Hydrogen-assisted laser-induced resonant transitions between metastable states of antiprotonic helium atoms

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 \htwo\ 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 \pbar\hep\ atoms. This was demonstrated by observing the transitions $(n,l)=(38,l)\rightarrow (39,l+1),\ l=35,36,37$ and $(n,l)=(37,l)\rightarrow (38,l+1),\ l=34,35,36$.