Control of He*-Li chemi-ionization

Ultracold mixtures of different atomic species are used to obtain dense samples of ultracold heteronuclear molecules which may feature long-range and anisotropic interactions. Such interactions allow for new physics and chemistry studies in a regime purely dominated by quantum effects. To achieve the co-trapping of ultracold atoms, reactive collisions must be efficiently suppressed.

As a first step towards co-trapping, we study the chemi-ionization of ultracold Li by metastable He (He*). For this, we combine a supersonic-beam source for He* with a magneto-optical trap for Li [1]. To distinguish in between the contributions of He(2³S₁) and He(2¹S₀) to the ionization rate, we deplete the He* population in the 2¹S₀ state using a novel laser-excitation scheme [2]. We also use laser-optical pumping to prepare both He(2³S₁) and Li(2²S_1/2) in selected magnetic sub-levels prior to the collision [3].

Here, we demonstrate the efficient control of He*-Li chemi-ionization at thermal energies using spin- and quantum-state preparation. Our results imply a strong suppression (enhancement) of chemi-ionization for non-spin-conserving (spin-conserving) reaction channels. These results are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the molecular reaction channels. Small deviations from the model are indicative for a violation of spin-conservation rules. The ionization rate also decreases when Li is laser-excited to the 2²P_1/2,3/2 states. This is due to the conservation of the projection of the total molecular orbital angular momentum along the internuclear axis [4].

[1] Grzesiak, J. et al., J. Chem. Phys. 150, 034201 (2019).

[2] Guan, J. et al., Phys. Rev. Appl. 11, 054037 (2019).

[3] Sixt, T. et al., Rev. Sci. Instrum. 92, 073203 (2021).

[4] Dulitz, K. et al., Phys. Rev. A 102, 022818 (2020).