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Studying 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 merging beam collision experimental method is a powerful approach to achieving cold collisions of mK collision energy, by deflecting one reactant beam to collide with another reactant beam in a collinear manner. In this work, based on the Zeeman effect, the interaction between atomic magnetic moments and a magnetic field, a permanent-magnet “magnetic guide” system is developed to deflect metastable helium atom beams, with the aim of achieving collinear transport of neutral helium atoms and molecules in cold merged-beams collisions. Metastable helium atoms He(23S1) are produced through RF discharge. Utilizing this “magnetic guide”, the quantum-state-resolved neutral helium atoms (He(23S1), $ {M_J} = + 1 $) are prepared. Helium flux measurements demonstrate about 10°deflection of metastable helium atoms with a flux exceeding 106 atoms/s, accompanied by successful preparation of $ {M_J} = + 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 are analyzed. This work lays an experimental foundation for quantum-state-resolved cold collisions between excited-state helium and molecules below 1 K, advancing the understanding of cold reaction mechanisms governing the evolution of interstellar media 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 23S1 helium atoms into the $ {M_J} = + 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. 
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Keywords:
- metastable helium atoms /
- cold collisions /
- merged-beams
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图 1 (a) 亚稳态氦原子束线平台示意图. 黑色虚线框内为合并束的永磁体“磁导”装置; (b) 氦原子能级结构图; (c) 磁导三维图
Figure 1. (a) Schematic drawing of metestable helium atoms beam, dashed box is the “magnetic guide” system; (b) energy level diagram of helium atoms; (c) 3D schematic of the magnetic guide.
图 2 激光系统部分的光路示意图(AMP, 放大器; LPF, 低通滤波器; PBS, 偏振分束镜; GT, 格兰泰勒棱镜; EOM, 电光调制器; LO, 本地振荡信号)
Figure 2. Schematic drawing of optical layout. AMP, amplifier; LPF, low-pass filter; PBS, polarizing beam splitter; GT, Glan-Taylor prism; EOM, electro-optic modulator; LO, local oscillator.
图 3 (a) 单个磁组件的结构示意图; (b) 单个磁组件的实物图; (c) 单个磁组件的径向磁场分布模拟结果
Figure 3. (a) Schematic diagram of the magnetic assembly; (b) photograph of magnetic assembly; (c) simulated 2D magnetic field distribution.
图 4 (a) 磁组件的一维径向磁场曲线图, 其中红色空心圆点为有限元模拟结果, 蓝色和黑色曲线为实验测量的磁场分布曲线; (b) 磁组件的径向磁场梯度分布
Figure 4. (a) Radial magnetic field of the magnetic assembly, where red open circles denote the simulation results, and blue/black curves represent measured magnetic field; (b) radial magnetic field gradient of the magnetic assembly.
图 5 800 m/s $ {M_J} = - 1 $磁子能级亚稳态氦原子在单个磁组件内的运动轨迹模拟
Figure 5. Simulated trajectory of metastable helium atoms (23S1, $ {M_J} = - 1 $) propagating through the magnetic assembly at 800 m/s.
图 6 800 m/s $ {M_J} = + 1 $磁子能级亚稳态氦原子穿过磁导的运动轨迹模拟
Figure 6. Simulated trajectory of $ {M_J} = + 1 $ 23S1 metastable helium atoms propagating through the magnetic guide at 800 m/s.
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