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量子态选择的低温原子分子反应动力学研究是从原子分子层面探究分子间及分子内的微观反应机理, 理解散射量子效应的关键研究手段之一. 合并束低温碰撞实验方法通过将一反应物偏转后与沿直线飞行的另一反应物发生共线碰撞, 获得毫开尔文量级的冷碰撞实验条件, 并开展毫开尔文至百开尔文碰撞能的反应动力学研究. 本文采用自主发展的永磁体“磁导”引导特定量子态的中性原子偏转后与分子束共线, 通过对氦原子穿过磁导的通量测量, 实验实现了三重态亚稳态23S1氦原子约10°角度偏转, 并制备了$ {M_J} = + 1 $ 磁子能级激发态氦原子. 本工作为发展亚稳态氦原子与分子低于开尔文量级的量子态选择激发态冷碰撞研究提供实验基础, 可以促进对激发态反应在星际介质演化中重要贡献的理解以及化学反应调控的研究. 本研究中发展的“磁导”也在原子速度滤波和冷原子输运等领域具有重要的应用前景.
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
Fig. 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)
Fig. 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
Fig. 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
Fig. 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
Fig. 5. Simulated trajectory of metastable helium atoms (23S1, $ {M_J} = - 1 $) propagating through the magnetic assembly at 800 m/s.
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