Electronic Fine Structure in Helium Bands

Abstract

EXPERIMENTAL evidence for the triplet nature of the 2π3∏a and 2π3∏b terms of the helium molecule has recently been given by Prof. Mulliken and Dr. Monk with their grating spectrograms of the band 6400 A. (Phys. Rev., 34, 1530; 1929). The resolution into do ublets (two of the triplet components being too near to be resolved) is more clearly shown on our interference spectrogram of the same band (Fig. 1). The photograph was taken by projecting horizontal fringes formed by a 40-plate echelon (resolving power, c. 400,000) on to the fairly wide slit of a plane grating spectrograph, the latter giving a dispersion of 3.2 A. per mm. in the first order. The exposure was one hour. The neon (impurity) line 6402.246 A. served as the standard of measurements, as this line appears in two consecutive orders, their distance Δλmax being 0.64057 A.