Radiative lifetimes of the bound excited states of ${\mathrm{Pt}}^{-}$

The intrinsic radiative lifetimes of the $5d^{10}6s^{}{}^{2}S_{1/2}$ and $5d^{9}6s^2 {}^{2}D_{3/2}$ bound excited states in the platinum anion ${\text{Pt}}^{-}$ have been studied at cryogenic temperatures at the Double ElectroStatic Ion Ring Experiment (DESIREE) facility at Stockholm University. The intrinsic lifetime of the higher-lying $5d^{10}6s^{} {}^{2}S_{1/2}$ state was measured to be $2.54\pm 0.10\mathrm{s}$, while only a lifetime in the range of 50–200 ms could be estimated for the $5d^{9}6s^2 {}^{2}D_{3/2}$ fine-structure level. The storage lifetime of the ${\text{Pt}}^{-}$ ion beam was measured to be a little over 15 min at a ring temperature of $13\mathrm{K}$. The present study reports the lifetime of an atomic negative ion in an excited bound state with an electron configuration different from that of the ground state.

Figure 1

Energy-level diagram of the three bound states of ${\text{Pt}}^{-}$. The vertical scale indicates total energies with respect to the ground state of the neutral Pt atom. Magnetic dipole ($M1$) and electric quadrupole ($E2$) transitions are the lowest-order transitions possible between the ground and excited states of ${\text{Pt}}^{-}$.

Figure 2

Schematic of one of the ion storage rings of DESIREE. $10\mathrm{keV}$ beams of ${\text{Pt}}^{-}$ are injected and stored in the ring. The output of a continuous wave (cw) Ti:sapphire laser propagates along the ion-beam direction in one of the straight sections of the ring and can be overlapped with the stored ions in a collinear geometry, while the output of a pulsed optical parametric oscillator (OPO) can be crossed with the ion beam at ${90}^{\circ }$ in the other straight section. Neutral Pt atoms produced through photodetachment in the collinear interaction region are detected by a resistive anode encoder (RAE) detector, while those produced in the crossed interaction region are detected by an imaging detector.

Figure 3

Mass scan of ${\text{Pt}}^{-}$ ion beam in the range of 190–199 u. The ${}^{194}\mathrm{Pt}^{-}$ isotope was used throughout the experiment to avoid contamination from other anionic species that share the same mass as the 195, 196, and 198 u isotopes of Pt.

Figure 4

Decay curves of the ${\text{Pt}}^{-}$ beam as a function of time at the base storage ring temperature of $13\mathrm{K}$. The top plot shows the decay signal from only the $5d^{10}6s^{} {}^{2}S_{1/2}$ state, while the bottom plot contains contributions from all three states of ${\text{Pt}}^{-}$. Note the difference in time scales between the top and bottom plots.

Figure 5

Decay curve and residual signal of the ${\text{Pt}}^{-}$ beam as a function of time at a Ti:sapphire wavelength of $750\mathrm{nm}$. The right vertical axis shows the original neutral counts of the decay curve ($△$) along with the exponential function fitted to the 15-s measurement window data (dashed line), while the left vertical axis shows the residual neutral counts after subtraction of the long-decay component corresponding to decay from the $5d^{10}6s^{} {}^{2}S_{1/2}$ excited state ($•$).