1.Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 2.University of Chinese Academy of Sciences, Beijing 100084, China 3.Key Laboratory of Atomic and Molecular Physics & Functional Material of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730000, China 4.Joint Laboratory of Atomic and Molecular Physics in Extreme Environments, Northwest Normal University and Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 5.Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100084, China
Fund Project:Project supported by the State Key R&D Program of China (Grant Nos. 2017YFA0402400, 2017YFA0402300) and the Strategic Leading Science and Technology Project of Chinese Academy of Sciences (Grant No. XDB34020000)
Received Date:12 October 2020
Accepted Date:09 December 2020
Available Online:23 March 2021
Published Online:20 April 2021
Abstract:Charge exchange, or electron capture, between highly charged ions and atoms and molecules has been considered as one of important mechanisms controlling soft X-ray emissions in many astrophysical objects and environments. However, to model charge exchange soft X-ray emission, astrophysicists commonly use principal quantum number n and angular momentum quantum numberl resolved state-selective capture cross section data, which are usually obtained by empirical and semi-classical theory calculations. The accuracy of the theoretical model is the key to constructing an accurate X-ray spectrum. With a newly-built cold target recoil ion momentum spectroscopy apparatus, we perform a series of precise state-selective cross section measurements on Ne8+ ions’ single electron capture with He targets, with the projectile energy ranging from 1.4 to 20 keV/u. The experimentally measured Q value spectrum shows that the process of electron captured to state of Ne7+ with n = 4 is the main reaction channel, and that with n = 3 and 5 are the small reaction channels. Using Gaussian curve to fit the area of each channel on the Q value spectrum and normalizing the area of all channels, we obtain the n-resolved relative state-selective cross section. By comparing the measured relative cross sections with the results calculated by the multichannel Landau-Zener method and molecular Coulomb over-barrier model, significant difference among the strengths of small reaction channels is found. Specifically, the multichannel Landau-Zener method overestimates the contribution of n = 2 channel and n = 3 channel, and underestimates the contribution of n = 5 channel. The molecular Coulomb over-barrier model overestimates the contribution of n = 5 channel and underestimates the contribution of n = 3 channel. The significant difference between the theoretical model calculation and experimental measurement is due to the limitations of semiclassical theoretical method and classical theoretical method. Furthermore, with l distribution models commonly used in the astrophysical literature, including the statistical model, separable model, Landau-Zener-I