Isotope Shift in the Mo Spectrum

The isotope shift in the molybdenum spectrum was measured with a Fabry-Perot interferometer, using separated isotopes. For the line $16578K$, a definite minimum isotope shift of - 8.9 mK was observed between ${\mathrm{Mo}}^{96}$ and ${\mathrm{Mo}}^{97}$. The negative sign indicates that the direction of the shift is opposite to that predicted by the volume effect.