Time-dependent Feshbach method to study resonant photoionization of He with ultrashort laser pulses

ABSTRACT: A time-dependent Feshbach formalism is proposed to study the resonant photoionization of the helium atom using ultrashort laser pulses. This spectral method consist in solving the time-dependent Schr¨odinger equation by expanding the time-dependent wavepacket in terms of eigenfunctions defined in two orthogonal halfspaces: a bound-like resonant Q and a non-resonant scattering-like P. The latter eigenfunctions for the projected Hamiltonians QHQ and PHP are not indeed eigenfunctions of the total Hamiltonian, so that the electrostatic coupling QHP acts as a leaking operator Q→P responsible for the temporal decay of resonances into the underlying continuum, keeping the physical insight of the Fano-Feshbach time independent formalisms. This method allows not only for accurate descriptions of the resonance parameters (energies, widths and Fano shape parameters) but also for the temporal evolution of the photodynamics involved in the resonant photoionization when using short laser pulses. We illustrate the performance of the method by analyzing the temporal formation of i) the one-photon ionization cross section below the second ionization threshold and the buildup of Fano profiles and ii) the up-down asymmetry of photoelectron angular distributions resulting from interferences of S-, P- and D-waves after simultaneous photoexcitation and decay of the lowest 1Se, 1Po and 1De resonant states, by using two sequential laser pulses with XUV harmonic frequencies separated by a time delay

An analysis of Helium resonant states in terms of entropy, information, complexity and entanglement measures

ABSTRACT: Shannon entropies and Fisher information calculated from one-particle density distributions and von Neumann and linear entropies (the latter two as a measure of entanglement) computed from the reduced oneparticle density matrix are analyzed for the 1,3Se,1,3 Po and 1,3De Rydberg series of He doubly excited states below the second ionization threshold. We find that both Fisher information and entanglement measures are able to discriminate resonances pertaining to different (K, T)A series

Information and entanglement measures applied to the analysis of complexity in doubly excited states of helium

ABSTRACT: Shannon entropy and Fisher information calculated from one-particle density distributions and von Neumann and linear entropies (the latter two as measures of entanglement) computed from the reduced one-particle density matrix are analyzed for the 1,3 Se, 1,3 Po, and 1,3 De Rydberg series of He doubly excited states below the second ionization threshold. In contrast with the Shannon entropy, we find that both the Fisher information and entanglement measures are able to discriminate low-energy resonances pertaining to different 2(K,T )An2 series according to the Herrick-Sinano˘glu-Lin classification. Contrary to bound states, which show a clear and unique asymptotic value for both Fisher information and entanglementmeasures in their Rydberg series 1sn for n→∞ (which implies a loss of spatial entanglement), the variety of behaviors and asymptotic values of entanglement above the noninteracting limit value in the Rydberg series of doubly excited states 2(K,T )A n2 indicates a signature of the intrinsic complexity and remnant entanglement in these high-lying resonances even with infinite excitation n2→∞, for which all known attempts of resonance classifications fail in helium

Computation of resonant states using explicitly correlated coordinates in Be-like atomic systems

ABSTRACT: We present an efficient computational method to obtain accurate values for energy positions and widths of autoionizing states in Be-like atomic systems. The two-active (outer) electron wavefunction is expanded in terms of Hylleraas-type correlated configurations. The interaction with the 1s2 frozen core is represented through a model potential and the unphysical 1sn` series of virtual core states are removed by using a Phillips-Kleinman pseudopotential projector. A novel feature is that all matrix elements can be written in closed form. We illustrate the performance of our approach in computing doubly-excited states in Be and Ne6+ by using the stabilization method

Time-dependent theoretical description of molecular autoionization produced by femtosecond xuv laser pulses

ABSTRACT: We present a nonperturbative time-dependent theoretical method to study H2 ionization with femtosecond laser pulses when the photon energy is large enough to populate the Q1 (25–28 eV) and Q2 (30–37 eV) doubly excited autoionizing states. We have investigated the role of these states in dissociative ionization of H2 and analyzed, in the time domain, the onset of the resonant peaks appearing in the proton kinetic energy distribution. Their dependence on photon frequency and pulse duration is also analyzed. The results are compared with available experimental data and with previous theoretical results obtained within a stationary perturbative approach. The method allows us as well to obtain dissociation yields corresponding to the decay of doubly excited states into two H atoms. The calculated H(n=2) yields are in good agreement with the experimental ones

Plasma screening effects in molecular hydrogen: modified energies and lifetimes of doubly excited states

We perform a systematic study on the behaviour of energy positions and autoionization widths of metastable resonance states in the Hydrogen molecule subject to screened Coulomb interactions among all particles using an ab initio Feshbach configuration interaction method. We only focus on the Q1 series of doubly excited states lying between the first H2 +(1sσg) and the second H2 +(2pσu) ionization thresholds, for several spectroscopic molecular symmetries 1,3Σg u, 1,3Πg, u and 1,3Δg, u. Special attention is given to the efficient and accurate method for the evaluation of screened Coulomb two-electron integrals in molecules when using configurations in terms of molecular orbitals described using a B-spline basi

Using ultrashort xuv laser pulses to investigate symmetry breaking in one-photon single-ionization of H2

ABSTRACT: We have evaluated photoelectron angular distributions from xed-in-space molecular hydrogen exposed to ultrashort xuv laser pulses. The theoretical method is based on the solution of the time-dependent Schrodinger equation in a basis of stationary states that include all electronic and vibrational degrees of freedom. We conclude that the origin of the asymmetry in these angular distributions is the interference of the two dissociative ionization channels (1s(sigma)g and 2p(sigma)u) due to delayed ionization from the H2 doubly excited states

Molecular structure of one-electron diatomic molecules subject to plasma screening and its e ect on the dynamics

ABSTRACT: The effect of plasma screening on the electronic and vibrational structure of one-electron diatomic molecules is analyzed by including screened Coulomb interactions among all their particles in the solution of the Schrodinger equation. The breakdown of the well-known separability of the Schrodinger equation in confocal elliptical coordinates, the lost of long-range Coulomb interaction and the non-degeneracy in both the united atom and separated atom limits bring new elements into the analysis of the modified binding energies (electronic and vibrational), adiabatic correlation rules, molecular Stark mixings due to polarization effects, unexpected presence of molecular shape resonances and changes in non-adiabatic and radiative couplings upon the variation of the screening strength

Effect of potential screening on the H2 autoionizing states

ABSTRACT: We study the behavior of autoionizing states of the hydrogen molecule subject to screened Coulomb interactions using an ab initio Feshbach configuration interaction method. Special attention is given to the algorithms developed for the evaluation of (i) screened molecular orbitals expressed in terms of one-center expansions using B-spline polynomial basis functions and (ii) screened two-electron integrals between configurations expressed in terms of such molecular orbitals, by solving the screened Poisson equation. As an illustration of the method we focus on the lowest Feshbach resonance of the Q1 1+g series of doubly excited states of H2, which lies between the first H2 + (1sσg) and the second H2 + (2pσu) ionization thresholds. We show that Coulomb screening in the electron-proton interaction and between electrons may significantly alter the resonance position and autoionizing decay as a function of internuclear distance. In general, screening increases the resonance lifetime. However, when electron-proton screening dominates over electron-electron screening, we find that the Q1 resonance acquires a pronounced shape-resonance character at internuclear distances where the resonance approaches the lower ionization threshold, thus leading to a pronounced decrease of its lifetime