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

We report on saturated absorption spectroscopy measurements of the $(4s^2{)}^1{S}_0\to (4s4p{)}^1{P}_1$ 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 kHz, improving previously reported values by a factor of about five. A King plot analysis employing literature values of the 729-nm transition in Ca ions shows excellent linearity. The field and mass shift parameters are determined from King plots with other transitions.