Mass Ratio of the Lithium Isotopes from the Spectrum ofLi2

It is shown from a study of the ${}_{}{}^1{}_{}{}^{}\Sigma \to {}_{}{}^1{}_{}{}^{}\Sigma {\mathrm{Li}}_2$ band system that the relative masses of the two isotopes of Li as calculated from the observed isotope effect in each of the two lowest states of ${\mathrm{Li}}_2$ is in excellent agreement with the mass ratio obtained by other precise methods. Various methods of treating the data lead consistently to the conclusion that the mass-coefficient $\rho$ is 1.04077±0.00004 and that the mass ratio is 1.16640±0.00016. This result disagrees with that of McKellar and Jenkins who, from a less extensive study of the ${}_{}{}^1{}_{}{}^{}\Pi \to {}_{}{}^1{}_{}{}^{}\Sigma$ system of ${\mathrm{Li}}_2$, concluded that the mass ratio was 1.1678±0.0008, a result which cast suspicion upon the band spectrum method of determining relative atomic masses. There has also been found a small but definite electronic isotope shift: $\nu _e^{}{}_{}{}^i-{\nu }_e=-0.064\mathrm{}\pm 0.010$ ${\mathrm{cm}}^{-1\mathrm{}}$.