Publications (50)111.35 Total impact

Article: Theoretical methods for attosecond electron and nuclear dynamics: applications to the H 2 molecule
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ABSTRACT: Attosecond science, born at the beginning of this century with the generation of the first bursts of light with durations shorter than a femtosecond, has opened the way to look at electron dynamics in atoms and molecules at its natural timescale. Thus controlling chemical reactions at the electronic level or obtaining timeresolved images of the electronic motion has become a goal for many physics and chemistry laboratories all over the world. The new experimental capabilities have spurred the development of sophisticated theoretical methods that can accurately predict phenomena occurring in the subfs timescale. This review provides an overview of the capabilities of existing theoretical tools to describe electron and nuclear dynamics resulting from the interaction of femto and attosecond UV/XUV radiation with simple molecular targets. We describe one of these methods in more detail, the timedependent Feshbach closecoupling (TDFCC) formalism, which has been used successfully over the years to investigate various attosecond phenomena in the hydrogen molecule and can easily be extended to other diatomics. In addition to describing the details of the method and discussing its advantages and limitations, we also provide examples of the new physics that one can learn by applying it to different problems: from the study of the autoionization decay that follows attosecond UV excitation to the imaging of the coupled electron and nuclear dynamics in H2 using different UVpump/IRprobe and UVpump/UVprobe schemes. 
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ABSTRACT: 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 twoelectron integrals in molecules when using configurations in terms of molecular orbitals described using a Bspline basis.  [Show abstract] [Hide abstract]
ABSTRACT: Shannon entropy and Fisher information calculated from oneparticle density distributions and von Neumann and linear entropies (the latter two as measures of entanglement) computed from the reduced oneparticle density matrix are analyzed for the Se1,3,Po1,3, and De1,3 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 lowenergy resonances pertaining to different (K,T)n2A2 series according to the HerrickSinanoğluLin classification. Contrary to bound states, which show a clear and unique asymptotic value for both Fisher information and entanglement measures in their Rydberg series 1snℓ 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 (K,T)n2A2 indicates a signature of the intrinsic complexity and remnant entanglement in these highlying resonances even with infinite excitation n2→∞, for which all known attempts of resonance classifications fail in helium.  [Show abstract] [Hide abstract]
ABSTRACT: The presence of net circular dichroism in the photoionization of nonchiral homonuclear molecules has been put in evidence recently through the measurement of molecularframe photoelectron angular distributions in dissociative photoionization of ${\text{H}}_{2}$ [Dowek et al., Phys. Rev. Lett. 104, 233003 (2010)]. In this work we present a detailed study of circular dichroism in the photoelectron angular distributions of ${\text{H}}_{2}$ and ${\text{D}}_{2}$ molecules, oriented perpendicularly to the propagation vector of the circularly polarized light, at different photon energies (20, 27, and 32.5 eV). Circular dichroism in the angular distributions at 20 and to a large extent 27 eV exhibits the usual pattern in which inversion symmetry is preserved. In contrast, at 32.5 eV, the inversion symmetry breaks down, which eventually leads to total circular dichroism after integration over the polar emission angle. Timedependent ab initio calculations support and explain the observed results for ${\text{H}}_{2}$ in terms of quantum interferences between direct photoionization and delayed autoionization from the ${\mathcal{Q}}_{1}$ and ${\mathcal{Q}}_{2}$ doubly excited states into ionic states $(1s{$\sigma${}}_{g}$ and $2p{$\sigma${}}_{u})$ of different inversion symmetry. Nevertheless, for ${\text{D}}_{2}$ at 32.5 eV, there is a particular case where theory and experiment disagree in the magnitude of the symmetry breaking: when ${\text{D}}^{+}$ ions are produced with an energy of around 5 eV. This reflects the subleties associated to such simple molecules when exposed to this fine scrutiny.  [Show abstract] [Hide abstract]
ABSTRACT: Shannon entropies and Fisher information calculated from oneparticle 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 S1,3(e),P1,3(o) and D1,3(e) 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. 
Article: Timedependent Feshbach method to study resonant photoionization of He with ultrashort laser pulses
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ABSTRACT: A timedependent Feshbach formalism is proposed to study the resonant photoionization of the helium atom using ultrashort laser pulses. This spectral method consist in solving the timedependent Schrodinger equation by expanding the timedependent wavepacket in terms of eigenfunctions defined in two orthogonal halfspaces: a boundlike resonant and a nonresonant scatteringlike . The latter eigenfunctions for the projected Hamiltonians H and H are not indeed eigenfunctions of the total Hamiltonian, so that the electrostatic coupling H acts as a leaking operator → responsible for the temporal decay of resonances into the underlying continuum, keeping the physical insight of the FanoFeshbach 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 onephoton ionization cross section below the second ionization threshold and the buildup of Fano profiles and ii) the updown asymmetry of photoelectron angular distributions resulting from interferences of S, P and Dwaves after simultaneous photoexcitation and decay of the lowest 1Se, 1P° and 1De resonant states, by using two sequential laser pulses with XUV harmonic frequencies separated by a time delay.  [Show abstract] [Hide abstract]
