Laser resonance transitions between normally metastable states of antiprotonic helium atoms were observed making use of state dependent quenching effects caused by small admixtures of ${\mathrm{H}}_2$ molecules. By selectively shortening the lifetimes of states with higher principal quantum number $n$ as compared to those of lower $n$, this method for the first time provides access to all initially populated metastable states of $\overline{p}{\mathrm{He}}^{+}$ atoms. This was demonstrated by observing the transitions $(n,l)=(38,l)\to (39,l+1)$, $l=35,36,37\mathrm{}$ and $(n,l)=(37,l)\to (38,l+1)$, $l=34,35,36\mathrm{}$.

• Received 21 October 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1671