First Observation of the Blue Optical Lines of Francium

We report here the first wave-length measurements in the second resonance doublet of francium, D 1'(7 s 2 S 1/2 -8 p 2 P 1/2 ) and D 2'(7 s 2 S 1/2 -8 p 2 P 3/2 ), carried out by collinear fast-beam laser spectroscopy. The transition wave numbers are D 1' = 23112.9603(50) cm -1 and D 2' = 23658.3058(40) cm -1 , corresponding to a 8 p fine-structure splitting of δ W 8 p = 545.3454(70) cm -1 . In addition the hyperfine structure in both lines and the isotope shift in the D 2' line for the isotopes 212,213,220,221 Fr have been measured. The results are discussed with special emphasis on the analysis of the atomic structure in the heaviest alkali element and compared with theoretical predictions, as well as the only earlier spectroscopic data on francium obtained in the first resonance doublet.