ABSTRACT: The stability of resonant states in Heliumlike atoms while under the influence of a plasma environment is studied. We analyze the variation of resonance parameters (energies and lifetimes) against the strength of the screening parameter within the DebyeHückel model. We use a Feshbachlike projection formalism with explicitly correlated CIwave functions to uncover the evolution of resonance parameters until they cross the upper Aq+(N=2) threshold, and a complex scaling method to analyze the survival of these Feshbach resonances across the threshold, which eventually makes them behave as shape resonances. 
Conference Paper: Reply to comment on 'Timeresolved resonant photoionization of He using a timedependent Feshbach method with ultrashort laser pulses'
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ABSTRACT: We study the resonant photoionization of the Helium atom subject to ultrashort laser pulses by using a Feshbach formalism in the time domain. We solve the projected timedependent Schrodinger equation in terms of a configuration interaction spectral method, with a total wave function expanded with configurations defined within boundlike ( ##IMG## [http://ej.iop.org/images/17426596/488/3/032023/jpconf14_488_032023_eqn001.gif] ) and scatteringlike ( ##IMG## [http://ej.iop.org/images/17426596/488/3/032023/jpconf14_488_032023_eqn002.gif] ) halfspaces. The method allows for accurate descriptions of both the atomic structure (energy positions and widths) as well as for the resonant photodynamics using ultrashort laser pulses. Special attention is given to the temporal formation of Fano profiles in the one and twophoton ionization cross sections. 
Article: Feshbach projection approach to study plasma effects on doubly excited autoionizing states in helium
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ABSTRACT: We have implemented a method based on the Feshbach formalism along with an explicitly correlated configuration interaction method to perform a systematic study on the behavior of resonance parameters (energies and lifetimes) of the autoionizing states of plasmaembedded He 1,3Se, 1,3Po, and 1,3De, as a function of the screening strength. In particular, we study the evolution of the lowest states in the series located below the He+(N=2) ionization threshold in the unscreened case. At variance with oneelectron atoms (where shape resonance widths vary monotonically with the screening strength) the evolution of the Auger width with respect to screening is found to be different for each series represented by (K,T)A pseudoquantum numbers until resonances merge into the upper electronic continuum, when crossing the He+(2s) threshold. We conclude from our ab initio calculations that, although resonances pertaining to the same (K,T)A series share a similar tendency in their widths against the screening strength, general propensity rules for the robustness of lifetimes, based on the isomorphic series in the (K,T)A classification, cannot be established in plasmaembedded helium.  [Show abstract] [Hide abstract]
ABSTRACT: Atomic autoionization following photoabsorption is a typical example of quantum interferences governed by electronelectron correlation. Coherence between direct photoionization and autoionization paths results in "Fano profiles", widely explored in atoms in the last 60 years. The advent of femto and attosecond laser technology made timeresolved images of the delayed electron ejection in autoionization accessible, leading to the reemergence of such studies in atomic systems. The counterpart molecular phenomena show the richness, as well as the complexity, added by nuclear motion, which may proceed on similar time scales. However, Fano profiles are usually absent in measured molecular photoionization cross sections and an unequivocal parametrization of molecular autoionization signatures, similar to that introduced by Fano in atoms [U. Fano, Phys. Rev. 1961, 124, 1866] has not yet been achieved. In this work we introduce a simple semiclassical model that accounts for all the features observed in H2 photoionization and demonstrate that the interference structures observed in dissociative ionization spectra are almost exclusively due to the phase accumulated in the nuclear motion. Furthermore, we show that the temporal buildup of these structures in the energydifferential cross sections is also determined by nuclear motion. We validate our models by comparing with fulldimensional ab initio calculations solving the timedependent Schrödinger equation.  [Show abstract] [Hide abstract]
ABSTRACT: (IOP Select 2013  Also LabTalk available) We study the photoionization and autoionization of Helium atom subject to ultrashort laser pulses by using a Feshbach formalism in the time domain. We solve the timedependent Schr\"odinger equation in terms of a configuration interaction (CI) spectral method, in which the total wavefunction is expanded with configurations defined within boundlike ($\mathcal{Q}$) and scatteringlike ($\mathcal{P}$) halfspaces. The method allowsone to provide accurate descriptions of both the atomic structure (energy positions and widths) and the photodynamics. We illustrate our approach by i) calculating the timeresolved onephoton ionization below the He$^+$ ($n$=2) ionization threshold, from $1^1S^e$ and $2 ^1P^o$ initial states, then reaching the lowest autoionizing states of $^1S^e$, $^1P^o$ and $^1D^e$ final symmetries ii) studing the temporal formation of the Fano profile of $^1P^o$ resonances and iii) showing its performance in obtaining the perturbative longtime limit of one and twophoton ionization cross sections using ultrashort laser pulses following a recently developed procedure in Phys. Rev. A, {\bf 77}, 032716 (2008).  [Show abstract] [Hide abstract]
