We report the transition energies and probabilities of satellite lines due to 2p2(3P0,1,2)→1s2p(3P0,1,2) transitions of He-like C,Ne,Al and Ar ions embedded in strongly coupled plasma (SCP) environment. The SCP environment is represented by the ion-sphere (IS) model and the IS radius is varied in such a way that plasma electron densities (∼1020−1026 per cm3) cover a wide range of experimental interest. Both relativistic and non-relativistic energies of 2p2(3P) and 1s2p(3P) states are reported. The relativistic MCDF results are obtained by modified GRASP2K code and the non-relativistic variational results are done by using explicitly correlated Hylleraas basis. The relativistic results are in good agreement with experimental data available in literature. The positions of intercombination lines in presence of dense plasma can be calculated from present results which are comparable with experimental measurements. The partial pressures experienced by the plasma-embedded ions are analyzed in the purview of available experimental observations. An empirical relation is reported to estimate partial pressure on ions in dense plasma environment.

We applied a relativistic configuration-interaction (CI) framework to the stabilization method as an approach for obtaining the autoionization resonance structure of heliumlike ions. In this method, the ion is confined within an impenetrable spherical cavity, the size of which determines the radial space available for electron wave functions and electron-electron interactions. By varying the size of the cavity, one can obtain the autoinization resonance position and width. The applicability of this method is tested on the resonances of He atom while comparing with benchmark data available in the literature. The present method is further applied on the determination of the resonance structure of heliumlike uranium ion, where a relativistic framework is mandatory. In the strong-confinement region, the present method can be useful to simulate the properties of an atom or ion under extreme pressure. An exemplary application of the present method to determine the structure of ions embedded in dense plasma environment is briefly discussed.

Bound and resonance states of symmetric three-body exotic \(pXX\) negative atomic ions \((X=\mu^{-}, \pi^{-}, K^{-})\) as well as exotic \(ppX\) positive molecular ions for total angular momentum \(J=0\), are studied in details under the framework of Stabilization method. The resonance states under consideration lie below \(N=2\) ionization threshold of the corresponding \(pX\) atom. The wave-function is expanded in correlated multi-exponent Hylleraas type basis set for explicit incorporation of \(p\)-\(p\), \(\mu\)-\(\mu\), \(\pi\)-\(\pi\) or \(K\)-\(K\) correlations. The methodology has been tested by estimating the parameters of the resonance states of \((p\mu\mu)^{-}\), \((pp\mu)^{+}\), \((p\pi\pi)^{-}\) and \((pp\pi)^{+}\) and comparing with the results existing in the literature. The interparticle interactions for all the systems under consideration are purely Coulombic.

Precise energy eigenvalues of metastable bound doubly excited 1,3Fe states originating from 2pnf (n = 4–6) configuration of helium-like ions (Z = 2–4) under weakly coupled plasma (WCP) environment have been estimated within the framework of Ritz variational method. The wavefunction is expanded in explicitly correlated Hylleraas type basis set. The screened Coulomb potential is considered mimic the WCP environment. The atomic systems tend towards gradual instability and the number of excited metastable bound states reduces with increasing plasma strength. The wavelengths corresponding to 2pnf (1,3Fe) → 2pn′d (1,3Do) (n = 4–6; n′ = 3–6) transitions occurring between doubly excited states of plasma embedded two-electron ions are also reported.

The ground state energy eigenvalues of the spatially confined symmetric three-body exotic ions \(Ps^-\) and \(H^+_2\) have been determined under the framework of Rayleigh-Ritz variational method. The spatial confinement is simulated by considering an impenetrable spherical box of varying radius around the ion. The electron-electron correlation in \(Ps^-\) and the protonproton correlation in \(H_+^2\) are properly taken care of by expanding the trial wavefunction in explicitly correlated Hylleraas type basis set. The critical radius and the corresponding critical pressure, at which the respective ions destabilize are also reported.

