Qu Yi-Zhi

Positron-Impact Excitation of Hydrogen Atoms in Debye Plasmas

The positron-impact excitation of hydrogen atoms embedded in plasma environments is investigated using the close-coupling approximation from the low to intermediate energy region without including any positronium formation channel, and the excitation cross sections for 1s → 2s, 1s → 2p and 2s → 2p processes are calculated in a wide Debye parameter range. The screening interactions, described by the Debye–Huckel model, decrease the coupling matrix elements, resulting in the reduction of the excitation cross sections from a few percent to one magnitude of ten. This will alter remarkably the spectroscopy of hydrogen in intensity and position, which should be considered in the simulation and diagnostics under some specific plasma conditions.

Polarization and Angular Distribution of Lℓ X-Ray Following Inner-Shell 2p3/2 Photoionization of Magnesium-Like Ions

The inner-shell 2p3/2 photoionization and the subsequent decay of Mg-like Fe14+, Cd36+, W62+ and U80+ ions are studied theoretically within the multiconfiguration Dirac—Fock method and the density matrix theory. Special attention is paid to exploring the influence of the non-dipole terms which arise from the multipole expansion of the electron-photon interaction in the photoionization process. The results show that the non-dipole contribution to the total cross section, the magnetic sublevels cross section of the photoionization process, the degree of linear polarization and angular distribution of the subsequent characteristic x-ray radiation become more important with the increase of photons energy and atomic nuclear Z. Especially for the cross section and the degree of linear polarization, the non-dipole contribution arrives at 50% for U80+ at four time energy threshold units. However, for the angular distribution, the maximum contribution does not exceed 4%, even for U80+ ions.

Radiative Decay of Proton Colliding with Rb at Low Energies

The radiative decay and radiative charge transfer cross sections for H+ +Rb(5s) collisions are calculated by using the optical potential approach, the semiclassical and the fully quantum-mechanical methods, respectively, for the energy range 10−6−10eV. The radiative association cross sections are obtained by the cross section differences between the radiative decay and radiative charge transfer processes. The relevant molecular data are calculated from the multi-reference single- and double-excitation configuration interaction approach. The emission spectra at resonant and non-resonant energies are analyzed, then the isolated sharp and broad resonances can be identified by their rotational and vibration quantum numbers.

Resonant charge transfer in slow Li+−Li(2s) collisions*

The resonant charge transfer process for Li+–Li(2s) collision is investigated by the quantum-mechanical molecular orbital close-coupling(QMOCC) method and the two-center atomic-orbital close-coupling(AOCC) method in an energy range of 1.0 e V/u–104e V/u. Accurate molecular structure data and charge transfer cross sections are given. Both the allelectron model(AEM) and one-electron model(OEM) are used in the QMOCC calculations, and the discrepancies between the two models are analyzed. The OEM calculation can also give a reliable prediction of the cross sections for energies below 1 ke V/u.

Fast-electron-impact ionization process by 3p of hydrogen-like ions in Debye plasmas*

The plasma screening of fast-electron-impact-ionization by excited state (3p) of Hydrogen-like ions was investigated in the first Born approximation with a plasma screening length δ varying from 1000a0 to 10a0. The generalized oscillator strength densities showed dramatic changes: some accessional minima occurred along with a remarkable enhancement in certain continuum-energy domains. The double-differential cross sections exhibit not only the same structures as the Bethe surface for moderate and large momentum transfers, but also a broadened enhancement for small momentum transfers. The single-differential cross sections exhibit a near-zero-energy-enhancement and prodigious multiple-shape resonances, depending on the continuum energy and the plasma screening length. These features are analogous to those of the photo-ionization cross section. These findings, for both types of cross section, can be explained by processes associated with continuum electrons, as long as the potential has a short-range character.

Polarization Transfer in the 2p3/2 Photoionization of Magnesium-Like Ions

The inner-shell 2p3/2 ionization of Mg-like Fe14+, Cd36+, W62+ and U80+ ions by linear polarized light and the subsequent radiative decay are studied theoretically with the multiconfiguration Dirac—Fock (MCDF) method and the density matrix theory. Special attention is paid to exploring the influence of the polarization properties of the incident photon and the non-dipole term which arises from the multipole expansion of the electron-photon interaction on the properties of the subsequent x-ray radiation. The results show that there is a linear relationship between the degree of linear polarization of the radiative decay and ones of the incident light, which can be used for diagnosing the polarization of a light source. In addition, with the increasing photon energies, the non-dipole contribution to the alignment of the residual ions, the degree of linear polarization, as well as the angular distribution of the radiative decay following the inner-shell photoionization will also be increased.

Relationship between electromagnetically-induced transparency and Autler?Townes splitting in a Doppler-broadened system

We study the relationship between electromagnetically-induced transparency (EIT) and Autler–Townes (AT) splitting in a cascade three-level Doppler-broadened system. By comparing the absorption spectrum with the fluorescence excitation spectrum, it is found that for a Doppler-broadened system, EIT resonance cannot be explained as the result of quantum interference, unlike the case of a homogeneously broadened system. Instead, the macroscopic polarization interference plays an important role in determining the spectra of EIT and AT splitting, which can be explained within the same framework when being detected by the absorption spectra.

Macroscopic effects in electromagnetically-induced transparency in a Doppler-broadened system

We study the electromagnetically-induced transparency (EIT) in a Doppler-broadened cascaded three-level system. We decompose the susceptibility responsible for the EIT resonance into a linear and a nonlinear part, and the EIT resonance reflects mainly the characteristics of the nonlinear susceptibility. It is found that the macroscopic polarization interference effect plays a crucial role in determining the EIT resonance spectrum. To obtain a Doppler-free spectrum there must be polarization interference between atoms of different velocities. A dressed-state model, which analyzes the velocities at which the atoms are in resonance with the dressed states through Doppler frequency shifting, is employed to explain the results.

Nonradiative charge transfer in collisions of protons with rubidium atoms

The nonradiative charge-transfer cross sections for protons colliding with Rb(5s) atoms are calculated by using the quantum-mechanical molecularorbital close-coupling method in an energy range of 10−3 keV–10 keV. The total and state-selective charge-transfer cross sections are in good agreement with the experimental data in the relatively low energy region. The importance of rotational coupling for chargetransfer process is stressed. Compared with the radiative charge-transfer process, nonradiative charge transfer is a dominant mechanism at energies above 15 eV. The resonance structures of state-selective charge-transfer cross sections arising from the competition among channels are analysed in detail. The radiative and nonradiative charge-transfer rate coefficients from low to high temperature are presented.

Theoretical analysis of ionic autoionization spectra of lanthanum in the energy region of 90650-91500 cm-1

Eigenquantum defects µα and transformation matrix Uiα of La+ are calculated from the first principles by relativistic multichannel theory, and dipole matrix elements Dα are obtained by fitting the experimental spectra. With these parameters, ionic autoionization spectra of lanthanum via an intermediate state (Xe)5d6d 1P1 of La+ in the energy region of 90650-91500 cm -1 are calculated within the framework of multichannel quantum defect theory. Our calculated spectra are in general agreement with the experimental data.