Measurement of the lifetimes of the lowest ${}^{3}P_1$ state of neutral Ba and Ra

The lifetimes of the lowest ${}^{3}P_1$ states of Ba and Ra were determined to be $1345\pm 14\mathrm{ns}$ and $422\pm 20\mathrm{ns}$, respectively, by measuring the exponential decay of fluorescence after illuminating a thermal atomic beam with pulses of laser light. In addition, the ${}^{1}S_0(F=1∕2)\text{−}{}^{3}P_1(F=3∕2)$ transition frequency in ${}^{225}\mathrm{Ra}$ was measured to be $13999.269\pm 0.001{\mathrm{cm}}^{-1}$ by referencing a nearby ${\mathrm{I}}_2$ transition.

Figure 1

Experimental layout geometry (not to scale). The laser beam is shown passing transversely across the atomic beam with linear polarization and the applied magnetic field, $B$, in the $+y$ direction. The laser beam diameter (1.0–4.5 cm) was larger than the diameter of the fluorescence collection region (0.3 cm). Additional measurements were made on barium with the laser beam directed in the $-y$ direction and linearly polarized in the $x$ direction. In the atomic energy diagrams, the relative branching ratios shown in the rectangles [10] for the dominant decays to dark states (dotted arrows) and the transition of interest (solid arrows).

Figure 2

Fluorescence detected from the ${}^{1}S_0(F=1∕2)\text{−}{}^{3}P_1(F=3∕2)$ transition in ${}^{225}\mathrm{Ra}$ is shown in (a). The frequency of nearby ${\mathrm{I}}_2$ transitions are shown in (b). During the experiments, the indicated ${\mathrm{I}}_2$ transition was used as a frequency reference for the probe beam.

Figure 3

Typical distribution of photon arrival times after shuttering the laser are shown in (a) for barium and in (d) for radium. In the “off resonance” data, the laser detuning was $\approx 3\mathrm{GHz}$ from the atomic transitions of interest. The background-subtracted data with fits to an exponential decay with a time-independent background are shown in (b) for barium and in (e) for radium. The fits give $1344\pm 3\mathrm{ns}$ and $422\pm 3\mathrm{ns}$ (statistical uncertainty only) for the lifetimes of the displayed barium and radium data, respectively. The residuals for the fit, normalized by the square root of the number of counts in each bin, are shown in (c) for barium and (f) for radium.