Bound and resonance states of helium atom have been investigated inside a quantum dot by using explicitly correlated Hylleraas type basis set within the framework of stabilization method. To be specific, precise energy eigenvalues of bound 1sns (1Se) (n = 1–6) states and the resonance parameters i.e. positions and widths of 1Se states due to 2sns (n = 2–5) and 2pnp (n = 2–5) configurations of confined helium below N = 2 ionization threshold of He+ have been estimated. The two-parameter (Depth and Width) finite oscillator potential is used to represent the confining potential due to the quantum dot. It has been explicitly demonstrated that the electronic structural properties become sensitive functions of the dot size. It is observed from the calculations of ionization potential that the stability of an impurity ion within a quantum dot may be manipulated by varying the confinement parameters. A possibility of controlling the autoionization lifetime of doubly excited states of two-electron ions by tuning the width of the quantum cavity is also discussed here.

Rayleigh–Ritz variational method has been employed to estimate precise energy-eigenvalues of spherically compressed two-electron atoms ( Z=1−10) embedded in Debye plasma with a view to modelling atom under dense plasma environment. The trial wave function is expanded in terms of explicitly correlated Hylleraas-type basis set satisfying Dirichlet's boundary condition. The combined effect of decrease in the size of spatial confinement domain and increase in Debye screening parameter pushes the system towards gradual destabilization and subsequent ionization or complete fragmentation of the system. Present results are in reasonable agreement with other results existing in literature. Within finite domain, the thermodynamic pressure experienced by the ions due to the plasma electrons is also estimated.

The Rayleigh-Ritz variational technique with a Hylleraas basis set is being tested for the first time to estimate the structural modifications of a lithium atom embedded in a weakly coupled plasma environment. The Debye-Huckel potential is used to mimic the weakly coupled plasma environment. The wave functions for both the helium-like lithium ion and the lithium atom are expanded in the explicitly correlated Hylleraas type basis set which fully takes care of the electron-electron correlation effect. Due to the continuum lowering under plasma environment, the ionization potential of the system gradually decreases leading to the destabilization of the atom. The excited states destabilize at a lower value of the plasma density. The estimated ionization potential agrees fairly well with the few available theoretical estimates. The variation of one and two particle moments, dielectric susceptibility and magnetic shielding constant, with respect to plasma density is also been discussed in detail.

A detailed analysis on the effect of spherical impenetrable confinement on the structural properties of two-electron ions in S-states have been done. The energy values of 1sns [n = 2-4] (3Se) states of helium-like ions (Z = 2-5) are estimated within the framework of Ritz variational method by using explicitly correlated Hylleraas-type basis sets. The correlated wave functions used here are consistent with the finite boundary conditions due to spherical confinement. A comparative study between the singlet and triplet states originating from a particular electronic configuration shows incidental degeneracy and the subsequent level-crossing phenomena. The thermodynamic pressure felt by the ion inside the sphere pushes the energy levels towards continuum. Critical pressures for the transition to strong confinement regime (where the singly excited two-electron energy levels cross the corresponding one-electron threshold) as well as for the complete destabilization are also estimated.

Calculation of energies and radiative decays was performed for several highly charge states of sulphur in order to identify experimental lines associated to double excited KK atomic states.

The analytic form of the electrostatic potential felt by a slowly moving test charge in quantum plasma is being derived. It has been shown that the potential composed of two parts: Debye-Huckel screening term and near-field wake potential which depends on the velocity of the test charge and the number density of the plasma electrons. Rayleigh-Ritz variational calculation has been done to estimate precise energy eigenvalues of hydrogen-like ion under such plasma environment. A detailed analysis shows that the energy levels are gradually moves to the continuum with increasing plasma electron density while level crossing phenomenon have been observed with the variation of ion velocity.

We have measured the x-ray spectra from highly charged Si, S and Cl ions in collisions with thin foils using a high-resolution x-ray spectrometer. The observed lines have been assigned to various transitions in H-, He- and Li-like ions. For proper identification of line positions, the theoretical calculations have been carried out using a state-of-the-art MCDF code including QED effects, with which the experimental data is in excellent agreement. We have also observed, for the first time, x-rays arising out of the decay of long-lived resonant states in the He-like ions of each species. Details will be presented.

