orthohelium
(stress variable)
[f. ortho- + helium, as an antonym of the earlier par(a)helium.]
The form of helium whose spectrum exhibits a fine structure of triplets owing to the spins of the two orbital electrons being parallel.
| 1922 A. D. Udden tr. Bohr's 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. Ibid. 87 The metastable state..is the final stage of the process giving the orthohelium spectrum. 1961 Powell & Crasemann Quantum Mech. xii. 458 Spectroscopically, the singlet and triplet systems are independent of one another, and have been given the names parahelium (singlets) and orthohelium (triplets). The ground state of the orthohelium system..is therefore stable with respect to optical transitions of the usual kind, and has a correspondingly long lifetime. |