1.Ion Beam and Optical Physical Joint Laboratory of Xianyang Normal University and Institute of Modern Physics, Chinese Academy of Sciences, Xianyang 712000, China 2.Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11605147, 11075135) and the Natural Science Basic Research Plan in Shaanxi Province, China (Grant No. 2020JM-624)
Received Date:06 April 2020
Accepted Date:02 July 2020
Available Online:22 October 2020
Published Online:05 November 2020
Abstract:During the interaction of highly charged ions with solid target in the energy region near the Bohr velocity, the potential energy of the projectiles will be deposited on a nanometer-scale target surface within the time on the order of femtoseconds. That will lead the target atoms to be ionized into ions and the ions to be excited, resulting in the multiple ionization states and the complex configuration of energy levels. The de-excitation radiations of these levels cover the radiations from near-infrared spectral line to X-ray. Investigation of these spectral lines is significant for investigating the mechanism of such an interaction, diagnosing plasma and studying astrophysics. The experimental results show that the near-infrared spectral lines and X-ray spectra are produced by the 129Xeq+ (q = 21, 23, 25, 27) with kinetic energy of 1360 keV and 129Xe20+ with kinetic energy of 4 MeV impacting on the Cu surface, separately. The experiment is carried out in the National Laboratory of Heavy Ion Research Facility in Lanzhou, HIRFL. The beam intensity is on the order of nA. The highly charged ions capture the electrons of the Cu target and thus being neutralized in a femtosecond time. The energy of the highly charged ions is deposited on the target surface, and the target atoms are excited or ionized, resulting in the transition between complex configurations, such as the dipole forbidden transition (magnetic dipole and quadrupole transition) and magnetic dipole transition of the Cu22+. The infrared spectral lines of the atoms and ions from deexcitation radiation are measured. With the 4 MeV 129Xe20+ ions impacting on solid Cu surfsce, the X-rays are measured, such as, the magnetic dipole deexcitation radiation transition of Cu22+, the X-rays of the L1 edge transition and Lβ3 of the Cu I, Lη and Lβ3 X-rays of the Xe ions. The results show that during the neutrilization of highly charged Xe ions with lower energy above the Cu surface, the infrared lines are mainly from the deexcitation of the incident ions and the ionized or excited target atoms. The increasing trend of the the single ion fluorescence yield of the infrared spectral line is the same as that of the potential energy of the projectile. The characteristic L X-rays of the Xe atom are emitted by the second generation of hollow atoms formed below the surface. Keywords:highly charged ion/ forbidden transition/ near-infrared spectral lines
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3.1.动能一定、不同电荷态离子束激发靶表面的近红外光谱线
动能1360 keV、电荷态分别取q = 21, 23, 25和27的129Xeq+离子束入射Cu靶, 测量相互作用过程发射的近红外光谱线. 图3(a)给出Xe21+入射Cu表面激发的标识的近红外光谱线, 图3(b)给出Xe27+入射Cu表面激发的标识的近红外光谱线, 其中有部分谱线未认定. Cu II 829.78 nm(829.8461)是靶原子被离化激发到151757.601 cm–1 能级 (电子组态3d9(2D5/2)7d, 态项为2[5/2], 角动量为2), 退激到139710.491 cm–1 能级(电子组态 3d8(3F)4s4p(1Po), 态项为3Fo, 角动量为2)辐射的近红外光谱线. Cu I 1665 nm是靶原子被激发到55387.667 cm–1能级(电子组态3d105d, 态项为2D, 角动量为3/2), 退激到49383.264 cm–1能级(电子组态 3d105p, 态项为$^2{\rm{P}}^\circ_{1/2}$, 角动量1/2)辐射的近红外光谱线. Xe I 1435.46 nm是炮弹离子Xe21+俘获靶表面电子中性化后处于激发态92153.279 cm–1能级(电子组态$5{\rm{p}}^5(^2{\rm{P}}^\circ_{3/2})8{\rm{p}}$, 态项为2[1/2], 角动量为1), 退激到85188.777 cm–1能级(电子组态$5{\rm{p}}^5(^2{\rm{P}}^\circ_{3/2})7{\rm{s}}$, 态项为2[3/2]°, 角动量为1)辐射的红外光谱线. 红外谱线的能量不确定度主要来源于多次测量结果的统计误差, 详细的跃迁见表1.
表1129Xeq+入射Cu靶激发的红外光谱线 Table1.Measured near-infrared spectral lines induced by 129Xeq+ ions on Cu surface
图 3 动能一定(1360 keV)的129Xeq+离子入射Cu靶表面激发的近红外光谱线 (a) 129Xe21+; (b) 129Xe27+ Figure3. Measured near-infrared spectral lines induced by 129Xeq+ ions with 1360 keV kinetic energy impacting on Cu surface: (a) 129Xe21+; (b) 129Xe27+
23.2.动能为4 MeV的129Xe20+入射靶激发的X射线谱 -->
3.2.动能为4 MeV的129Xe20+入射靶激发的X射线谱
图4给出了离子束与靶原子相互作用辐射的X射线谱. 用Gauss函数拟合, 其中拟合的均方差w与X射线谱的半高全宽存在$2\sqrt {{\rm{ln4}}}\;w$的转换关系. 图 4 动能为4 MeV的129Xe20+离子入射Cu靶表面辐射的X射线谱 Figure4. X-ray spectra induced by 129Xe20+ ions with 4 MeV kinetic energy impacting on Cu surface.
表示, 其中R表示谱线的相对强度, N是入射离子总数, I为流强, Δt为积分时间, q为入射离子的电荷态, e为元电荷. 利用(4)式进行计算, 3条红外谱线的单粒子产额显示于图5. 从图5可以看出, 随着入射离子129Xeq+的电荷态增加, 其势能即为电离q个电子的电离能的总和, 因此随着离子的电荷态增加, 势能将会更快地增加(电荷态q = 21, 23, 25, 27时, 总势能分别为Ep = 5.83, 7.27, 8.95, 12.01 keV), 释放给靶原子的势能增加, 同时激发的靶原子和离子数目增加, 辐射的谱线强度随之增强. 同样, 一定流强的炮弹离子, 随着携带的势能增加, 释放势能中性化后处于激发态的Xe原子数增加, 因此, 离化和激发的靶原子以及炮弹中性化的原子辐射光谱线的相对强度也在增加, 增加的趋势同炮弹的势能增加趋势相同[5]. 图 5 (a) 129Xeq+离子携带势能随电荷态q增加的趋势; (b)近红外光谱线的单粒子产额随电荷态q增加的趋势 Figure5. (a) Potential energy of the 129Xeq+ ion vs. the charge q; (b) single ion fluorescence yield of the near-infrared spectral lines as a function of the projectile charge q where the near-infrared spectral lines are induced by 1360 keV 129Xeq+ (q = 21—27) ion impacting on a Cu surface.