Isotope-selective trapping of doubly charged Yb ions

We report isotope-selective loading and trapping of doubly ionized ytterbium into an rf quadrupole trap. Isotopically pure clouds of ${}^{174}\mathrm{Yb}$${}^{+}$ ions were first loaded into the rf trap via a multistage photoionization process. The Yb${}^{2+}$ ions were then produced by electron impact ionization of the trapped Yb${}^{+}$ ions. The Yb${}^{2+}$ ions were subsequently detected by rf excitation of their secular motion in the trap, which led to sympathetic heating and changes in the fluorescence of the laser-cooled Yb${}^{+}$ ions. The presence of doubly charged Yb ions was further verified by the appearance of a dark band in the center of Yb${}^{+}$ ion cloud after electron impact ionization. We discuss the possible formation of Yb${}^{2+}$H and similar compounds and the schemes for the direct optical detection of the Yb${}^{2+}$ ions.

Figure 1

Standard electrode configuration used in linear rf traps. An rf potential, $V$, and in some instances, a dc potential, $U$, are applied to two diagonally opposed rods, with the remaining two rods connected to ground. In these experiments, a low-amplitude drive signal, $V_D$, is applied to the ground rods to drive the motion of the ions in the trap, as described in subsequent sections of this article. Also shown in this figure are the endcap cylinders that provide axial confinement.

Figure 2

Dependence of the secular frequency of singly charged Yb ions on endcap potential.

Figure 3

Relevant energy levels for optical detection of Yb${}^{+}$.

Figure 4

Scan of Yb${}^{+}$ secular frequency.

Figure 5

(a) Results of sweeps of the secular frequency drive rf on mixed ion samples. (b) Linear fit results for the higher (circles) and lower (squares) frequency resonances are ${\omega }_{\sec }=1.1544(19)-2.6(2)U_{\mathrm{ec}}$ and ${\omega }_{\sec }=1.1544(5)-4.88(6)U_{\mathrm{ec}}$, respectively.

Figure 6

A mixed cloud of singly and doubly charged ions. The dark band at the center of the cloud is consistent with segregation of the more tightly bound, doubly charged ions.

Figure 7

A partial energy-level diagram for Yb${}^{2+}$. There exist several levels below the upper state of the cooling transition to which the ion can decay. Estimated lifetimes for the decay into and out of these levels is also shown. A Bates-Damgaard approximation was used to estimate the electric quadrupole transition rates.