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With the development of the intense light source, such as free electron lasers, the experiments on the nonlinear process in atomic photo absorption in the XUV and X-ray region became more and more feasible. As one of the simplest possible nonlinear processes, the sequential two-photon double ionization, in which the first photon produces an ion which is subsequently ionized by the second photon, attracts increasing attention of theorists and experimentalists. Study on the angular distributions and angular correlations of the photoelectrons in the sequential two-photon double ionization process are especially attractive, which provides valuable information about the electronic structure of atom or molecule systems and allows the obtaining of additional information about mechanism and pathway of the two-photon double ionization. In this paper, the expression for the photoelectron angular distribution in a sequential two-photon process is given based on the multi-configuration Dirac-Fock method and the density matrix theory. And then, the relativistic calculation program for photoelectron angular distribution is further developed with the help of the program packages GRASP2K and RATIP which are based on the multi-configuration Dirac-Fock method. By using this code, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically. The cross section, magnetic cross section, alignment of residual ions and the asymmetry parameter of the photoelectron angular distribution, each as a function of photon energy, for the first and the second step of sequential two-photon double ionization of argon are presented. The calculations predict that the alignment has a maximum value and the asymmetry parameter has a minimum value in the region of the cooper minimum. The angular distribution of the first step ionization for Ar atom and the second step ionization for Ar+ ion are given at 33.94 eV and 55.34 eV photon energy, respectively. In addition, the difference in property between the angular distributions of the first photoelectron in sequential two-photon double ionization and in conventional one-photon single ionization is discussed. The present calculated results are compared with other available results, showing that they are in good agreement with each other. The results of this paper will be helpful in studying nonlinear processes in the XUV range.

In this paper, the capabilities of laser-induced break down spectroscopy (LIBS) for rapid analysis to multi-component plant are illustrated using a 1064 nm laser focused onto the surface of folium lycii. Based on homogeneous plasma assumption, nine of essential micronutrients in folium lycii are identified. Using Saha equation and Boltzmann plot method electron density and plasma temperature are obtained, and the irrelative concentration (Ca, Mg, Al, Si, Ti, Na, K, Li, and Sr) are obtained employing a semi-quantitative method.

A detailed investigation of plasma screening effects on atomic structure and transition properties are presented for He-like ions embedded in dense plasma environment. Multi-configuration Dirac-Fock calculations were carried out for these ions by considering a Debye-Huckel potential. A large-scale relativistic configuration-interaction method is adopted to calculate transition energies and transition probabilities and to allow for a systematic improvement of the calculations. Comparison of the presently calculated results with others, when available, is made.

During the last decades, the electron impact excitation (EIE) process has aroused much interest in various research areas. This process is crucial to the diagnoses of astrophysical and laboratory plasmas. Moreover, the EIE studies play an important role in understanding the quantum electrodynamic, many-electron, and hyperfine interaction effects in heavy atomic systems. As is well known, when ions are excited by collisions with a unidirectional beam of electrons, the magnetic sublevels of the excited state may be populated with nonstatistical probability. In the decay of the excited state, the emitted radiation is found to be anisotropic and polarized. From the analysis of the polarization, valuable information can be obtained. These properties have become indispensable tools for the diagnosis of plasma state and the analysis of complex spectrum formation mechanism. Up to now, however, most of studies have dealt with the linear polarization of X-ray radiation. Fewer publications have reported the circular polarization. Moreover, theoretical studies of the characteristic X-ray emission have just dealt with ions having zero nuclear spin, or have simply omitted all contributions that arise from such a spin. It is known that some kinds of ions each have a nuclear spin I 0. Owing to the hyperfine coupling, new decay channel will be open, namely, hyperfine-induced transition. It is thus important to analyze how the hyperfine interaction affects the polarization properties of X-ray radiation. In this study, we present a systematically theoretical analysis of the polarization and angular distribution of X-ray radiation during the hyperfine-induced transition. The calculations are performed by using a fully relativistic distorted-wave method. Special attention is paid to the studies of angular correlations and polarization properties of the 1s2p 3P2 Fi=3/2 1s2 1S Ff=1/2 decay for highly charged He-like Sc19+ and 205Tl79+ ions with nuclear spin I=1/2 following impact excitation by a completely longitudinally-polarized electron beam. Two effects, i.e.the BI and the mutipole mixing between the leading M2 decay and hyperfine-induced E1 decay, on the polarization of the emitted radiation are discussed. Our results show that both the BI and the E1-M2 interference effects may significantly affect the polarization and angular emission pattern of the transition line. For example, the BI and the E1-M2 mixing lead the circular polarization to increase by about 50% and 40% for 205Tl79+ ions, respectively. With the development of the X-ray detectors, the measurement on the polarization during the hyperfine-induced transition becomes feasible. We hope that the present results would be useful in resolving some disagreement between the theories and experiments relating to the polarization properties of the X-ray radiation.

