Optical frequency measurements with laser cooled Sr atoms

This paper reports the frequency measurement with an accuracy in the 100 kHz range of several optical transitions of atomic Sr. Measurements are performed with a frequency chain based on a femto-second laser and referenced to a hydrogen maser. They allowed the determination with a 70 kHz uncertainty of the energy of the 5s5p 3 Po state and the direct observation and frequency measurement of the doubly forbidden 1 S 0 rarr 3 P 0 transition at 698 nm. This line has a natural width of 10 - Hz and can be used for a new generation of optical frequency standards using atoms trapped in a light shift free dipole trap