[en] The photoionizaton processes of lithium isoelectronic sequence (Be⁺, B²⁺, C³⁺, N⁴⁺, O⁵⁺, and F⁶⁺) under the influence of plasma environments are explored using the method of complex coordinate rotation in combination with the model potential approximation. The photoionization cross sections compared to existing theoretical predictions and varied with Debye screening lengths are reported. Under the perturbation of plasmas with certain Debye screening lengths, Cooper minima are uncovered in photoionization cross-section curves of the ground-state Li-like ions, in which the Cooper minima are absent in the respective free ion cases. The relations between the appearance of Cooper minima and the instability of the ground states due to plasma environments are discussed.

[en] Although a great deal of effort has been devoted to investigating dipole photoionization of plasma-embedded atoms, far less is known about the corresponding quadrupole transitions. In the present work, quadrupole photoionization processes for the ground and excited states of hydrogen atoms in Debye plasma are explored using the method of complex coordinate rotation. The plasma shielding effects on the quadrupole photoionization cross sections are reported for a variety of Debye screening lengths and compared to the dipole results accordingly. Under the perturbation of plasma screening, shape resonances and Cooper-type minima occurring in both dipole and quadrupole photoionization cross sections are presented and discussed. Comparisons are made to other theoretical calculations for the dipole photoionization with good agreement. The present quadrupole results are the first predictions for hydrogen photoionization in Debye plasmas.

[en] This paper describes the research works performed in my group in recent years in the theoretical methods and computational schemes for investigations of resonances in few-body atomic systems. These methods include the complex absorbing potential method, the stabilization method, and the method of complex scaling. In addition to atomic resonances, this paper also describes some of our recent works on investigations of Borromean bindings in three-body atomic systems. By systematically changing the two-body interaction from the long range Coulomb potential to the short range screened Coulomb (Yukawa) potential, we have investigated the Borromean bindings for hydrogen molecule ion H₂⁺ and the muonic molecular ions. (author)

[en] Using three dimensional test particle simulations, the characteristics and essential conditions under which an electron, in a vacuum laser beam, can undergo a capture and acceleration scenario (CAS) have been examined. When a₀∼>100 the electron can be captured and violently accelerated to energies ∼>1 GeV, with an acceleration gradient ∼>10 GeV/cm, where a₀=eE₀/m_eωc is the normalized laser field amplitude. The physical mechanism behind the CAS is that diffraction of the focused laser beam leads to a slowing down of the effective wave phase velocity along the captured electron trajectory, such that the electron can be trapped in the acceleration phase of the wave for a longer time and thus gain significant energy from the field

[en] Multiphoton processes, where transparency appears, have long fascinated physicists. Plasma screening effects are investigated on three-color three-photon bound-bound transitions in hydrogen atom embedded in Debye plasmas; where photons are linearly and circularly polarized, two left circular and one right circular. All possible combinations of frequency and polarization are considered. Analytical wave functions are used for initial and final states along with the pseudostate wave functions for intermediate states. The analytical wave functions are obtained from the modified wave functions for screening Coulomb potential (Debye model) using Ritz variational method. Here, we have found three-photon transparency for lower values of Debye length. This type of phenomenon occurs due to energy level shifting in the presence of Debye plasma environments. The description of resonance enhancements and three-photon transparency is reported in the present context along with the region of resonance enhancements and three-photon transparency.

[en] The resonance states of systems consisting of three identical bosons and three fermions interacting with an attractive 1/r potential are investigated using highly correlated exponential basis functions. The method of complex-coordinate rotation is used to extract energies and widths for the resonances lying below the N = 3 excited state threshold of the respective two-body subsystem. We have obtained a total of 24 S-wave resonances for the three-boson system and a total of 19 S-wave resonances in total spin-3/2 (the ⁴S state) and a total of 15 S-wave resonances in total spin-1/2 (the ²S state) below the N = 3 threshold. For the resonances associated with the N 3 threshold, we have obtained two, three and four series of Rydberg states for the three-boson system, three-fermion system for ²S states and three-fermion system for ⁴S states, respectively. The resonance energies and widths of these states for the three-body self-gravitating bosons and fermions are reported

[en] We consider weakly coupled plasmas, characterized by Debye-Huckel model potential, and an external electric field along z-axis. Due to plasma environment the energy levels of atom are shifted up, bound states are merged to continuum. For external electric field the excited energy levels also split up; degenerate energy eigenvalues become nondegenerate. In the presence of external electric field, energy levels are shifted up and down, except ground state. The ground state energy value is shifted only down. Therefore, it is very interesting to study the combined effect of plasmas and external electric field on a simple atom (hydrogen). To calculate the energy levels and the corresponding states, we expand the wave function in terms of linear combination of the basis functions. The basis is generated by hydrogenic wave functions. Here, we estimate various plasma surroundings and electric field strengths. We observe converged results for the basis size 45, with angular momentum states up to eight.

[en] The asymptotic ratios of double-to-single photoionization cross sections of plasma-embedded helium atoms at very high but nonrelativistic photon energies are calculated for different shielding parameters that are based on the Debye model for weakly coupled plasmas. The ratio of double-to-single photoionization for free atom is determined as 1.644%, in good agreement with the synchrotron measurements. The asymptotic ratio of double-to-single photoionization cross sections for different screening parameters are reported

[en] Two S-wave resonances in the positron-helium system are calculated using the stabilization method. A model potential for helium is used to represent the He⁺ ion core and the outer electron. Hylleraas-type wavefunctions are used to represent the correlation effects between the outer electron, the positron and the He⁺ core. Resonance energies and widths for two resonances lying below the Ps (n = 2) threshold are reported

[en] We have made an investigation on the ground states and the 2s²¹S^e resonance states of H^- in dense quantum plasmas. Exponential-cosine-screened Coulomb potentials (ECSCP) are used to represent the effective potential for a test charge in dense quantum plasmas. Ground-state energies and wavefunctions are determined within the framework of Ritz's variational principle by employing highly correlated wavefunctions to take into account the correlation effect of the charged particles. Ground-state energies are shown to converge with the increase of terms in the wavefunctions. We also report various expectation values of the coordinates of electrons in H^-. Resonance energies and widths for the doubly excited H^- for various values of the screening parameter are determined using the stabilization method by calculating the density of the resonance states. Results for resonance energies and widths are reported for the screening parameter in the range 0.0-0.15. Such a calculation for H^- is reported for the first time in the literature.