model, Landau-Zener-II model and even model, we calculate the soft X-ray emissions in the charge exchange between 1.6 and 2.4 keV/u Ne8+ and He. It is found that the calculated intensities of X-ray spectra significantly deviate from the existing measurements, and only the separable model can partly match the laboratory simulated solar wind charge exchange X-ray measurement. Furthermore, we find that the intensity of the charge exchange X-ray emission spectrum measured experimentally is dependent on the collision energy, while the emission spectrum calculated based on the model seems to be unchanged with the increase of the collision energy. These results indicate that if the classical and semi-classical models are applied to the astrophysical plasma for studying diffusive soft X-ray background, the obtained parameters of the astrophysical plasma will be inaccurate. Keywords:reaction microscopes/ charge exchange/ state selective capture/ soft X-ray
2.实验方法中国科学院近代物理研究所除已有的320 kV高电荷态离子综合实验平台[24], 近期又成功建立了离子能量在102 eV—30q keV范围的紧凑灵活的低能高电荷态离子实验平台[25], 其中q 为离子的电荷态. 平台主要由电子束离子源(EBIS)、20 kV的高压平台、维恩速度选择器、束流线、束流诊断及传输系统等组成. 结合自主研制的反应显微成像谱仪[26], 可以系统开展太阳风速度范围的低能高电荷态离子与原子分子碰撞电荷交换过程的实验室模拟研究. 开展电荷交换实验用的实验装置布局如图1所示. 图 1 电荷交换实验装置布局图, 其中包括离子源系统与反应显微成像谱仪, 超声射流的方向是从下往上的. ETOF是TOF谱仪的引出电场 Figure1. Layout of CX experimental setup with ion source system and reaction microscope spectroscopy, the supersonic gas jet flow direction is from down to top. ETOF represents the electric field of TOF spectrometer
3.结果与讨论图2给出了在Ne8+-He单电子俘获中测得的Q值谱. 实验结果表明, 电子主要被俘获到炮弹主量子数n = 4的态上, 这与先前的实验结果一致[18,27-32]. 从当前的测量结果来看, 随着碰撞能量增加, n = 3和5的态的布居比例也同样增加, 其中n = 5的通道增加最明显, 即俘获态随着炮弹能量增加趋向于布居到更高的量子态上, 该趋势类似于Abdallah等[33]报道的Ar16+-He单电子俘获结果, 即更高的态比低的态具有更大的密度, 平均Q值趋向于更小值, 这点在文献[33]中给出了较好的解释. 在理论方面, Otranto等[34]介绍了一种关于n态布居的简单经验公式, 我们利用该公式预测电子被俘获后的布居态为n = 3.5, 这与当前实验结果存在一定偏差. 此外, 图2还给出了基于分子库仑过垒模型(MCBM)[35]计算的反应窗, 并将其峰值归一到实验测量的峰值以进行比较. 可以看出, MCBM计算结果与实验测量结果符合较好, 并且随着碰撞能量增加, 符合程度更好. 图2(d)中黑色粗线与MCBM曲线交叉, 黑线的长短反映了不同反应通道的相对态选择截面大小, 可以给出MCBM模型计算的单电子俘获到不同主量子数n的态分辨截面. 图 2 不同入射炮弹能量下Ne8+-He单电子俘获的Q值谱 (a) 1.6 keV/u; (b) 2.4 keV/u; (c) 7.2 keV/u; (d) 20 keV/u. 黑色空心方块和红色实线是实验测量的结果, 蓝色实线为归一到实验测量峰值的MCBM计算的反应窗. 图(d) 中的黑色粗线与MCBM计算的反应窗的交点反映了MCBM计算的态选择截面的大小 Figure2. Measured Q spectrum of single electron capture between Ne8+ and He with different incident projectile energies: (a) 1.6 keV/u; (b) 2.4 keV/u; (c) 7.2 keV/u; (d) 20.0 keV/u. The black hollow square and the red solid line are the results of the experimental measurement, and the blue solid line is the response window calculated by MCBM normalized to the peak of the experimental measurement. The heavy black thread in panel (d) represents the intensity of state selected cross sections for MCBM calculations
