Absolute frequency and isotope shift measurements of mercury S-P transition.

We report the measurement of the absolute frequencies of the 6s S-6s6p P transition (253.7 nm) and the relevant isotope shifts in five mercury isotopes  Hg,  Hg,  Hg,  Hg, and  Hg. The Doppler-free saturated absorption measurements were performed in an atomic vapour cell at room temperature with a four-harmonic generated (FHG) continuous-wave (cw) laser digitally locked to the atomic transition. It was referenced with a femtosecond optical frequency comb synchronized to the frequency of local representation of the International Atomic Time to provide traceability to the SI second by the 330 km-long stabilized fibre optical link. The transition frequencies and isotope shifts have been determined with an accuracy of a few hundred kHz, at least one order of magnitude better than any previous measurement. By making a King plot with the isotope shifts of 6s6p P-6s7s S transition (546 nm) we determined the accurate value of the ratio of the electronic field-shift parameters E/E and estimated the electronic field-shift term E.