High Resolution Laser Spectroscopy of Radioactive Atoms

Abstract

Until very recently no optical lines of the francium atom were known. Using basically the same techniques which had enabled us to study long series of radioactive isotopes of alkali atoms by means of the high resolution laser spectroscopy /1/, we have undertaken the systematic study of the spectral characteristics of the francium atom. Francium atoms are provided by the ISOLDE facility at CERN and formed as an atomic beam. Illuminated at a right angle by a CW single mode tunable laser, the francium atoms may undergo op tical resonances which induce an optical pumping in the two hyperfine sub levels of the ground state ; this optical pumping can in turn be detected magnetically through a six-pole magnet analyser /2/. In a first series of experiments we have been able to detect the two first resonance lines connec ting the 7s ground state to the 7p excited states, and to measure accurate ly the corresponding wavelengths which lie in the red and near infrared re gion /3/, and also to detect the second resonance lines connecting the ground state to the 8p excited states which lie in the blue region /4/. In a second series of experiments, we analyzed the hyperfine structure and iso tope shifts of 16 isotopes of francium, and we obtained the results which are schematically reported in the figure 1. A careful study of the isotope shift results has shown us an unexpected inversion of the so-called “odd even staggering” effect, as can be observed in the figure 2 for those iso topes having a neutron number N located between 133 and 137.