Preparation of Single-Quantum-State-Selected Helium for Neutral Atom-Molecule Merged-Beams Collisions

The study of low-temperature atomic and molecular reaction dynamics in quantum state selection is one of the key research methods for exploring the collision reaction mechanisms and revealing quantum effects in scattering processes. The merged beams collision experiment method is a powerful approach to achieving cold collisions at mK collision energy, by deflecting one reactant beam to collide collinearly with another reactant beam. In this work, based on the Zeeman effect, the interaction between atomic magnetic moments and a magnetic field, we developed a permanent-magnet “magnetic guide” system to deflect metastable helium atom beams, with the objective of achieving collinear transport of neutral helium atoms and molecules in cold merged-beams collisions. Metastable helium atoms He(2³S₁) were produced via RF discharge. Utilizing this "magnetic guide", the quantum-state-resolved neutral helium atoms (He(2³S₁), MJ=+1) have been prepared. Helium flux measurements demonstrate ~10°deflection of metastable helium atoms with a flux exceeding 10⁶ atoms/s, accompanied by successful preparation of MJ=+1 magnetic sublevel helium atoms. Furthermore, by combining the magnetic field measurements and magnetic force calculations for 2³S₁ metastable helium atom, the simulated trajectories propagating through the magnetic guide were analyzed. This work provides an experimental foundation for quantum-state-resolved cold collisions between excited-state helium and molecules below 1 K, advancing our understanding of cold reaction mechanisms governing interstellar medium evolution and promoting chemical reaction control. The developed magnetic guidance technology in this study also has important application prospects in fields such as atomic velocity filtering and cold atom transport. In the future, optical pumping experimental methods will be employed to pump 2³S₁helium atoms into the MJ=+1 magnetic sublevel helium atoms, enhancing the population of single quantum state. Moreover, two-dimensional magneto-optical traps and optical molasses will be implemented to optimize beam, which is expected to further improve the beam flux of helium atoms.