Spectroscopy of the 7s-8s highly forbidden transition in laser-trapped francium

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The fundamental forces of nature are one of the fascinating subjects that can be studied at the ISAC radioactive beam facility at TRIUMF, Canada's national laboratory for nuclear and particle physics. In particular, the weak force, causing the beta-decay of nuclei, is unique in exhibiting the phenomenon of parity-violation: the mirror images of physical processes mediated by the weak force are not observed in nature. ISAC offers the possibility to study atomic parity-violation in francium (Fr), an element combining a very heavy nucleus (Z=87) with the simple, essentially hydrogenlike, valence structure of an alkali. Fr nuclei are the least stable among the first 103 elements in the periodic table, the longest-lived isotope decays in only 23 minutes. To make optimum use of the minute sample, the Fr atoms are slowed down with laser light and captured into a `magneto-optical' laser trap, suspended in free space at temperatures millions of times colder than room temperature. We have established a francium trapping facility at ISAC; in collaboration with researchers from the United States, Mexico, and China, we are now using this francium source to carry out a series of precision experiments using laser and microwave spectroscopy. In one of them, we will use the laser equipment requested in this application to excite an exceedingly faint electronic transition in francium, and study its properties in detail. The strength of this transition is very sensitive to relativistic effects in the francium atom and its measurement allows a comparison to state-of-the-art atomic theory. In addition, the 7s-8s transition in Fr is the basis for a possible, future atomic parity-violation experiment.