We describe astra (attosecond transitions), a close-coupling approach to molecular ionization that uses many-body transition-density matrices between ionic states with arbitrary spin and symmetry, in combination hybrid integrals Gaussian numerical orbitals, efficiently evaluate photoionization observables. Within the transition-density-matrix approach, evaluation of interchannel coupling is exact does not depend on size configuration-interaction space ions. Thanks these two crucial features,...

The perturbed relativistic coupled-cluster (PRCC) theory is applied to calculate the electric dipole polarizabilities of alkaline Earth metal atoms. Dirac-Coulomb-Breit atomic Hamiltonian used and we include triple excitations in (RCC) theory. theoretical issues related excitation cluster operators are described detail also provide details on computational implementation. PRCC results good agreement with experimental previous results. We, then, highlight importance considering Breit interaction for

We numerically study models of ${\mathrm{H}}_{2}$ and $\mathrm{LiH}$ molecules, aligned collinearly with the linear polarization external field, to elucidate possible role correlation in enhanced-ionization (EI) phenomena. Correlation is considered at different levels approximation time-dependent generalized-active-space configuration-interaction method. The results our studies show that enhanced ionization occurs multielectron molecules important, they also demonstrate significant...

We use perturbed relativistic coupled-cluster (PRCC) theory to calculate the electric dipole polarizability of noble-gas atoms Ar, Kr, Xe, and Rn. also provide a detailed description nonlinear terms in PRCC consider Dirac-Coulomb-Breit atomic Hamiltonian for calculations. find that largest contribution from Breit interaction is 0.1$%$, case As we go Ar Rn, based on pattern random-phase-approximation effects, contraction outermost ${p}_{1/2}$ orbitals due corrections discernible without any ambiguity.

We use the perturbed relativistic coupled-cluster (PRCC) theory to compute electric dipole polarizabilities $\ensuremath{\alpha}$ of Zn, Cd, and Hg. The computations are done using Dirac-Coulomb-Breit Hamiltonian with Uehling potential incorporate vacuum polarization corrections. To assimilate self-energy corrections we model operator Shabaev et al. [Phys. Rev. A 88, 012513 (2013)]. triple excitations included perturbatively in PRCC nonperturbatively unperturbed sector. Our results for all...

We develop a method based on the relativistic coupled-cluster theory to incorporate perturbative interaction no-pair Dirac-Coulomb atomic Hamiltonian. The is general and suitable any perturbation Hamiltonian in many electron atom or ion. Using this perturbed (PRCC) we calculate electric dipole polarizability, $\alpha$, of Neon. linearized PRCC results are very good agreement with experimental value. However, nonlinear shows larger uncertainty but it consistent observations from earlier works.

Attosecond noncollinear four-wave-mixing spectroscopy with one attosecond extreme ultraviolet (XUV) pulse and two few-cycle near-infrared (NIR) pulses was used to measure the autoionization decay lifetimes of inner valence electronic excitations in neon atoms. After a 43--48-eV XUV photon excites $2s$ electron into $2s2{p}^{6}\phantom{\rule{0.16em}{0ex}}(np)$ Rydberg series, broadband NIR couple $2s2{p}^{6}3p$ XUV-bright state neighboring $2s2{p}^{6}3s$ $2s2{p}^{6}3d$ XUV-dark states....

Using perturbed relativistic coupled-cluster (PRCC) theory we compute the ground state electric dipole polarizability, $\alpha$, of doubly ionized alkaline earth metal ions $\rm{Mg}^{2+}$, $\rm{Ca}^{2+}$, $\rm{Sr}^{2+}$, $\rm{Ba}^{2+}$ and $\rm{Ra}^{2+}$. In present work use Dirac-Coulomb-Breit atomic Hamiltonian also include Uehling potential, which is leading order term in vacuum polarization corrections. We examine correction to orbital energies arising from potential self-consistent...

We compute the ground-state electric dipole polarizability of group-IIIA ions using perturbed relativistic coupled-cluster (PRCC) theory. To account for effects and QED corrections, we use Dirac-Coulomb-Breit Hamiltonian with corrections from Uehling potential self-energy. The triple excitations are considered perturbatively in PRCC. Our PRCC results $\alpha$ good agreement previous theoretical all ions. From our computations find that nonlinear terms have significant contributions must be...

As a step toward the full \emph{ab-initio} description of two-photon double ionization processes, we present finite-pulse version virtual-sequential model for polyelectronic atoms. The relies on \emph{ab initio} single scattering states both neutral and ionized target system. proof principle benchmark, is applied to helium atom using {\tt NewStock} atomic photoionization code. results angularly integrated observables, which are in excellent agreement with existing TDSE (time-dependent...

