Lifetime of the metastable ${}^{2}P_{1/2}$ state of F-like ${\mathrm{Ar}}^{9+}$ isolated in a compact Penning trap

Multiply ionized atoms of a single charge state can be captured at low energy in a compact Penning trap, allowing the measurement of the radiative lifetime of an atomic state that decays via weakly allowed transitions. Such a measurement is reported here for the radiative decay lifetime of the metastable ${}^{2}P_{1/2}$ state in fluorinelike ${\mathrm{Ar}}^{9+}$, wherein the $n=2$ shell has only one vacancy (hole) in a $2p$ orbital. Highly ionized atoms are extracted from an electron beam ion trap and guided into the ion capture apparatus. Upon capturing the ${\mathrm{Ar}}^{9+}$ ions, light from the magnetic-dipole transition is detected with a photomultiplier tube and counted with a multichannel scaler. Using this technique, the radiative lifetime of the metastable ${}^{2}P_{1/2}$ state in ${\mathrm{Ar}}^{9+}1s^{2}2s^{2}2p^5$ is measured to be $\tau =9.31\pm 0.08$ ms, consistent with a previous intra-EBIT measurement.

Figure 1

Schematic diagram for the detection of radiative decay from ions captured in a unitary Penning trap. Ion bunches are periodically injected into the trap to build up statistics. A small aspheric lens in the ring electrode collects photons emitted by stored ions. A set of lenses and a filter centered at 550 nm allows a 40 nm band of light to be relayed to a photon detector. After each measurement cycle, the trapped ions are counted by ejection to a time-of-flight (TOF) detector. (Inset) Optical transfer function simulation for the absolute number of detected photons per data acquisition cycle. The calculations are performed in a coordinate system which is aligned with the optical axis.

Figure 2

Measured radiative decay lifetime of the ${}^{2}P_{1/2}$ state of ${\mathrm{Ar}}^{9+}$ for a base pressure of $8.0\times {10}^{-8}$ Pa. Top: observed photon counts (black) as a function of time after ion capture along with a fit (red) described by Eq. (1). Bottom: deviations from the best-fit function, plotting normalized residuals (${\mathrm{\Delta }}_i/{\sigma }_{\mathrm{\Delta }}$), where ${\mathrm{\Delta }}_i$ is the difference between the exponential fit result $N\left(t_i\right)$ and the measured photon data $N_i$ and ${\sigma }_{\mathrm{\Delta }}$ is the global standard deviation of the residuals. For this data set ${\chi }^2/\nu =1.05$.

Figure 3

Stern-Volmer analysis of the ${}^{2}P_{1/2}$ radiative decay rate in ${\mathrm{Ar}}^{9+}$. An extrapolation of the measured decay rate as $\text{P}\to 0$ approaches the ideal vacuum limit.

Figure 4

Comparison of calculations and measurements of the radiative decay lifetime of the ${\mathrm{Ar}}^{9+}{}^{2}P_{1/2}$ level. Yellow-filled triangles represent calculated values as reported; red-filled squares correspond to wavelength-adjusted results. The error bars of each measurement (blue circle) indicate one standard error. The average of the adjusted theoretical lifetime calculations is shown in the solid green line, with the standard deviation of the predictions denoted by the green dashed lines.