Observation of Cold, Long-Lived Antiprotonic Helium Ions

Cold, two-body antiprotonic helium ions $\overline{p}{}^4\mathrm{H}\mathrm{e}^{2+}$ and $\overline{p}{}^3\mathrm{H}\mathrm{e}^{2+}$ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers $n_i=28–32$ and ${\ell }_i=n_i-1$ have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature $T\sim 10\mathrm{K}$ by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of $\overline{p}{}^3\mathrm{H}\mathrm{e}^{2+}$ against annihilation induced by collisions were shorter than those of $\overline{p}{}^4\mathrm{H}\mathrm{e}^{2+}$, and decreased for larger-$n_i$ states.

Figure 1

Portion of energy level diagram of $\overline{p}{\mathrm{H}\mathrm{e}}^{2+}$ ions and $\overline{p}{\mathrm{H}\mathrm{e}}^{+}$ atoms. Wavelengths of the laser transitions are indicated in nanometers The curved arrows indicate Auger transitions to ionic states, and the straight arrows radiative $\overline{p}{\mathrm{H}\mathrm{e}}^{2+}$ transitions.

Figure 2

Annihilation spike produced by inducing the $\overline{p}{}^4\mathrm{H}\mathrm{e}^{+}$ transition $(n,\ell )=(39,35)\to (38,34)$, measured at a high target density (a). A prolongation of the tail is observed at ultralow densities (b), indicating the formation of long-lived $\overline{p}{}^4\mathrm{H}\mathrm{e}^{2+}$ ions populating state $(n_i,{\ell }_i)=(32,31)$.

Figure 3

Decay rates of annihilation signals measured at various target densities. Data corresponding to $\overline{p}{}^4\mathrm{H}\mathrm{e}^{2+}$ states are indicated by filled circles, and those of $\overline{p}{}^3\mathrm{H}\mathrm{e}^{2+}$ by triangles.

Figure 4

Gradients of the decay rates per target density of four $\overline{p}{}^4\mathrm{H}\mathrm{e}^{2+}$ (indicated by filled circles) and $\overline{p}{}^3\mathrm{H}\mathrm{e}^{2+}$ (triangles) ionic states, as a function of the principal quantum number $n_i$.