为了定量显示出单电子俘获相对态选择截面与碰撞能量的变化关系, 并对比实验与理论之间的差异, 在图3中给出了Ne8+-He单电子俘获态选择截面, 并与多通道Landau-Zenner (MCLZ)和MCBM计算的结果进行了比较. 实心点是实验测量的结果, 空心点是MCBM计算的结果, 不同的颜色与形状代表不同的俘获通道, 实线是MCLZ[36]计算的结果. 从图3可以看出, 实验测量的主要俘获态(n = 4)的相对截面随碰撞能量增加而减小, 而两个弱反应通道(n = 3, 5)截面随能量增加而增加. MCLZ和MCBM理论计算的结果在n = 4的态上与实验测量结果符合很好, 但在两个弱通道则有明显的差异. MCLZ极大低估了n = 5截面, 而高估了n = 2和n = 3的截面, 理论计算n = 5截面随碰撞能量增加而减小[36], 而实验测量显示随能量增加而增加. MCBM则高估了n = 5的截面, 低估了n = 3的截面, 但截面随能量增加的变化趋势与实验测量一致. 这两种方法都属于经典的理论方法, 他们对弱反应通道描述不准确, 特别是MCBM方法只与激活电子在反应前后的束缚能以及碰撞速度有关, 因此经典方法的局限性必然导致理论与实验测量存在一定的差异. 总的来说, 两种理论模型对主要反应通道的预测与实验测量符合较好, 对弱反应通道的预测与实验测量存在较大差异, 因此需要量子描述的理论模型的支持. 图 3 Ne8+-He单电子俘获的相对态选择截面, 实心点和实线是实验测量的结果, 空心点和点线是MCBM计算的结果, 不同的颜色与形状代表不同的俘获通道, 实线是MCLZ计算的结果 Figure3. Ne8+-He single electron capture relative state selection cross section, the solid shape and solid line is the result of experimental measurement, the hollow shape and dot line is the result of MCBM calculation, different colors and shapes represent different capture channels, and the solid line is the result of MCLZ calculation
我们希望利用实验准确测量的态选择截面, 构建电荷交换X射线发射谱, 来检验应用于天体物理的角动量分布模型的准确性. 软X射线发射对天体物理源中的物理条件非常敏感, 因此, 软X射线谱具有较高的诊断实用性[37]. 被俘获的电子退激发射谱强度取决于电荷交换过程所填充的l态的分布. 对于给定主量子数n分辨的截面, 天体物理学界通常使用几种估计l分布的分析模型来将n分辨的态选择截面展开为nl分辨的态选择截面, 这五种模型分别是Statistical, Separable, Landau-Zener-I (LZ-I), Landau-Zener-II (LZ-II) 以及Even模型[38,39]. 通过实验测量的n分辨态选择截面, 以及天体物理常用的l分布模型展开成l分辨的态选择截面, 在考虑退激分支比和级联效应后, 就可以计算出发射的软X射线谱. 在当前研究的能量范围内, Ne8+和He之间的电荷交换中电子会布居到炮弹的n = 3, 4, 5的态, 俘获后的电子会快速地退激并发射X射线, 这些俘获之后退激发射的软X射线在100—220 eV的能量范围内. 我们发展了一个计算电荷交换后的发射谱的程序(photo emission following charge exchange, PhECX), 并计算了1.6和2.4 keV/u Ne8+与He电荷交换后的软X射线级联发射谱. 图4给出了计算的软X射线谱与Zhang等[18]测量的X射线谱之间的比较, 不同颜色的实线代表不同模型重构的发射谱, 实心点是Zhang等测量的结果. Zhang等[18]的实验是在美国橡树岭国家实验室离子研究装置上开展的, 实验利用一个垂直于束流安置的微卡计, 测量气室中的He靶与太阳风速度的Ne8+束流电荷交换的X射线发射谱. 为便于比较, 将Zhang等测量的发射谱与文献[18]报道的探测效率对谱线强度进行了校正, 并把计算结果用7.9 eV的实验分辨进行了卷积. 结果如图4所示, 可以看出, Statistical模型的计算结果高估了3s$ \rightarrow $2p和3d$ \rightarrow $2p的贡献, 且所有l分布模型都低估了3p$ \rightarrow $2s的贡献. 对比图中两个反应能量下的发射谱, 模型计算的发射谱几乎不随碰撞能量变化而改变, 主要是因为所有l分布模型都是与速度无关的, 而实际测量的发射谱[18]以及l分布则依赖于炮弹能量[34], 这直接导致实验测量与模型计算出现明显差异. 从n分辨测量中还可以看出, 靶电子被俘获到n = 4的态占所有俘获通道的90%以上. 这表明n = 4俘获的级联退激对来自3s, 3p和3d的发射谱起着决定性作用. 因此计算的3p$ \rightarrow $2s谱线强度低于实验测量的原因可能是来自不合适的退激分支比. 图 4 1.6和2.4 keV/u的Ne8+-He俘获电子后的归一化$ {\rm{Ne}}^{7+*}$发射谱 (a) 1.6 keV/u; (b) 2.4 keV/u. 黑色、红色、蓝色、品红、绿色实线分别代表Statistical, Separable, Landau-Zenner-I, Landau-Zenner-II, 以及Even模型计算的结果, 黑色实心点代表Zhang等[18]测量的结果, 半高全宽是7.9 eV Figure4. Normalized $ {\rm{Ne}}^{7+*}$ emission spectrum after electron capture of Ne8+-He at 1.6 and 2.4 keV/u: (a) 1.6 keV/u; (b) 2.4 keV/u. The black, red, blue, magenta, and green solid lines represent the results calculated by the Statistical, Separable, Landau-Zenner-I, Landau-Zenner-II, and Even models, respectively. The black solid points represent the results measured by Zhang 2019, the full width at half maximum is 7.9 eV.
图 1 电荷交换实验装置布局图, 其中包括离子源系统与反应显微成像谱仪, 超声射流的方向是从下往上的. ETOF是TOF谱仪的引出电场
图 2 不同入射炮弹能量下Ne8+-He单电子俘获的Q值谱 (a) 1.6 keV/u; (b) 2.4 keV/u; (c) 7.2 keV/u; (d) 20 keV/u. 黑色空心方块和红色实线是实验测量的结果, 蓝色实线为归一到实验测量峰值的MCBM计算的反应窗. 图(d) 中的黑色粗线与MCBM计算的反应窗的交点反映了MCBM计算的态选择截面的大小
图 3 Ne8+-He单电子俘获的相对态选择截面, 实心点和实线是实验测量的结果, 空心点和点线是MCBM计算的结果, 不同的颜色与形状代表不同的俘获通道, 实线是MCLZ计算的结果
图 4 1.6和2.4 keV/u的Ne8+-He俘获电子后的归一化
