Suppression of Penning ionization by orbital angular momentum conservation

In our experiment, we study quantum-state-controlled Penning collisions between lithium atoms (Li) and metastable helium atoms (He*) to investigate new ways of controlling the outcome of Penning-ionizing collisions. For this, we have combined a supersonic beam source for He* atoms with a magneto-optical trap (MOT) for Li atoms. In order to get full quantum-state control of the reaction partners, the Li atoms are optically pumped into selected electronic hyperfine and magnetic substates. Additionally, we produce a pure beam of He(2³S₁) by the optical depletion of the He(2¹S₀) state using a novel excitation scheme.
In this contribution, we report on the efficient suppression of He*-Li Penning ionization by laser excitation of the Li atoms. The results illustrate that not only the electron spin, but also Λ - the projection of the total molecular orbital angular momentum along the internuclear axis - is conserved during the ionization process. Our findings suggest that Λ 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 He* and alkali atoms.