Spin-state-controlled chemi-ionization reactions between metastable helium atoms and ground-state lithium atoms

We demonstrate the control of ⁴He(2³S₁)-⁷Li(2²S_1/2) chemi-ionization reactions by all-optical electron-spin-state preparation of both atomic species prior to the collision process. Our results demonstrate that chemi-ionization is strongly suppressed (enhanced) for non-spin-conserving (spin-conserving) collisions at thermal energies. These findings are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the quasi-molecular states. Small deviations from the model indicate the contribution of the ⁴Σ⁺ channel to the reaction rate, which is in violation of spin conservation.