The energy-dependent population of fine quantum states in single electron capture (SEC) reflects the intrinsic collision dynamics. Here we report experimental studies of Ar7+ ions colliding with He in the energy range of 1.05-17.5 keV/u. Owing to the high resolution of a recoil-ion momentum spectrometer, the n-, l-, and spin-state electron capture populations are well resolved, and a strong energy dependence of the SEC cross sections is observed. Most importantly, a clear inversion of the cross-section ratio between the spin-resolved triplet and singlet 3s3d configurations is found, demonstrating a breakdown of spin statistics. Together with recent spin-resolved studies of C3+-He collisions (PRL 133, 173002 (2024)), these results suggest that the breakdown of spin statistics is likely a general feature of charge exchange in open-shell highly charged ion systems.

Penning traps, renowned for their unparalleled precision in determining fundamental properties, such as mass and magnetic moments, are cornerstone instruments in modern physics. Their applications span from nuclear structure studies to stringent tests of quantum electrodynamics and CPT invariance. Although Penning traps have been demonstrated for fundamental studies, often employing superconducting magnets, their high cost and operational complexity remain challenges. In this work, we report the development of a compact cryogenic Penning trap that utilizes a permanent magnet to provide a confining magnetic field, offering a more economical and flexible alternative. We have successfully demonstrated all core functionalities of this system, including ion generation, transport, confinement, manipulation, and signal detection. This compact trap not only serves as a vital technical testbed for the development of the Shanghai Penning Trap but also establishes a cryogenic Penning-trap experiment platform for ion trapping and cooling applications and envisaged spectroscopic studies applications.

Spectra of Sr-like Sn12+, Xe16+, La19+, Pr21+, Nd22+, and Sm24+ ions in the wavelength range of 200-600 nm were measured using an electron beam ion trap. A total of thirty four magnetic dipole transition lines were identified with twenty four lines newly assigned. Theoretical calculations were performed using the multi-configuration Dirac-Hartree-Fock and relativistic configuration interaction methods, incorporating the Breit interaction and the dominant quantum electrodynamics effects. The calculated results show good agreement with experiment, with an average deviation below 1.0%. The experimental spectra reported provide reliable reference data for further studies.

The hyperfine structure constants of the 1s(2)2s(2)2p P-2(1/2) and P-2(3/2) states in the boron-like isoelectronic sequence with nuclear charges Z = 6 to 36 were calculated using the multiconfiguration Dirac-Hartree-Fock (MCDHF) method. In these calculations, we included electron correlation, Breit interaction, and one-electron quantum electrodynamics (QED) corrections. It was found that taking into account the 1s core electron correlation and higher-order electron correlation (i.e., triple and quadrupole excitations) effects improves the accuracy of the hyperfine structure constants to the order of 10(-5) similar to 10(-6). Additionally, we obtained fitting formulas for the hyperfine structure constants as a function of Z, useful to predict hyperfine structure constants for any isotopes of boron-like ions in the region of 6 <= Z <= 36.

The spectroscopic investigations on 3 p 3 2D5/2-2D3/2 transitions of phosphorus-like Ge17+,As18+, Se19+, Br20+, and Kr 21+ ions at an electron beam ion trap were presented. The direct wavelength measurements were reported for the first time for Ge 17+ - Br 20+ ions. All the measurements reached precision levels of a few ppm. The theoretical calculations were carried out using multi-configuration Dirac-Hartree-Fock and relativistic configuration interaction methods including a large set of configuration state functions, in which the Breit interaction and QED effects were taken into account. The present results showed a good agreement between the theory and the experiment, and the divisions were less than 0.6%. Especially for the Kr 21+ ions, meticulous scrutiny of line strengths with charge state distributions and continuity of results with isoelectronic sequence were performed to identify the measured spectral line as 3 p 3 2D5/2-2D3/2 transition. The present work resolved the long-standing confusion of the Kr 21+ spectral line. Our accurate experimental results could be reference data for further calculations.

Weakly bound systems provide possibilities to study nonlocal electronic decay processes, which present unique physical and chemical properties. Here we report the experimental observation of a nonlocal decay process, i.e., nonadiabatic-coupling-mediated charge transfer (NCMCT), in argon dimer. This process can be effectively produced and manipulated by the double electron capture in low-charge-state oxygen ion collisions. Compared to other common decay mechanisms of Ar2 dication, the NCMCT process results in notably different kinetic energy releases for the Ar+ + Ar+ channel. It is due, revealed by theoretical calculations, to the special potential energy curve of the responsible Ar2+ - Ar state that possesses many crossing points and thus nonadiabatically couples with the Ar+ - Ar+& lowast; states in a wide internuclear distance range. Such an NCMCT process is expected to be a general process occurring in weakly bound systems when highly excited. The present work provides a prime collision system to explore the charge transfer in ion-molecule reactions and also brings insight into cluster dissociation mechanism.

