Third Edition, June 2005; most recently modified version published online March 2022).
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