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 $(n,l)=(37,l)\rightarrow (38,l+1),\ l=34,35,36$

Quenching of metastable states of antiprotonic helium atoms by collisions with H2_2 molecules

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of \mbox{H$_2$}\ to the target helium gas. With this method of ``\mbox{H$_2$}-assisted inverse resonances'' the decay rates of the states $(n,l)=(39,l),\ l=36,37,38$ and $(38,l),\ l=35,36,37$ of \mbox{$\overline{\mathrm{p}}$}\mbox{He$^{+}$}\ were determined as a function of the \mbox{H$_2$}\ admixture. The quenching cross sections at 30~K deduced therefrom for the states with $n=39$ were found to be of the order of the geometrical cross section for \mbox{$\overline{\mathrm{p}}$}\mbox{He$^{+}$}-\mbox{H$_2$}\ collisions ($2\cdot 10^{-15}$~cm$^2$), with a moderate decrease with increasing $l$. Within a given cascade with constant $v=n-l-1$, the quenching cross sections for states with $n=38$ are smaller by a factor of 4--6 than those for states with $n=39$