A cold target recoil-ion momentum spectroscopy (COLTRIMS) mounted on a 3 MV tandetron accelerator is introduced, which is a powerful tool to study atomic and molecular reaction dynamics induced by few-megaelectronvolt (MeV) ions. The setup mainly consists of a supersonic gas jet, a time of flight mass spectrometer (TOFMS), three position-sensitive detectors (PSDs), and a data acquisition (DAQ) system. Using the TOF and position information on the PSDs, the 3-D momenta of recoil ions can be reconstructed. The diameter of the supersonic gas jet is measured to be 1.3 +/- 0.3 mm. The TOFs of Ar(1-4)+ ions are measured under 2 MeV He2+ impact and are proportional to the square root of the mass-to-charge ratio. The TOFMS achieves a mass resolution of 1004 +/- 21 at mass m = 40 (Ar+). The fragmentation of nitrogen by electron loss collisions of 1.33 MeV He+ is investigated. Two complete Coulomb fragmentation channels, N-2 (2+)-> N+ + N+ and N-2(3+)-> N2++ N+, are identified. The kinetic energy release (KER) distribution for channel N-2 (2+)-> N+ + N+ agrees well with previous studies. Systematic studies of impact ionization and fragmentation dynamics of atoms and molecules induced by few-MeV ion beams can be taken up using this setup.

The fragmentation dynamics of allene dications induced by electron loss collisions of 3 MeV He+ ions is studied. Utilizing cold target recoil ion momentum spectroscopy, the momenta of ionic fragments are measured and the kinetic energy release (KER) distributions for two complete and six incomplete fragmentation channels are obtained. For two complete two-body channels, the fragmentation mechanisms are analyzed by comparing the experimental KER distributions with available results of ab initio reaction path calculations. We analyze the fragmentation mechanisms of incomplete channels, with one undetected hydrogen atom, using the island slope in ion-ion coincidence time-of-flight maps and Newton diagrams. The data indicate that initial charge separation [s(i)] is the dominant mechanism. In particular, the KER peak values for the channels considered are found to be lower than those reported in previous experiments involving 50 keV u-1 Ne8+ ion impact and intense laser ionization. This may suggest that the e-e process dominates simultaneous projectile-target ionization in the present experiment. This process predominates at large impact parameters.

Accurate atomic mass data hold significant application value in various research fields, in which Penning trap mass spectrometry is considered the most precise experimental method. A cryogenic detection system is a key component for reading out the image charge of charged particles in Penning traps using the Fourier transform ion cyclotron resonance technique. In this paper, we present the development and characteristics of this detection system, which includes a superconducting resonator and cryogenic low-noise amplifiers. The resonator consists of delicately woven thin NbTi wires configured into a multilayer helical coil, offering a quality factor of 98004 at around 1 MHz. Low-noise amplifiers are developed based on GaAs field effect transistors, exhibiting amplification factors greater than 27 dB with a power consumption of approximately 6 mW in the frequency range of 0.1 to 10 MHz. The lowest input voltage noise is 0.8 nV/ √(Hz) at 1 MHz. The fabrication process, operation, and measurements are elucidated in detail.

The collision of highly charged ions with neutral atoms or molecules is a fundamental process of quantum transition among multi-centers,which is characterized by unique features of a strong Coulomb field condition at the atomic scale,multi-channel involvement,and highly excited state population.Such collision processes are prevalent in various extreme matter environments,such as hot astrophysical plasmas,fusion,and fission processes,and serve as crucial diagnostic tools.Consequently,studying atomic collisions with highly charged ions is vital for testing fundamental collision theories and understanding complex physics in extreme-matter environments.Here,we reviewed theoretical and experimental studies of charge exchange dynamics and high-fidelity data production related to total cross-sections,state-selective cross-sections,and X-ray emission over recent decades.Additionally,we highlight the significance of laboratory studies in modeling cometary X-ray emission in the solar system and diffuse X-ray emission from other astrophysical objects.Finally,we are able to conclude that charge exchange is ubiquitous,and its studies can enhance the understanding of quantum transition dynamics in strong Coulomb fields and astrophysical X-ray modeling.

