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Kossin
Kalita, M. ., Behr, J. A., Gorelov, A. ., Pearson, M. R., DeHart, A. ., Gwinner, G. ., … Flambaum, V. V. (2018). Isotope shifts in the 7s→8s transition of francium: Measurements and comparison to ab initio theory. Physical Review A, 97. http://doi.org/10.1103/physreva.97.042507 (Original work published April 2018)