Résumé : The 23S1 state of helium is the longest-lived neutral atomic state and the most energetic metastable state of an atom. These characteristics render this species an important source of stored energy in ionospheric and discharge plasmas. In our experiments, we study quantum-state-controlled Penning collisions between lithium atoms and metastable helium atoms at various collision energies, e.g., to study the influence of electron-spin polarization on the reaction rate and to observe quantum resonance effects at low collision energies. For this, we use an experimental apparatus which consists of a discharge source for the production of metastable helium atomic beams and a magneto-optical trap (MOT) for ultracold lithium atoms .
In this presentation, I will show results illustrating that the reaction rate dramatically depends on the initialelectronic state of the Li scattering target. I will also describe the experimental characterization of a new optical quenching scheme  which makes it possible to distinguish the relative contribution of the He(21S 0 ) and He(23S1) states to the reaction rate. This technique makes it possible to fully deplete the population of He(21S0) in the supersonic beam via optical excitation to the 41P1 state. Since it is based on simple and inexpensive diode laser technology, the method can be implemented in many laboratories in a straightforward manner. I will also show results on the reactive scattering of He(23S1) with Li atoms obtained using the optical quenching scheme.
 J. Grzesiak, T. Momose, F. Stienkemeier, M. Mudrich and K. Dulitz, J. Chem. Phys. 150 (3), 034201, 2019.
 J. Guan, V. Behrendt, P. Shen, S. Hofsäss, T. Muthu-Arachchige, J. Grzesiak, F. Stienkemeier, and K. Dulitz, Phys. Rev. Appl. 11 (5), 054073, 2019.