  The efficient suppression of Penning-ionizing collisions is a stringent
requirement to achieve quantum degeneracy in metastable rare gases. Thus far,
such loss processes have been avoided by electron-spin polarizing the collision
partners. Here, we report on the efficient suppression of Penning ionization in
collisions between metastable He and laser-excited Li atoms. The results
illustrate that not only the electron spin, but also $\Lambda$ - the projection
of the total molecular orbital angular momentum along the internuclear axis -
is conserved during the ionization process. Our findings suggest that $\Lambda$
conservation can be used as a more general means of reaction control, for
example, to improve schemes for the simultaneous laser cooling and trapping of
metastable He and alkali atoms.
