arXiv subjects

S. F. Zhang

Publications and source records attributed to S. F. Zhang.

10 recordsLinked to original sources

arXiv2026

Positronium breakup versus hydrogen ionization in collisions with fast charged projectiles: a comparative study

We perform a comparative study of the breakup of positronium and ionization of atomic hydrogen by projectile-nuclei in the weak perturbation collision regime, $Z_p e_0^2/\hbar \ll v < c $ ($v$ is the collision velocity, $Z_p$ the projectile atomic number, $e_0$ the elementary charge and $c$ the speed of light). In this regime the only principal difference between the collisions with these atomic systems lies in the masses of their positively charged constituents. We have shown that the corresponding mass effects strongly influence the spectra of the target fragments and the total cross sections. This influence manifests itself via i) the significantly smaller binding energy in positronium resulting in smaller momentum transfers necessary to break the system, ii) a strong constructive interference between the inelastic scattering of the projectile on the electron and the positron in collisions with positronium that also increases the chances for the breakup and iii) the "passive" role of the hydrogen nucleus caused by its heavy mass that prohibits hydrogen ionization to proceed via the interaction between the projectile and the nucleus.

physics.atom-ph↗

arXiv2022

Precise determination of the 2s22p5-2s2p6 transition energy in fluorine-like nickel utilizing a low-lying dielectronic resonance

High precision spectroscopy of the low-lying dielectronic resonances in fluorine-like nickel ions were determined by employing the merged electron-ion beam at the heavy-ion storage ring CSRm. The measured dielectronic resonances are identified by comparing with the most recent relativistic calculation utilizing the FAC code. The first resonance at about 86 meV due to the dielectronic recombination via (2s2p6[2S1/2]6s)J=1 intermediate state was recognized. The experimental determination of the resonance position at 86 meV reaches an uncertainty of 4 meV, which allows precise determination of the 2s22p5[2P3/2] - 2s2p6[2S1/2] transition energy. The Rydberg binding energy of the 6s electron in the (2s2p6[2S1/2]6s)J=1 state is calculated by the multi-configurational Dirac-HartreeFock and stabilization methods. The determined transition energies are 149.056(4)exp(10)theo and 149.032(4)exp(6)theo, respectively. Moreover, the transition energy has also been calculated by fully relativistic and ab initio approaches. Individual theoretical contributions are evaluated by employing the core-Hartree and Kohn-Sham screening potentials, respectively. High-order QED and correlation effects contribute prominently to the total transition energy. The present DR precision spectroscopy study at the CSRm paves the way for future precision measurements of atomic energy levels with heavier highly charged ions.

physics.atom-ph↗

arXiv2022

Fragmentation of the $^4$He$_2$ dimer by relativistic highly charged projectiles in collisions with small kinetic energy release

We study theoretically the fragmentation of the helium dimer, $^4$He$_2$, into singly charged ions in collisions with relativistic highly-charged projectiles. We discuss the main mechanisms driving this process with the focus on the fragmentation caused by direct ionization of both atomic sites of the dimer in a single collision with the projectile. This direct mechanism dominates the He$_2$ $\to $ He$^+$ + He$^+$ breakup events with the kinetic energies of the emerging ionic fragments below $4$--$5$ eV. We explore the energy and angular distributions of the He$^+$ ions produced in collisions with $1$ and $7$ GeV/u U$^{92+}$ projectiles and show that their shape is significantly affected by relativistic and higher-order effects in the interaction between the projectile and the dimer. We also show that the shape of the energy spectrum is quite sensitive to the binding energy of the He$_2$ dimer which can be exploited for its precise determination. The contribution of the direct mechanism to the total cross section for the He$_2$ fragmentation by $1$ and $7$ GeV/u U$^{92+}$ was calculated to be $3.65$ Mb and $2.4$ Mb, respectively, representing roughly half of this cross section.

physics.atom-ph↗

arXiv2021

Measurement of n-resolved State-Selective Charge Exchange in Ne(8,9)+ Collision with He and H2

