Mass Ratio of the Carbon Isotopes from the Spectrum of CN

With a source containing carbon enriched about ten times in C¹³, 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 C¹³{N}¹⁴$, as well as of ${C}¹²N¹⁴. The vibrational constants of the normal states of both molecules are accurately determined, and give a value of the isotope mass coefficient ρ=ωₑⁱωₑ of 0.97898±{}0.00002, corresponding to a mass for C¹³ 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.