Isotope Shifts of Radium Monofluoride Molecules

Isotope shifts of ${}^{223–226,228}\mathrm{Ra}{}^{19}\mathrm{F}$ were measured for different vibrational levels in the electronic transition $A^2{\mathrm{\Pi }}_{1/2}\leftarrow X^2{\mathrm{\Sigma }}^{+}$. The observed isotope shifts demonstrate the particularly high sensitivity of radium monofluoride to nuclear size effects, offering a stringent test of models describing the electronic density within the radium nucleus. Ab initio quantum chemical calculations are in excellent agreement with experimental observations. These results highlight some of the unique opportunities that short-lived molecules could offer in nuclear structure and in fundamental symmetry studies.

Figure 1

Isovibrational spectra of the $A^2{\mathrm{\Pi }}_{1/2}\leftarrow X^2{\mathrm{\Sigma }}^{+}$ electronic transition. (a) Spectra of the ${}^{224}\mathrm{Ra}{}^{19}\mathrm{F}$ (top) and ${}^{228}\mathrm{Ra}{}^{19}\mathrm{F}$ (bottom) isotopologues. The wave number axis was shifted such that the origin coincides with the center of the $0\leftarrow 0$ transition of the reference ${}^{226}\mathrm{Ra}{}^{19}\mathrm{F}$ isotopologue. Moving in the negative direction on the wave number axis, the four peaks correspond to the $0\leftarrow 0$, $1\leftarrow 1$, $2\leftarrow 2$, and $3\leftarrow 3$ vibrational transitions. The histograms represent the experimental data, while the continuous black curves depict the best fits for each isotopologue. The vertical bands mark the central values of the transitions, while the width of each band corresponds to the associated uncertainty. (b) $0\leftarrow 0$ peaks of the five isotopologues ${}^{223–226,228}\mathrm{Ra}{}^{19}\mathrm{F}$. The plot at the bottom marks the position of the center of each peak relative to the one corresponding to ${}^{226}\mathrm{Ra}{}^{19}\mathrm{F}$. The width of each vertical band shows the uncertainty on the given central value.

Figure 2

Isotope shifts as a function of the changes in the charge radius of the five analyzed RaF isotopologues with respect to the reference ${}^{226}\mathrm{Ra}{}^{19}\mathrm{F}$ molecule. Each color corresponds to one of the four investigated transitions and the black line shows the best linear fit to the $0\leftarrow 0$ transition. The inset in the bottom left shows the values of the slope of this fit, $F$, corresponding to experimental data (black) and theoretical predictions (cyan for FSCCSD method and magenta for FSCCSDc). See main text for details.