We use the perturbed relativistic coupled-cluster theory to compute static electric dipole polarizabilities of singly ionized alkali-metal atoms, namely, Na${}^{+}$, K${}^{+}$, Rb${}^{+}$, Cs${}^{+}$, and Fr${}^{+}$. The computations Dirac-Coulomb-Breit Hamiltonian with no-virtual-pair approximation we also estimate correction polarizability arising from Breit interaction.

We employ a fully relativistic coupled-cluster theory to calculate the ground-state electric dipole polarizability and electron correlation energy of superheavy elements Cn, Nh$^+$ Og. To assess trend as function $Z$, we also energies for three lighter homologs--Zn, Cd Hg; Ga$^+$, In$^+$ Tl$^+$; Kr, Xe Rn--for each elements. The effects quantum electrodynamical corrections are included using Dirac-Coulomb-Breit Hamiltonian with from Uehling potential self-energy. triple excitations...

The linear correlations of Rydberg energy versus ionization potential [S. Chattopadhyay, P. Hochmann, and S. McGlynn, J. Chem. Phys. 65, 3341 (1976)] for p6, s2, d10s2 atoms are extended to open-shell s1, p5, p4, p3 systems. intercepts lead the concept an ’’average (or zero-order) member a group’’. slopes exhibit 1/(n̄*)2 dependency, where n̄* is average effective quantum number. This fact establishes connection with defect approach. molecular [P. H. Templet, H.-t. Wang, 62, 2588(1975)]...

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of ${\mathrm{H}}_{2}$, LiH, and HF molecules by intense near-infrared laser pulses using a three-dimensional description the systems with time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron quantum many-electron system under consideration. Our correlated diatomic shows that occurs at certain critical internuclear...

We have developed an all-particle Fock-space relativistic coupled-cluster method to calculate the properties of two-valence atoms and ions. Using we compute associated with a hyperfine-induced ${}^{1}{S}_{0}\ensuremath{\rightarrow}{}^{3}{P}_{0}^{o}$ clock transition in ${\mathrm{Al}}^{+}$. Our result ${}^{3}{P}_{0}^{o}$ metastable-state lifetime, $20.20\ifmmode\pm\else\textpm\fi{}0.91$ s, is excellent agreement experimental value, $20.60\ifmmode\pm\else\textpm\fi{}1.4$ s [T. Rosenband et...

It is shown, for several groups of atoms, that the term values low-energy Rydberg excitations are linearly correlated with corresponding ionization potentials. The form this relation T (μ,α,n)/Z2=anαIμ/Z2+bnα, where (μ,α,n) value nth lowest-energy state αth type (i.e., α a symmetry index) derived from μth cationic parent; Iμ potential; Z core charge; and anα bnα correlation coefficients. A discussion intercepts leads to concept an average (or zero-order) member given group atoms...

The pump-probe experiments enabled by x-ray free-electron lasers (XFEL) will allow us to directly observe correlated electronic motion with attosecond time resolution detecting photoelectron pairs in coincidence. In helium, the transition between nonsequential and sequential regime two-photon double ionization (TPDI) is well explained a virtual-sequential model. Much less known, however, about TPDI process more complex atoms. Recently, we extended model arbitrary light pulses [Chattopadhyay...

As a step toward the full \emph{ab-initio} description of two-photon double ionization processes, we present finite-pulse version virtual-sequential model for polyelectronic atoms. The relies on \emph{ab initio} single scattering states both neutral and ionized target system. proof principle benchmark, is applied to helium atom using {\tt NewStock} atomic photoionization code. results angularly integrated observables, which are in excellent agreement with existing TDSE (time-dependent...

We describe ASTRA (AttoSecond TRAnsitions), a new close-coupling approach to molecular ionization that uses many-body transition density matrices between ionic states with arbitrary spin and symmetry, in combination hybrid integrals Gaussian numerical orbitals, efficiently evaluate photoionization observables. Within the TDM approach, evaluation of inter-channel coupling is exact does not depend on size configuration-interaction space ions. Thanks these two crucial features, opens way...

The pump-probe experiments enabled by X-ray free-electron lasers (XFEL) will allow us to directly observe correlated electronic motion with attosecond time resolution detecting photoelectron pairs in coincidence. In helium, the transition between non-sequential and sequential regime two-photon double ionization (TPDI) is well explained a virtual-sequential model. Much less known, however, about TPDI process more complex atoms. Recently, we extended model arbitrary light pulses [Chattopadhyay...