ABSTRACT: We study the dissociative photoionization of D2 by an attosecond pulse train (APT) in the presence of a nearinfrared (IR) field. Strong oscillations in the D+ kinetic energy release spectrum with a half period of the optical cycle of the infrared field are observed and attributed to interferences between ionization pathways involving different harmonic orders of the APT due to the IRinduced coupling between the 1sσg and 2pσu ionization channels.  [Show abstract] [Hide abstract]
ABSTRACT: A time dependent Feshbach method is implemented to study the correlated electron dynamics of He doubly excited states below the He+ (N=2) ionization threshold, using ultrashort laser pulses in XUVpump IRprobe delay schemes.  [Show abstract] [Hide abstract]
ABSTRACT: In this work, circular dichroism in H2 (D2) photoionization is studied in detail. We have selected several photon energies for a case study: 19 eV for which only direct ionization to the 1sσg ionization channel is present, 27 eV where autoionization of Q1 1Σ+u doubly excited states takes place, and 32.5 eV for which autoionization from Q1 and doubly excited states and direct ionization to 1sσg and 2pσu channels strongly interfere. The latter case shows clear evidence of different behavior of the photoionization against radiation helicity.  [Show abstract] [Hide abstract]
ABSTRACT: We present a study on the 1,3Se, 1,3P° and 1,3 De He resonant states below the He+ (N = 2) threshold under the influence of a plasma environment, when the Debye screening length is varied. The interaction between all charged particles is modeled with Yukawalike screened Coulomb potentials. We make use of the Feshbach projector method to characterize resonant states, implemented by using explicitly correlated CIwave functions.  [Show abstract] [Hide abstract]
ABSTRACT: We report experiments where hydrogen molecules were dissociatively ionized by an attosecond pulse train in the presence of a nearinfrared field. Fragment ion yields from distinguishable ionization channels oscillate with a period that is half the optical cycle of the IR field. For molecules aligned parallel to the laser polarization axis, the oscillations are reproduced in twoelectron quantum simulations, and can be explained in terms of an interference between ionization pathways that involve different harmonic orders and a laserinduced coupling between the 1sσ(g) and 2pσ(u) states of the molecular ion. This leads to a situation where the ionization probability is sensitive to the instantaneous polarization of the molecule by the IR electric field and demonstrates that we have probed the IRinduced electron dynamics with attosecond pulses.  [Show abstract] [Hide abstract]
ABSTRACT: The delayed autoionization of H2 doubly excited states into channels of different inversion symmetry gerade and ungerade is investigated by using pulses of attosecond duration (isolated or packed in trains), linearly polarized along the molecular axis. It has been shown in previous work, by using XUV laser pulses with durations of 4 fs or longer, that the molecular frame photoelectron angular distributions (MFPAD) associated with the dissociative channel H+ + H(nℓ) are not symmetric with respect to the inversion center of the molecule. In contrast, the MFPADs become symmetric for shorter fs pulses. Here we show that, although this is still the case for pulses of attosecond duration, the combination of two of these pulses with a controlled time delay may still lead to asymmetric MFPADs. From the analysis of the time evolution of the calculated MFPADs, we propose a way to elucidate autoionization lifetimes of molecular resonant states. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2462–2471, 2010  [Show abstract] [Hide abstract]
ABSTRACT: Applications of the Feshbach formalism to systems of more than two active electrons are very scarce due to practical limitations in the construction of the projection operators P and Q that are inherent to the theory. As a consequence, most previous applications rely on the use of approximate quasiprojection operators, whose theoretical justification is not yet clear. In this work, an implementation of the Feshbach formalism for threeelectron atoms is presented that includes all the ingredients of the original formalism. Energy positions and autoionization widths of the lowest 2Se, 2Po, and 2De autoionizing states of Li and Ne7+ have been evaluated. The results show that the use of quasiprojection operators is justified for the evaluation of resonant positions. However, for the 2Se states of Li, the use of quasiprojection operators can lead to errors in the autoionization widths of the order of 100%.  [Show abstract] [Hide abstract]
ABSTRACT: Circular dichroism is a consequence of chirality. However, nonchiral molecules can also exhibit it when the measurement itself introduces chirality, e.g., when measuring molecularframe photoelectron angular distributions. The few such experiments performed on homonuclear diatomic molecules show that, as expected, circular dichroism vanishes when the molecularframe photoelectron angular distributions are integrated over the polar electron emission angle. Here we show that this is not the case in resonant dissociative ionization of H2 for photons of 3035 eV, which is the consequence of the delayed ionization from molecular doubly excited states into ionic states of different inversion symmetry.
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647  Citations  
111.35  Total Impact Points  
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Institutions

20032015

University of Antioquia
 Instituto de Física
Medellín, Antioquia, Colombia


20062007

Universidad Autónoma de Madrid
 Department of Chemistry
Madrid, Madrid, Spain
