Lifetime of the bound excited level in ${\mathrm{Ni}}^{-}$

The intrinsic lifetime of the upper level in the bound-bound $3d^{9}4s^2$ ${}^2{D}_{3/2} \to 3d^{9}4s^2$ ${}^2{D}_{5/2}$ radiative transition in ${\text{Ni}}^{-}$ was measured to be $15.1\pm 0.4\mathrm{s}$. The experiment was performed at cryogenic temperatures in one of the ion-beam storage rings of the Double ElectroStatic Ion Ring ExpEriment facility at Stockholm University. The storage lifetime of the ${\text{Ni}}^{-}$ ion beam was measured to be close to 5 min at a ring temperature of 13 K.

Figure 1

Schematic energy level diagram for the bound states of ${\text{Ni}}^{-}$. The vertical scale indicates energies with respect to the ground state of the neutral Ni atom. In the present work we measure the spontaneous decay rate ${\mathrm{\Gamma }}_{\text{sp}}$ of the (radiative) ${}^2{D}_{3/2} \to {}^2{D}_{5/2}$ transition.

Figure 2

A schematic of the ion storage ring DESIREE. ${\text{Ni}}^{-}$ ions (10 keV) are injected and stored in the ring. A chopped laser beam crosses the ion beam at right angles to yield neutral Ni atoms through electron detachment processes. The neutral atoms are detected by a microchannel plate (MCP).

Figure 3

Neutral yield for ${\text{Ni}}^{-}$ photodetachment at $1075\mathrm{nm}$ as a function of time. The full line is a fit to experimental data as described in the text.

Figure 4

Yields of neutral Ni atoms due to photodetachment of stored ${\text{Ni}}^{-}$ ions as functions of time. The upper curves in each panel are for laser probing at 910 nm (detachment of the $J=5/2$ and $J=3/2$ levels) while the lower curves are for probing of only the $J=3/2$ level at $\lambda =1075\mathrm{nm}$. The upper and lower panels show data for ring temperatures of 13 and 19 K, respectively. The full curves are obtained from the fitting procedure described in the text.

Figure 5

Stern-Vollmer plot of the effective decay rate of the ${}^2{D}_{3/2}$ level plotted as a function of the effective decay rate of the ${}^2{D}_{5/2}$ level (the ground state). The line is a fit to the data, and its intercept with the vertical axis (${\mathrm{\Gamma }}_{5/2}=0$) gives the spontaneous decay rate ${\mathrm{\Gamma }}_{\text{sp}}$ as collisions with the residual gas dominate the beam losses.