Pseudopolar reaction of hydrogen atoms

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Abstract

Anomalous terms with both the electrons excited are known since a long time to occur in atoms with two valence electrons. In particular, the following are well known in numerous neutral or ionized atoms: 2p2p 3 P 012, 2p2p 1 D, 2p2p 1 S. According to a recent interpretation(1) the X term of the hydrogen molecule is formally analogous to these terms and should be precisely assigned to the configuration (2)21Σg(⋆). The analogy, however, breaks down in regard to the energies: whereas in atoms the frequency of the line 2p2p → 1s2p is of the same order of magnitude as the frequency of the line 1s2p → 1s2s, the X term is instead relatively deep, slightly above the normal term 12 1Σu (⋆⋆) with which it intensely combines in the infrared region; but the second one is in turn much higher than the ground state (1)21Σg (ca. 12 volts). The problem is then to justify theoretically the abnormal energy level of such an anomalous term and even to justify its existence. WEIZEL, in his attempt to solve the problem, provided a rather questionable evaluation based on dubious analogies. We have attacked the problem directly and our calculations seem to confirm WEIZEL’s assumption about the existence of a deep term (2)21Σg, although the theoretical equilibrium distance between the two nuclei is in better agreement with the term K (according to WEIZEL 23 1Σg)than with the term X.

Presented by the member O. M. Corbino at the meeting held on January 4, 1931.
Translated from “Rendiconti dell’Accademia dei Lincei”, vol. 13, 1931, pp. 58–61, by P. Radicati di Brozolo.
The lower indices g and u are shorthand notations for “gerade” and “ungerade”, which are the German for “even” and “odd”. (Note of the Editor in E. Amaldi, op. cit.)
In “Rendiconti dell’Accademia dei Lincei” it is erroneously printed Σn, here and in the following, in place of Σu. (Note of the Editor, see also E. Amaldi, op. cit.)