Spectra of Isotopes

Abstract

THE quantum theory of line spectra developed by Bohr has been most successful in explaining the spectrum of hydrogen and helium, and by a further hypothesis the spectra of the alkali metals. By asserting that elliptic orbits are possible, as well as circular orbits, Sommerfeld succeeded in explaining the Stark and Zeeman effects and the fine structure of spectral lines. It is significant that Bohr's equation for the frequency of the spectral series also explains the difference between the series spectra of isotopes of the same element. The experiments of Aronberg and Merton on the structure of 4058 Å. of isotopes of lead showed, however, a remarkable discrepancy between the shift predicted by the theory and that actually observed. Similarly Merton's experiments on the line 6708 of lithium showed that the line consisted of two components 0·151 Å. apart, while the theoretical shift was 0·087 Å. The quantum theory is unable to account for this large separation observed.