Accurate wavenumbers of the first resonance doublet of francium atoms and relativistic contribution to isotope shifts in the resonance doublets of francium and caesium atoms

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Abstract
The results of a new experiment using high-resolution Doppler-free laser spectroscopy are reported. Firstly, the wavenumbers of the D1 and D2 lines of Fr atoms have been accurately measured: sigma (D1)-12237.409 (2) cm-1 and sigma (D2)=13923.998 (2) cm-1. Secondly, the isotope shifts of the D1 and D2 lines have been compared for Fr and Cs atoms. The contribution of the p1/2 electron to the volume shift has been measured relative to that of the s1/2 electron as 4.75(25)*10-2 for Fr and 1.4(3)*10-2 for Cs. They are discussed in terms of semi-empirical calculations as well as relativistic Dirac-Fock ones.