Frequency measurements of $5s5p^{3}$P$_{0}\to5s6d^{3}$D$_{1}$ and observation of nonlinearities in King plot with Sr

We report the first precision measurement of the absolute frequency of $5s5p^{3}$P$_{0}\to5s6d^{3}$D$_{1}$ for all four stable Sr isotopes with an accuracy of $\sim$25 kHz employing repumping induced spectroscopy. By combining the isotope shifts of this transition with the existing measurement data on the intercombination line, the King plot is established which reveals a deviation from the linearity at the 5.2$\sigma$ level.