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: 1S0 - 3P1 at 689 nm, 3P1 - 3S1 at 688 nm and 3P0 - 3S1 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 5s21S0 - 5s5p 3P0 transition of Sr87 at 698 nm and in a second step its direct observation. Frequency measurements are performed for Sr88 and Sr87, allowing the determination of 3P0, 3P1 and 3S1 isotope shifts, as well as the 3S1 hyperfine constants. (authors)