Mass Ratio of the Carbon Isotopes from the Spectrum of CN

With a source containing carbon enriched about ten times in ${\mathrm{C}}^{13}$, the violet CN bands have been photographed with a dispersion of 0.63A/mm. Measurements are given of the lines of low rotational quantum number in the 0,0, 0,1 and 0,2 bands of ${\mathrm{C}}^{13}$ ${\mathrm{N}}^{14}$, as well as of ${\mathrm{C}}^{12}$ ${\mathrm{N}}^{14}$. The vibrational constants of the normal states of both molecules are accurately determined, and give a value of the isotope mass coefficient $\rho =\frac{{{\omega }_e}^i}{{\omega }_e}$ of 0.97898±0.00002, corresponding to a mass for ${\mathrm{C}}^{13}$ of 13.0088. This is in essential agreement with the mass-spectrograph value, and it is shown that the finer corrections to the isotope effect are negligible in this case.