Variations in the Photo-Electric Sensitivity of Platinum.

in the case of the singlet P states of the atom. That this sub-hydrogenic character appears in the molecule for a D instead of for a P term as in the atom, may prove instructive in determining the cause of this hitherto unexplained type of deviation from the Balmer formula. The various electron levels and transitions now known for the He2 molecule are shown in the form of an energy-level diagram in figure 1. A comparison of molecular and atomic terms, as given in the table, shows distinctly, in spite of certain irregularities, that the "main" series and related terms parallel the "doublet" terms of the atom-which presumably are really triplet terms"-while the "second" series terms parallel the singlet terms of the atom. These relations form the basis of the suggestion in the second paragraph as to the formation of helium molecules of two types from the two types of excited atoms. But in both cases, as already noted, the molecular terms, in the bands hitherto studied, appear to be of a singlet .type. The possibility that any of these terms belong to systems of higher multiplicity can almost certainly be excluded by various considerations; this is true even though the triplet or other separations were unresolved-but appreciable separations would seem probable, at least for the 2P level. The future discovery of helium bands involving triplet and quintet electronic terms is, however, likely. 1 R. S. Mulliken, Physic. Rev., 25, 290-1 and 26, 561 (1925). 2 R. T. Birge, Nature, 1926 (in press). 3 A. Fowler, Proc. Roy. Soc., A, 91, 208 (1915). 4 W. E. Curtis and R. G. Long, Proc. Roy. Soc., A, 108, 513 (1925). W. E. Curtis, Proc. Roy. Soc., A, 101, 38 (1922). 6 A. Kratzer, Zeit. Physik., 16, 353 (1923). 7R. Mecke, Physik. Zeit., 26, 227 (1925); cf. also Zeit. Physik., 31, 709 (1925). 8 R. Mecke, Zeit. Physik., 32, 823 (1925). 9 R. S. Mulliken, Physic. Rev., 25, 279 (1925). '0 W. Heisenberg, Zeit. Physik., 33, 879 (1925). 1' J. C. Slater, these PROCZEDINGs, Dec., 1925.