Laser-probing measurements and calculations of lifetimes of the $5d{}^{2}D_{3∕2}$ and $5d{}^{2}D_{5∕2}$ metastable levels in $\mathrm{Ba}\mathrm{II}$

The two metastable levels $5d{}^{2}D_{3∕2}$ and $5d{}^{2}D_{5∕2}$ in $\mathrm{Ba}\mathrm{II}$ both show extremely long lifetimes of the order of several tens of seconds each. This has been found both by experiments and by theoretical predictions. The small transition probabilities associated with these two levels make them interesting and challenging for theoreticians as well as for experimentalists. Several calculations and measurements of these two lifetimes have been made previously but discrepancies between the results are present. This article presents values of $\tau =89.4\pm 15.6\mathrm{s}$ for the ${}^{2}D_{3∕2}$ level and $\tau =32.0\pm 4.6\mathrm{s}$ for the ${}^{2}D_{5∕2}$ level measured in a beam-laser experiment performed at the ion storage ring CRYRING. These values are supported by our new calculations resulting in $\tau =82.0\mathrm{s}$ for the ${}^{2}D_{3∕2}$ level and $\tau =31.6\mathrm{s}$ for the ${}^{2}D_{5∕2}$ level.

Figure 1

Level scheme for $\mathrm{Ba}\mathrm{II}$.

Figure 2

Unnormalized fluorescence measured by probing the ${}^{2}D_{5∕2}$ level at base pressure. Each time point represents one value of the delay time between ion injection and the probe pulse.

Figure 3

Unnormalized repopulation, measured by probing the ${}^{2}D_{5∕2}$ level at base pressure.

Figure 4

Particle normalization curve recorded while measuring the lifetime of the ${}^{2}D_{5∕2}$ level at high pressure. The plot shows the number of particles stored at a fixed time after injection. The time indicated on the $x$ axis is the corresponding time point in Figs. 2, 3, 5.

Figure 5

The ion beam current decay at base pressure recorded while measuring the ${}^{2}D_{5∕2}$ level.

Figure 6

Normalized fluorescence (empty circles), normalized repopulation (crosses), and the final lifetime curve (filled circles) for a measurement of ${}^{2}D_{5∕2}$ at base pressure.

Figure 7

Stern-Vollmer plot for the ${}^{2}D_{5∕2}$ level.

Figure 8

Stern-Vollmer plot for the ${}^{2}D_{3∕2}$ level.

Figure 9

Comparison between calculated and measured lifetimes for the ${}^{2}D_{3∕2}$ level.

Figure 10

Comparison between calculated and measured lifetimes for the ${}^{2}D_{5∕2}$ level.