Suppression of Penning ionization by orbital angular momentum conservation

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.