Effect of strongly coupled plasma on the excitation energies and transition probabilities for the respective transitions 1s2:1Se → 1sns:3Se (n = 2, 3, 4) and 1s2:1Se → 1snp:3Po (n = 2, 3, 4) allowed by magnetic dipolar and quadrupolar excitations have been analyzed for the first time for the two-electron ions C4+, O6+, Ne8+, Mg10+, Si12+, and S14+. Time dependent Hatree-Fock theory within variational approach has been adopted for such a study. The effect of surrounding plasma has been treated through the standard Ion-Sphere (IS) model of the plasma where the plasma density is varied systematically from a low value to a pretty high value such that the respective excited states go over to continuum due to such a confinement. The effect of external pressure generated due to plasma confinement on the estimated spectral properties has been analyzed systematically.

The effect of spherical confinement on the ground state of helium-like ions (Z = 1-5) and the first four (S-1(e)) excited states for Z = 2-5 have been analyzed in detail by using correlated Hylleraas basis sets within the variational framework. The correlated wave functions used here are consistent with the finite boundary conditions due to spherical confinement. The present energy values and the thermodynamic pressure generated for the confined ions as well as the critical values of the confining radii close to the fragmentation limit can be set as a benchmark for future references.

We present an experimental determination of the 2p3d(P-1(0)) -> 1s3d(D-1(e)) x-ray line emitted from He-like Si, S, and Cl projectile ions, excited in collisions with thin carbon foils, using a high-resolution bent-crystal spectrometer. A good agreement between the observation and state-of-the-art relativistic calculations using the multiconfiguration Dirac-Fock formalism including the Breit interaction and QED effects implies the dominance of fluorescent decay over the autoionization process for the 2p3d(P-1(0)) state of He-like heavy ions. This is the first observation of the fluorescence-active doubly excited states in He-like Si, S, and Cl ions.

We present an experimental determination of the 2p3d(1Po)→1s3d(1De) x-ray line emitted from He-like Si, S, and Cl projectile ions, excited in collisions with thin carbon foils, using a high-resolution bent-crystal spectrometer. A good agreement between the observation and state-of-the-art relativistic calculations using the multiconfiguration Dirac-Fock formalism including the Breit interaction and QED effects implies the dominance of fluorescent decay over the autoionization process for the 2p3d(^{1}P^{o}) state of He-like heavy ions. This is the first observation of the fluorescence-active doubly excited states in He-like Si, S, and Cl ions.

The ground state energy eigenvalues of the symmetric three-body exotic negative ions p+π−π− and p+K−K− have been determined variationally for the first time using an explicitly correlated Hylleraas basis set. Ground state energies of Ps− and p+μ−μ− were also determined to check the accuracy of the present methodology by comparing those results to a few accurate earlier results for these systems.

The effect of strongly coupled plasma occurring in astrophysical context has been studied for the first time to estimate the energy levels of the autoionizing states of highly stripped astrophysically important ions Al11+, Si12+, P13+, S14+ and Cl15+ and also C4+ isoelectronic to helium. The transition energies corresponding to 1s 2:1Se → 2s 2:1Se, 2p 2:1De, 2s2p:1Po, 2s3d:1De and 2p3d:1Fo are analyzed with respect to different plasma densities using the ion sphere (IS) model of strongly coupled plasma. Transition energies are obtained from the position of the poles of a variational functional based on frequency dependent perturbation calculation probing the collective oscillation modes of the plasma embedded two electron ions. For the free ions corresponding to zero plasma coupling our calculated data agree well with those obtained from spectroscopic data while for the plasma embedded ions the data are new but follow systematic trend. The work has been performed in the domain of linear response theory. The analytical wave function of the doubly excited states have been obtained and may be useful for calculating various transition properties of the plasma embedded ions and also for estimating the rate coefficients for dielectronic recombination processes which play a major role in maintaining equilibrium in high temperature astrophysical or laser produced plasmas.