High-Precision Measurement of the Laser-Trapping Frequencies for ${}^{209,210,211}$Fr Atoms

We present the accurate measurement of the frequency of the $7S-7P$ laser-trapping transition for three francium isotopes. Our approach is based on an interferometric comparison to deduce the unknown laser frequency from a secondary laser frequency-standard. After careful investigation of systematics, with samples of about 100 atoms the final accuracy reaches 8 MHz, an order of magnitude better than the best previous measurement for ${}^{210}$Fr, and opens the way to improved tests of the theoretical computation of Fr atomic structure.