Precision Frequency Measurement of Visible Intercombination Lines of Strontium

We report the direct frequency measurement of the visible $5s^2{}^{1}S_0\mathrm{\text{−}}5s5p{}^{3}P_1$ intercombination line of strontium that is considered a possible candidate for a future optical-frequency standard. The frequency of a cavity-stabilized laser is locked to the saturated fluorescence in a thermal Sr atomic beam and is measured with an optical-frequency comb generator referenced to the SI second through a global positioning system signal. The ${}^{88}\mathrm{S}\mathrm{r}$ transition is measured to be at 434 829 121 311 (10) kHz. We measure also the ${}^{88}\mathrm{S}\mathrm{r}\mathrm{\text{−}}{}^{86}\mathrm{S}\mathrm{r}$ isotope shift to be 163 817.4 (0.2) kHz.