Accurate spectroscopy of Sr atoms

We report the frequency measurement with an accuracy in the 100 kHz range of several optical transitions of atomic Sr: S-1(0)- P-3(1) at 689 nm, P-3(1)- S-3(1) at 688 nm and P-3(0)- S-3(1) at 679 nm. Measurements are performed with a frequency chain based on a femtosecond laser referenced to primary frequency standards. They allowed the indirect determination with a 70 kHz uncertainty of the frequency of the doubly forbidden 5s(2) S-1(0) - 5s5p P-3(0) transition of Sr-87 at 698 nm and in a second step its direct observation. Frequency measurements are performed for Sr-88 and Sr-87, allowing the determination of P-3(0), P-3(1) and S-3(1) isotope shifts, as well as the S-3(1) hyperfine constants.