Charge exchange between highly charged ions and neutral atoms and molecules has been considered as one of the important mechanisms controlling soft X ray emissions in many astrophysical objects and environments. However, for modeling charge exchange soft X ray emission, the data of n and l resolved state selective capture cross sections are often obtained by empirical and semiclassical theory calculations. With a newly built cold target recoil ion momentum spectroscopy (COLTRIMS) apparatus, we perform a series of measurements of the charge exchange of Ne(8,9)+ ions with He and H2 for collision energy ranging from 1 to 24.75 keV/u. n resolved state selective capture cross-sections are reported. By comparing the measured state selective capture cross sections to those calculated by the multichannel Landau Zener method (MCLZ), it is found that MCLZ calculations are in good agreement with the measurement for the dominant n capture for He target. Furthermore, by using nl resolved cross sections calculated by MCLZ and applying l distributions commonly used in the astrophysical literature to experimentally derived n resolved cross sections, we calculate the soft X ray emissions in the charge exchange between 4 keV/u Ne8+ and He by considering the radiative cascade from the excited Ne7+ ions. Reasonable agreement is found in comparison to the measurement for even and separable models, and MCLZ calculations give results in a better agreement.

astro-ph.GA↗

arXiv2020

Photo ionization of an atom passed through a diffraction grating

We consider photo ionization of an atom which, due to passing through a diffraction grating, is prepared in a multi-site state possessing a periodic space structure with alternating maxima and minima. It has been found that this process qualitatively differs from photo ionization of a 'normal' atom. In particular, the spectra of emitted electrons and recoil ions in this process display clear one- and two-particle interference effects. Moreover, there are also striking differences between the momentum distributions of these particles, which no longer mirror each other. The origin of all these features is discussed in detail. It is also shown that the information about the diffraction grating, which is encoded in the multi-site state of the atom, can be fully decoded by exploring the spectra of recoil ions whereas the photo-electron spectra contain this information only partially.

physics.atom-ph↗

arXiv2020

Probing atomic 'quantum grating' by collisions with charged projectiles

The wave function of an atom passed through a diffraction grating acquires a regular space structure and the interaction of another particle with this atom can be thought of as scattering on a 'quantum grating' composed of a single atom. Probing this 'grating' by collisions with charged projectiles reveals interference effects due to coherent contributions of its 'slits' to the transition amplitude. In particular, the spectra of electrons emitted from the atom in collisions with swift ions exhibit a pronounced interference pattern whose shape can be extremely sensitive to the collision velocity.

quant-ph↗

arXiv2019

Experimental study for leptonic and semileptonic decays in the charm sector

Leptonic and semileptonic decays in the charm sector have been well studied in recent years. With the largest data sample near $D\bar D$ threshold, precision measurements of leptonic and semileptonic decays of charm meson and baryon are perfromed at BESIII. Test for letpon flavor universality is also performed. Sensitivity for rare leptonic and semileptonic charm decays is significantly improved taking advantage of the huge statistics in LHCb and the $B$ factories.

hep-ex↗

arXiv2019

State-selective single-electron capture in 30 keV N3+-He collisions

The Cold-target recoil-ion momentum spectroscopy (COLTRIMS) has been employed to study the single-electron capture processes in collisions of N3+ ions with He atoms at an impact energy of 30 keV. The relative differential cross sections for the capture to different orbitals of N3+ ions are obtained and are compared with other experiments at low energy. The predictions of the molecular Columbic barrier model have been made. From the longitudinal momentum spectrum of recoil ions, the different electronic configuration was identified and the metastable projectile ions were distinguished. The single electron capture into the N2+ (1s22s22p 2P) state from the ground state N3+ (1s22s2 1S) projectile is the dominant reaction channel. The fraction of the metastable state N3+(2s2p 3P) of the incident projectile beam is about 46%.

physics.atom-ph↗

arXiv2018

Dynamic two-center resonant photoionization in slow atomic collisions

An additional channel opens for photoionization of atom $A$ by an electromagnetic field if it traverses a gas of atoms $B$ resonantly coupled to this field. We show that this channel, in which $A$ is ionized via resonant photoexcitation of $B$ with subsequent energy transfer to $A$ through two-center electron correlations and which is very efficient when $A$ and $B$ constitute a bound system, can strongly dominate the ionization of $A$ also in collisions where the average distance between $A$ and $B$ exceeds the typical size of a bound system by orders of magnitude.

physics.atom-ph↗

arXiv2013

Mutual ionization in atomic collisions near the electronic threshold

We study mutual ionization in collisions between atomic hydrogen and helium at impact velocities near the electronic threshold for this process (determined by the condition that kinetic energy of an equivelocity free electron is approximately equal to the sum of binding energies of active electrons in the projectile and target). We show that this process is substantially influenced by the Coulomb repulsion between the emitted electrons and that the atomic nuclei are very strongly involved in the momentum balance along the collision velocity.

physics.atom-ph↗