The line spectrum of mercury in absorption. Occurrence of the “forbidden” line λ 2270 1 1 S 0 — 1 3 P 2

Abstract

The mercury line 1 1 S 0 — 1 3 P 2 , which is forbidden by the selection rule for inner quantum numbers, has none the less been observed to occur in emission under certain special conditions. It has not been observed in the spectrum of the ordinary mercury lamp, but Takamine and Fukuda* have observed it in the spectrum of a “branched arc ” in which a small fraction of the current of a mercury vacuum arc was taken off from a supplementary anode. The line was very faint compared with the resonance line 1 1 S 0 — 1 3 P 1 or other strong lines of the mercury spectrum. I have myself observed it in the excited vapour (afterglow) from a low-current discharge of 1/10 milliampere.