Isotope-shift measurement of the 423-nm transition in neutral Ca

We report on saturated absorption spectroscopy measurements of the $\left(4s^2\right){}^{1}S_0\to \left(4s4p\right){}^{1}P_1$ transition for the four most abundant even-mass isotopes in calcium. By referencing the laser locked to an ultralow expansion cavity and carefully investigating systematic errors, isotope shifts are determined with a precision below $100\mathrm{k}\mathrm{Hz}$, improving previously reported values by a factor of about 3. A King plot analysis, employing literature values of the 729-$\mathrm{n}\mathrm{m}$ transition in ${\mathrm{Ca}}^{+}$ ions, shows excellent linearity. The field and mass shift parameters are determined from King plots with other transitions.