Fernando Martín

Universidad Autónoma de Madrid, Madrid, Madrid, Spain

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Publications (106)489.27 Total impact

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    ABSTRACT: In the past few years, attosecond techniques have been implemented for the investigation of ultrafast dynamics in molecules. The generation of isolated attosecond pulses characterized by a relatively high photon flux has opened up new possibilities in the study of molecular dynamics. In this paper, we report on experimental and theoretical results of ultrafast charge dynamics in a biochemically relevant molecule, namely, the amino acid phenylalanine. The data represent the first experimental demonstration of the generation and observation of a charge migration process in a complex molecule, where electron dynamics precede nuclear motion. The application of attosecond technology to the investigation of electron dynamics in biologically relevant molecules represents a multidisciplinary work, which can open new research frontiers: those in which few-femtosecond and even subfemtosecond electron processes determine the fate of biomolecules. It can also open new perspectives for the development of new technologies, for example, in molecular electronics, where electron processes on an ultrafast temporal scale are essential to trigger and control the electron current on the scale of the molecule.
    IEEE Journal of Selected Topics in Quantum Electronics 09/2015; 21(5):1-12. DOI:10.1109/JSTQE.2015.2419218 · 3.47 Impact Factor
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    ABSTRACT: We present a combined experimental and theoretical study of the ionization of N- acetylglycine molecules by 48 keV O6+ ions. We focus on the single ionization channel of this interaction. In addition to the prompt fragmentation of the N-acetylglycine cation, we observe also the formation of metastable parent ions with lifetimes in the microsecond range. On the basis of density functional theory calculations, we assign these metastable ions to the diol tautomer of N-acetylglycine. In comparison to the simple amino acids, the tautomerization rate is higher due to the presence of the peptide bond. The study of a simple biologically relevant molecule containing a peptide bond allows us to demonstrate how increasing the complexity of the structure influences the behavior of the ionized molecule.
    The Journal of Physical Chemistry A 08/2015; DOI:10.1021/acs.jpca.5b06009 · 2.78 Impact Factor
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    ABSTRACT: We present a combined experimental and theoretical study of the fragmentation of doubly positively charged β-alanine molecules in the gas phase. The dissociation of the produced dicationic molecules, induced by low-energy ion collisions, is analysed by coincidence mass spectrometric techniques; the coupling with ab initio molecular dynamics simulations allows rationalisation of the experimental observations. The present strategy gives deeper insights into the chemical mechanisms of multiply charged amino acids in the gas phase. In the case of the β−alanine dication, in addition to the expected Coulomb explosion and hydrogen migration processes, we have found evidence of hydroxyl-group migration, which leads to unusual fragmentation products, such as hydroxymethyl cation, and is necessary to explain some of the observed dominant channels.
    Physical Chemistry Chemical Physics 05/2015; 17(26). DOI:10.1039/C5CP01628B · 4.20 Impact Factor
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    ABSTRACT: We present an analytical model based on the time-dependent WKB approximation to reproduce the photoionization spectra of an H2 molecule in the autoionization region. We explore the nondissociative channel, which is the major contribution after one-photon absorption, and we focus on the features arising in the energy differential spectra due to the interference between the direct and the autoionization pathways. These features depend on both the timescale of the electronic decay of the autoionizing state and the time evolution of the vibrational wavepacket created in this state. With full ab initio calculations and with a one-dimensional approach that only takes into account the nuclear wavepacket associated to the few relevant electronic states we compare the ground state, the autoionizing state, and the background continuum electronic states. Finally, we illustrate how these features transform from molecular-like to atomic-like by increasing the mass of the system, thus making the electronic decay time shorter than the nuclear wavepacket motion associated with the resonant state. In other words, autoionization then occurs faster than the molecular dissociation into neutrals.
    New Journal of Physics 05/2015; 17(5). DOI:10.1088/1367-2630/17/5/053011 · 3.67 Impact Factor
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    ABSTRACT: The present work combines experimental and theoretical studies of the collision between keV ion projectiles and clusters of pyrene, one of the simplest Polycyclic Aromatic Hydrocarbons (PAHs). Intra-cluster growth processes induced by ion collisions lead to the formation of a wide range of new molecules with masses larger than that of the pyrene molecule. The efficiency of these processes is found to strongly depend on the mass and velocity of the incoming projectile. Classical molecular dynamics simulations of the entire collision process -- from the ion impact (nuclear scattering) to the formation of new molecular species -- reproduce the essential features of the measured molecular growth process and also yield estimates of the related absolute cross sections. More elaborate density functional tight binding calculations yield the same growth products as the classical simulations. The present results could be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Saturn's moon Titan.
    Journal of Physical Chemistry Letters 04/2015; 6(9):1536-1542. DOI:10.1021/acs.jpclett.5b00405 · 7.46 Impact Factor
