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. 
S. Sanguinetti, R. Calabrese, L. Corradi, A. Dainelli, A. Khanbekyan,   E. Mariotti, C. de Mauro, P. Minguzzi, L. Moi, G. St...