Observation of cold, long-lived antiprotonic helium ions -: art. no. 063401

Cold, two-body antiprotonic helium ions (p) over bar He-4(2+) and (p) over bar He-3(2+) with 100-ns-scale lifetimes, occupying circular states with the quantum numbers n(i)=28-32 and l(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 Tsimilar to10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of (p) over bar He-3(2+) against annihilation induced by collisions were shorter than those of (p) over bar He-4(2+), and decreased for larger-n(i) states.