Absolute frequency measurement of the 5s5p 1P1 - 5s5d 1D2 transition in strontium

We report on the absolute frequency determination of the 5s5p ${}^1{P}_1$ - 5s5d ${}^1{D}_2$ transition in atomic strontium, achieved through frequency comb-referenced laser-induced-fluorescence (LIF) spectroscopy. We excite the $5s^2$ ${}^1{S}_0$ - 5s5p ${}^1{P}_1$ transition using an on-resonance laser at $\approx$461 nm, and then measure the variation in the LIF signal while scanning the laser at $\approx$767 nm across the 5s5p ${}^1{P}_1$ - 5s5d ${}^1{D}_2$ transition. We determine the absolute frequency of $390599571.7\pm 0.4$ MHz, with an accuracy that surpasses the previous most accurate measurement by two orders of magnitude. This measurement technique can be readily applied for precision spectroscopy of high-lying states not only in strontium, but also in other atomic species.