The multi-configuration Dirac?Fock (MCDF) method is implemented to study doubly excited 2s2p 1,3P1 resonances of the helium atom and the interference between photoionization and photoexcitation autoionization processes. In order to reproduce the total photoionization sprectra, the excited energies from the ground 1s2 1S0 state to the doubly excited 2s2p 1,3P1 states and the relevant Auger decay rates and widths are calculated in detail. Furthermore, the interference profile determined by the so-called Fano parameters q and p2 is also reproduced. Good agreement is found between the present results and other available theoretical and experimental results. This indeed shows a promising way to investigate the Fano resonances in photoionization of atoms within the MCDF scheme, although there are some discrepancies in the present calculations of the 2s2p 3P1 state.

This paper calculates the transition wavelengths and probabilities of the two-electron and one-photon (TEOP) transition from the (3s1/2-1 4dj) J = 1,2 to (3p3/2-1 4s1/2)J = 1 and the (3p1/2-1 4s1/2) J = 1 to (3dj-1 4dj')J = 1,2 for highly charged Ni-like ions with atomic number Z in the range 47 ≤ Z ≤ 92. In the calculations, the multi-configuration Dirac–Fock method and corresponding program packages GRASP92 and REOS99 were used, and the relativistic effects, correlation effects and relaxation effects were considered systematically. It is found that the TEOP transitions are very sensitive to the correlation of electrons, and the probabilities will be enhanced sharply in some special Z regions along the isoelectronic sequence. The present TEOP transition wavelengths are compared with the available data from some previous publications, good agreement is obtained.

In a recent XUV photoabsorption spectrum of Cs III ions by Cummings and O'Sullivan [2001 J. Phys. B 34 199], rather large linewidths were found for the 4d95s25p6 – 4d105s25p5 transition which are quite in disagreement with corresponding quasi-relativistic multiconfiguration Hartree–Fock (MCHF) calculation. In the present work, a detailed multiconfiguration Dirac-Fock study has been carried out to explore this discrepancy. Owing to the detailed consideration of electron correlation effects, some 'forbidden' Auger decay channels, such as 4d105s25p35d and 4d105s05p6, would become 'open'. As a result, remarkable improvement of the linewidths has been obtained in our calculation. Furthermore, the theoretical Auger spectrum of the 4d95s25p6 core-excited states of Cs III ions is given in the present work.

The photo-excitation and Auger decay processes of inner-shell double vacancy states 1s2s2p6(1,3S)3s3p of neutral neon atoms have been studied theoretically. Multi-configuration Dirac-Fock (MCDF) calculations have been carried out, with electron correlation effects taken into consideration. The relaxation of core and excited orbitals and configuration interaction are found to be crucial to creating the double vacancy states by single photo-absorption. The predominant decay paths for the double vacancy states turn out to be of the LLM Auger decay to 1s 2s22p53s(3p), KLL Auger decay to 1s22s2p43s3p, and KLM Auger decay to 1s22p63s(3p). They lead to further Auger decay, creating the neon ions of multiple charge states. For both double and single vacancy states the spectator type of Auger process is dominated in all the Auger decay processes. Theoretical Auger electron spectra are presented for further investigations, experimental and theoretical.