Investigating molecular fragmentation mechanisms and the kinetic energy distributions of fragments can offer crucial insights into their roles in plasma physics,radiation-induced damage in biological tissues,and interstellar chemistry.In this study,we conduct the experiments on collision between 3 keV/u Ar8+ions and CH3F molecules by using a cold target recoil ion momentum spectrometer(COLTRIMS). We focus on the three-body fragmentation channel H++CH2++F+resulting from C-F and C-H bond cleavage in CH3F3+ions,and measure the three-dimensional momentum vectors of all fragment ions.The fragmentation mechanism involved is analyzed using ion-ion kinetic energy correlation spectra,Newton diagrams,Dalitz plots,and other correlation spectra. Our results reveal two different dissociation mechanisms for the H++CH2++F+channel,i.e.concerted fragmentation and sequential fragmentation,with the former one being dominant.In the sequential fragmentation process,H+and the intermediate CH2F2+are firstly formed,followed by further fragmentation of the intermediates into CH2+and F+.No sequential pathways involving HF2+or CH32+intermediates are identified.Furthermore,we observe two types of concerted fragmentation processes with different dynamical characteristics,suggesting that hydrogen atoms in CH3F3+may occupy different chemical environments.This phenomenon can originate from either molecular isomerization producing different structural geometries or the Jahn-Teller effect leading to inequivalent C-H bonds.This study reveals the three-body dissociation dynamics of CH3F3+induced by highly charged ion collisions,highlighting the significant role of the Jahn-Teller effect or molecular isomerization in the ionic dissociation of polyatomic molecules.

The charge exchange (CX) collision cross sections of highly charged ions (HCIs) and neutral atoms play a crucial role in fundamental and applied physics. An experimental study on single-and double-electron capture (SEC and DEC) processes for C4+(1s2)- He in collision energy between 2.3 and 33.3 key u-1 was performed via the cold-target recoil-ion momentum spectroscopy (COLTRIMS) apparatus at Fudan University. For SEC, the measurement showed an increasing tendency of electrons to be captured into higher-n states because of the opening of new CX channels with the rise of impact energy. In the measurement of DEC, a similar gradually increasing trend was recognized from the contribution of highly excited states. A turning point was identified where the impact energy is around 20 key u-1, beyond which the contribution of DEC to the ground state 2s2 became minimal, while that from higher excited states became dominant. In addition, we also found significant contributions to DEC arising from asymmetric configurations. This might be linked to the vital role that electron-electron interaction plays in the formation of asymmetric configurations, as described by the two-active-electron semiclassical atomic-orbital close-coupling theory. These observations serve as experimental benchmarks for theoretical calculations and plasma spectral analysis in various environments.

Nuclear excitation by electron capture (NEEC) is an important nuclear excitation mechanism which still lacks conclusive experimental verification. This is primarily attributed to strong background x/gamma-ray noise and competing nuclear excitation processes which would overshadow the signals in various environments that NEEC takes place. Here, we propose an experimental approach to observe the NEEC process within a background-free environment. Through collisions with a highly compressed mono-energetic electron beam in an electron beam ion trap, nuclei may get excited to a long-lived isomeric state via the NEEC process. Subsequently, ions can be extracted and Penning-trap mass spectrometry employed to unambiguously detect the isomer. Our study focuses on the promising candidate 189Os, demonstrating measurable detection rates of the NEEC process and discussing the feasibility of the proposed approach. This new approach for observing the NEEC process may be realized in the near future.

We report an experimental investigation of three-body fragmentation of CO22+ into C+ + O+ + O induced by 1 MeV/u C4+ ions using a cold-target recoil-ion momentum spectroscopy. With ion-ion coincidence measurement, the C+ + O+ ion pair is identified. By taking advantage of the differences in momentum correlations among fragments from CO22+ and those from CO2q+ (q 3), we are able to disentangle the contributions from CO22+. Consequently, the kinetic energy release (KER) distribution for the channel CO22+ -> C+ + O+ + O is determined and compared with the available experimental and theoretical data. By filtering the events using an energy correlation map, the linear and molecular bending concerted fragmentation mechanisms, as well as the sequential fragmentation mechanism with CO+ as the intermediate, are identified with the aid of the kinetic energy distributions of the fragments and the Dalitz plots, and their relative branching ratios are evaluated. The linear concerted fragmentation with higher KER values was not observed in previous study of slow ion collisions. This difference indicates that the projectile velocity has a strong influence on the molecular fragmentation dynamics.

