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

We report on the absolute frequency determination of the 5s5p ¹P₁ - 5s5d ¹D₂ transition in atomic strontium, achieved through frequency comb-referenced laser-induced-fluorescence (LIF) spectroscopy. We excite the 5s² ¹S₀ - 5s5p ¹P₁ transition using an on-resonance laser at ≈461 nm, and then measure the variation in the LIF signal while scanning the laser at ≈767 nm across the 5s5p ¹P₁ - 5s5d ¹D₂ transition. We determine the absolute frequency of 390599571.7±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.