Precision frequency measurement of visible intercombination lines of strontium

We report the direct frequency measurement of the visible 5s$^2$ $^1$S$_0$-5s 5p$^3$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 GPS signal. The $^{88}$Sr transition is measured to be at 434 829 121 311 (10) kHz. We measure also the $^{88}$Sr-$^{86}$Sr isotope shift to be 163 817.4 (0.2) kHz.