The precise measurement of the fine structure and radiative transition properties of highly charged ions(HCI)is essential for testing fundamental physical models,including strong-field quantum electrodynamics(QED)effects,electron correlation effects,relativistic effects,and nuclear effects.These measurements also provide critical atomic physics parameters for astrophysics and fusion plasma physics.Compared with the extensively studied hydrogen-like and lithium-like ion systems,boron-like ions exhibit significant contributions in terms of relativistic and QED effects in their fine structure forbidden transitions.High-precision experimental measurements and theoretical calculations of these systems provide important avenues for further testing fundamental physical models in multi-electron systems.Additionally,boron-like ions are considered promising candidates for HCI optical clocks.This paper presents the latest advancements in experimental and theoretical research on the ground state 2P3/2-2P1/2 transition in boron-like ions,and summarizes the current understanding of their fine and hyperfine structures.It also discusses a proposed experimental setup for measuring the hyperfine splitting of boron-like ions by using an electron beam ion trap combined with high-resolution spectroscopy.This proposal aims to provide a reference for future experimental research on the hyperfine splitting of boron-like ions,to test the QED effects with higher precision,extract the radius of nuclear magnetization distribution,and validate relevant nuclear structure models.

Electron capture in the collision of highly charged ions with atoms and molecules is a fundamental process related to the electron transition between bound states belonging to two atomic-centers.The X-ray emission after electron capture is important for X-ray astrophysical modeling,fusion plasma diagnostics,and ion irradiated biophysics.In the past few decades,momentum-imaging cold-target recoil ion momentum spectroscopy has been a significantly developed technique and widely used to measure the quantum state-selective population in electron capture processes.Based on the cold target recoil ion momentum spectroscopy installed on the 150 kV highly charged ion platform in Fudan University,Shanghai City,China,the state-selectivity of double electron capture in the bombardment of 1.4-20 keV/u Ar8+on He is measured,and the relative cross sections of the 3l3l'to 3l7l'double excited states are obtained.It is found that with the increase of collision energy,more quantum state-selectivity channels are open in the double electron capture of Ar8+-He collision.It is also found that the relative cross section of the quantum state population is strongly dependent on the collision energy of the projectile ion.The present measurements not only enrich the state-selective cross-sectional library and collision dynamics of highly charged ion charge exchange processes,but also provide experimental benchmarks for existing theoretical calculations.

To extend the research on the atomic structure of tungsten ions with an open 4f-shell, we conducted an investigation into the M1 transitions originating from the ground configuration of W23+ using the SHHtscEBIT in the 420–600 nm range. Three different calculational methods, including the relativistic configuration interactions (RCI) and the relativistic many-body perturbation theory (RMBPT) implemented in FAC, as well as the multiconfiguration Dirac-Hartree-Fock (MCDHF) in GRASP2018, are performed to investigate the energy level structure of W23+. The excitation energies obtained through RCI and RMBPT are in good agreement, with an average difference of 0.62 %. When comparing RCI and MCDHF, this difference increases to 1.20 %. The observed 12 lines of W23+are identified using the detailed collisional-radiative model with the atomic data calculated by RCI. The average wavelength deviations from experimental results are 0.77 %, 1.38 %, and 2.02 % for RCI, RMBPT, and MCDHF, respectively.

As one kind of ubiquitous collision system, O6+ + He deserves attentive study due to its theoretical significance in fundamental physics and applied value in fields like astrophysics or plasma physics. Specifically, in the case of the electron capture process, while a considerable number of measurements and calculations have focused on single electron capture, research is still inadequate on the double electron capture which contributes nearly 10% to the overall electron capture. In this work, a two-active-electron semiclassical asymptotic-state close-coupling method is used to calculate the total and l-resolved state-selective double electron capture cross sections of O6+ + He collisions in the energy range 0.5-100 keV/u, accompanied by experimental measurements in the energy range 2.63-37.5 keV/u with an uncertainty of 16%, in good consistency. These theoretical and experimental data can fill gaps in the database of double electron capture in O6+ + He collisions and provide insights for improving theoretical models in further research.

The Flexible Atomic Code (FAC) is developed to extend the computational scale based on the Relativistic Configuration Interaction (RCI) in the theoretical study of W9+ atomic structure. The results of the lowest 144 energy levels are consistent with the GRASP calculations, with a mean deviation of 2.11 %. The reference configuration selections for central potential optimization in the RCI calculations at different computational scales are studied. The contributions of the inner core 4d to the energy levels of the RCI calculations are also investigated. The mean deviation of the theoretical wavelengths of five M1 transitions of W9+ from the experimental results is within 0.86 %. This work demonstrates the feasibility of improving the accuracy of RCI calculations of complex atomic systems through large-scale computation based on FAC, which is helpful for experimental spectral line identifications and spectrum synthesis in many fields of plasma spectroscopy.