University of Freiburg

Institute of Physics

Experimental Atomic and Molecular Physics and Cold Chemistry


Control of chemi-ionization between metastable helium and lithium by quantum state selection

The PhD project is focused on the study of chemi-ionizing collisions between electronically excited and long-lived (metastable) helium atoms (He*) and lithium atoms (Li). The reaction is enabled due to the fact that the electronic energy of the metastable levels of He* (≈ 20 eV) easily exceeds the ionization energy of most atoms and molecules, including Li.

The He*-Li reaction is very interesting from a fundamental point of view, because it is a prototypical electron transfer process, in which an electron from the 2s (or a higher-lying) orbital of Li is transferred to the 1s orbital of He, and the electron from the 2s orbital of He is ejected. Electron-transfer processes are ubiquitous in nature. Important processes, such as photosynthesis and catalysis processes, rely on electron transfer. Therefore, information from the He*-Li model system can help us to understand more complicated electron-transfer processes in nature.

We make use of two different techniques to prepare the reaction partners. The Li atoms are cooled by laser radiation and captured in a magneto-optical trap (MOT) at a temperature of around 1 mK. The Li atoms serve as an ultracold, dense and stationary target for collisions with He*. A beam of He* atoms is created by expanding He gas into the vacuum using a pulsed valve and by exciting the atoms using a gas discharge. The reaction rate is measured by counting the number of ions produced during the collision process. Owing to the simple and well-known internal energy level structure of the two atoms, they can be prepared in a variety of initial quantum states e.g. by laser light or by static electro-magnetic fields. This allows us to precisely investigate quantum-state-dependent effects on the reactivity, such as the influence of the mutual electron spin direction.


  • T. Sixt, F. Stienkemeier, and K. Dulitz, Spin-state-controlled chemi-ionization reactions between metastable helium atoms and ground-state lithium atoms, J. Chem. Phys. 156, 114306 (2022).
  • T. Sixt, J. Guan, A. Tsoukala, S. Hofsäss, T. Muthu-Arachchige, F. Stienkemeier, and K. Dulitz, Preparation of individual magnetic sub-levels of 4He(23S1) in a supersonic beam using laser optical pump-ing and magnetic hexapole focusing, Rev. Sci. Instrum. 92, 073203 (2021).
  • J. Guan, T. Sixt, K. Dulitz, and F. Stienkemeier, Sensitive detection of metastable NO and N2 by reactive collisions with laser-excited Li, J. Phys. B: At. Mol. Opt. Phys. 53, 245201 (2020).
  • K. Dulitz, T. Sixt, J. Guan, J. Grzesiak, M. Debatin, and F. Stienkemeier, Suppression of Penning ionization by orbital angular momentum conservation, Phys. Rev. A 102, 022818 (2020).
  • Sixt T, Chung T, Stienkemeier F, Dulitz K Symmetry Dependence of the Continuum Coupling in the Chemi-ionization of Li(2^2S_(1/2)) by He(2^3S_1, 2^3P_J) J. Phys. Chem. A 2023, 127, 4407-4414