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在少电子原子精密光谱测量中, 产生高强度、单一量子态的氦原子和类氦离子是实验研究的关键, 也是改善实验测量信噪比的决定性因素. 本文提出利用自由电子激光获得高强度亚稳态氦原子和类氦离子的实验方案. 激光的制备效率可以通过求解光和原子相互作用的主方程获得, 根据拟建设的深圳自由电子激光装置的设计参数和实验条件, 计算得到亚稳态He、Li+和Be2+的制备效率分别可达3%、6%和2%以上. 与常见的气体放电和电子轰击等制备方法相比, 激光激发产生亚稳态原子/离子不仅可以提高制备产率, 也可以降低放电时产生的电子、离子以及光子等高能杂散粒子的影响. 利用自由电子激光激发制备亚稳态氦原子和类氦离子有望应用于多个研究领域.
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关键词:
- 亚稳态氦原子和类氦离子 /
- 精密光谱 /
- 自由电子激光 /
- 测量信噪比 /
- 制备效率
In the precision spectroscopy of few-electron atoms, the generation of high-intensity metastable state helium atoms and helium-like ions is crucial for implementing experimental studies as well as a critical factor for improving the signal-to-noise ratio of experimental measurements. With the rapid development of free-electron laser (FEL) and technology, FEL wavelengths extend from hard X-rays to soft X-rays and even vacuum ultraviolet bands. Meanwhile, laser pulses with ultra-fast, ultra-intense and high repetition frequencies are realized, thus making it possible for FELs to prepare single-quantum state atoms/ions with high efficiency. In this work, we propose an experimental method for obtaining high-intensity single-quantum state helium atoms and helium-like ions by using FEL. The preparation efficiency can be calculated by solving the master equation of light-atom interaction. Considering the experimental parameters involved in this work, we predict that the efficiencies of preparing metastable 23S He, Li+ and Be2+ are about 3%, 6% and 2%, respectively. Compared with the common preparation methods such as gas discharge and electron bombardment, a state-of-the-art laser excitation method can not only increase the preparation efficiency, but also reduce the effects of high-energy stray particles such as electrons, ions, and photons generated during discharge. Furthermore, combined with the laser preparation technique, the sophisticated ion confinement technique, which can ensure a long interaction time between the ions and laser, increases the efficiency of metastable Li+ and Be2+ by several orders of magnitude. Therefore, the preparation of high-intensity metastable helium and helium-like ions can improve the measurement accuracy of precision spectroscopy of atoms and ions. A new experimental method, based on the FEL, to study the fine structure energy levels 23P of helium, has the potential to obtain the results with an accuracy exceeding the sub-kHz level. Thus, the high-precision fine structure constants can be determined with the development of high-order quantum electrodynamics theory. In order to measure energy levels with higher accuracy, a new detection technique, which can reduce or even avoid more systematic effects, must be developed. For example, the quantum interference effect, which has been proposed in recent years, seriously affects the accuracy of fine-structure energy levels. If the interference phenomenon of spontaneous radiation between different excited states can be avoided in the detection process, the measurement accuracy will not be affected by this quantum interference effect. High-intensity metastable atoms or ions in chemical reaction dynamics studies also have better chances to investigate reaction mechanisms. In summary, the FEL preparation of high-intensity metastable helium atoms and helium-like ions proposed in this work will lay an important foundation for developing cold atom physics and chemical reaction dynamics.
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Keywords:
- Metastable Helium and Helium-like Ions /
- Precision Spectroscopy /
- Free Electron Laser /
- Signal-to-Noise Ratio /
- Preparation Effciency
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图 1 (a)为制备亚稳态氦原子的相关能级, (b)为制备亚稳态氦原子/类氦离子的装置示意图
Fig. 1. (a) The energy levels for the preparation of metastable helium. (b) A schematic of designed apparatus for the preparation of metastable helium/helium-like ions.
图 2 单脉冲作用时单个氦原子不同能级布局数的时间演化结果
Fig. 2. Time evolution for different energy levels of helium by single-pulse excitation.
图 3 激光制备亚稳态氦原子的效率随光斑大小的模拟结果
Fig. 3. Simulation results of preparation efficiency of metastable helium with respect to spot size.
图 4 激发效率随同步辐射光通量的变化关系
Fig. 4. Excitation efficiency as a function of the photon flux for synchrotron radiation sources.
图 5 制备亚稳态$ {\rm{Li^{+}}} $和$ {\rm{Be^{2+}}} $的相关能级示意图
Fig. 5. The energy levels of metastable $ {\rm{Li^{+}}} $ and $ {\rm{Be^{2+}}} $.
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