The nuclear spin of arsenic

No fine structure has yet been recorded in any of the lines of the spectra of arsenic. The present paper gives an account of the fine structures of the majority of the visible lines of As II. This spectrum is very rich in strong lines and has been observed here in the region λλ 6400-4300 with high resolving power. The gross structure multiplet analysis of As II has been made by K. R. Rao, and the fine structure observations recorded here support this analysis. The terms expected and found in this spectrum are shown in Table I. All lines involving the 4s 2 . 4p 2 configuration lie in the deep ultra-violet region; therefore this configuration will be disregarded in the present investigation. According to the vector coupling scheme of White and Ritsehl it will be expected that the 4s 2 . 4p . 5s configuration will show wide hyperfine structure separations, since it involves an unpaired penetratings electron; this is actually verified here with some modification. As the 4s 2 . 4p . 5p configuration has no penetratings electron, only narrow structures are to be expected, but this, however, is not observed, since structures occur in this configuration which are of the same order as those found in the previous case. This has important theoretical significance and will be discussed later. The intervals in the 4s 2 . 4p . 5s triplet terms which are3 P0 —3 P1 = 397 cm.-1 and3 P1 —3 P2 = 2382 cm.-1 show that the electron coupling is by no means pure (LS) since the interval ratio deviates widely from the Landé interval rule. In the 4s 2 . 4p . 5p and in the 4s 2 . 4p 2 terms the deviations are much less marked. This incomplete (LS) coupling affects the structure and will be considered later.