Precision frequency measurement of visible intercombination lines of strontium

We report the direct frequency measurement of the visible 5s(2) S-1(0)-5s5p P-3(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 Sr-88 transition is measured to be at 434 829 121 311 (10) kHz. We measure also the Sr-88-Sr-86 isotope shift to be 163 817.4 (0.2) kHz.