Figure 2: Gas cell versus gas jet spectra.

(a) Ionization scheme with vacuum wavelengths for the excitation and ionization steps along with the expected hyperfine splitting for the ^212–215Ac isotopes indicating (not to scale) the 12 hyperfine transitions grouped in six (I to VI) multiplets. (b) Measured spectra (black dots) of the hfs in the 6d 7s² transition from the ground state shown as the α-counts collected in 50 s versus the frequency detuning ν with respect to the value of the centre of gravity. The gas-cell data has been corrected for the pressure shift (see Methods section). The red (blue) curve shows the best fit of a 12-peak Voigt profile to the data from laser spectroscopy studies in the gas cell (gas jet). (c,d) Zoom in of the triplet (number IV) and doublet (number V) hyperfine transitions in ²¹⁴Ac and ²¹⁵Ac, respectively, for the typical average energy per pulse of 0.8 μJ of the excitation laser radiation. The colour code used to show the different components of the multiplets indicates the corresponding hyperfine transitions as seen in the excitation scheme in a. One sigma s.d. as statistical errors are reported in all the data points.