Quenching of metastable states of antiprotonic helium atoms by collisions with H$_2$ 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 $n=39$.