We have observed the first laser-induced resonant transitions in antiprotonic helium atoms. These occur between metastable states and Auger dominated short lived states, and show that the anomalous longevity of antiprotons previously observed in helium media results from the formation of high-n high-l atomic states of p¯${\mathrm{He}}^{+}$. The observed transition with vacuum wavelength 597.259±0.002 nm is tentatively assigned to (n,l)=(39,35)→(38,34).

• Received 10 November 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.1180