The excitation of lines forbidden by the Laporte rule and corresponding to transitions between levels, all belonging to the ground configuration, was studied for the elements of the fifth group. The short-wave experimental technique developed permits the recognition of these multipole lines without any knowledge of the spectra investigated, from their behavior at the changeover point of the discharge. Neutral multipole spectra of Bi, Sb, As, and P (s2p3 configuration) were obtained with decreasing intensity in the order listed as well as singly ionized spectra of Bi and Sb (s2p2 configuration). Interesting changes in relative intensities of lines, in particular of the pair 2P3/2,½ → 4S3/2 were noted, and an anomalous effect for lines originating from the 2P3/20 level in bismuth observed. A high resolution study of lines in Sb and As gave previously unknown hf separations of levels 3P1, of Sb II, 2P½0 of As I and upper limits for those of the levels 2P3/20 and 4S3/20 of As I.
M. HULTS and S. MROZOWSKI, "Multipole Lines in Spectra of Elements from the Fifth Group," J. Opt. Soc. Am. 54, 855-857 (1964)