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    ABSTRACT: Time delays of electrons emitted from an isotropic initial state and leaving behind an isotropic ion are assumed to be angle-independent. Using an interferometric method involving XUV attosecond pulse trains and an IR probe field in combination with a detection scheme, which allows for full 3D momentum resolution, we show that time delays between electrons liberated from the $1s^{2}$ spherically symmetric ground state of He depend on the emission direction of the electrons with respect to the linear polarization axis of the ionizing XUV light. Such time delays can exhibit values as large as 60 attoseconds. With the help of refined theoretical models we can attribute the observed anisotropy to the interplay between different final quantum states, which arise naturally when two photons are involved in the photoionization process. Since most measurement techniques tracing attosecond electron dynamics have involved at least two photons so far, this is a general, significant, and initially unexpected effect that must be taken into account in all measurements dealing with photoionization dynamics on atomic, molecular and condensed matter systems.
  • Bo Y Chang · Seokmin Shin · Alicia Palacios · Fernando Martín · Ignacio R Sola
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    ABSTRACT: To create an oscillating electric dipole in an homonuclear diatomic cation without an oscillating driver one needs (i) to break the symmetry of the system and (ii) to sustain highly correlated electronic and nuclear motion. Based on numerical simulations in H2+ we present results for two schemes. In the first one (i) is achieved by creating a superposition of symmetric and antisymmetric electronic states freely evolving, while (ii) fails. In a second scheme, by preparing the system in a dressed state of a strong static field, both conditions hold. We then analyze the robustness of this scheme with respect to features of the nuclear wave function and its intrinsic sources of decoherence.
    Journal of Physics B Atomic Molecular and Optical Physics 02/2015; 48(4). DOI:10.1088/0953-4075/48/4/043001 · 1.92 Impact Factor
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    ABSTRACT: We present a theoretical study of the adsorption of benzene C6H6 on the Cu(100) metal surface. The insulating effect of ionic films on this system has also been investigated by adsorbing C6H6 on the same surface covered with 1, 2, and 3 monolayers of NaCl. For this purpose, we employed density functional theory (DFT) including the van der Waals dispersion forces via a DFT-D2 scheme. For all the studied systems we analyzed the adsorption energies and geometries as well as the density of states in order to get a complete description of the type of binding, the charge transfer between the molecule and the surface, and the electronic level alignment after adsorption. We show that the molecule-substrate interaction is weak and mainly governed by dispersion forces, with an almost insignificant charge transfer between the substrate and the adsorbate. We found a progressive decoupling of the molecule from the metal surface when the size of the ultrathin insulating NaCl film increases.
    The Journal of Physical Chemistry C 02/2015; 119:4062. DOI:10.1021/jp5106604 · 4.77 Impact Factor
  • Fernando Martín
    Nature Photonics 02/2015; 9(2):76-77. DOI:10.1038/nphoton.2014.325 · 29.96 Impact Factor
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    ABSTRACT: The concerted motion of two or more bound electrons governs atomic and molecular non-equilibrium processes including chemical reactions, and hence there is much interest in developing a detailed understanding of such electron dynamics in the quantum regime. However, there is no exact solution for the quantum three-body problem, and as a result even the minimal system of two active electrons and a nucleus is analytically intractable. This makes experimental measurements of the dynamics of two bound and correlated electrons, as found in the helium atom, an attractive prospect. However, although the motion of single active electrons and holes has been observed with attosecond time resolution, comparable experiments on two-electron motion have so far remained out of reach. Here we show that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium. The beat appears in attosecond transient-absorption spectra measured with unprecedentedly high spectral resolution and in the presence of an intensity-tunable visible laser field. We tune the coupling between the two low-lying quantum states by adjusting the visible laser intensity, and use the Fano resonance as a phase-sensitive quantum interferometer to achieve coherent control of the two correlated electrons. Given the excellent agreement with large-scale quantum-mechanical calculations for the helium atom, we anticipate that multidimensional spectroscopy experiments of the type we report here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems. They might also provide a route to the site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental chemical reactions.
    Nature 12/2014; 516(7531):374-8. DOI:10.1038/nature14026 · 42.35 Impact Factor
  • Alberto Sánchez Muzas · Cristina Díaz · Fernando Martín
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    ABSTRACT: Diffraction experiments of atoms and molecules under fast grazing incidence conditions have opened a new field in surface science. This experimental effort calls for complementary theoretical studies, which would allow a detailed analysis of experimental data. Here, we have analyzed the ability of classical dynamics simulations to reproduce experimental results. To perform this study, a DFT (density functional theory) based potential energy surface, describing the interaction between a H atom and a LiF(1 0 0) sur- face, has been computed. Diffraction probabilities have been simulated by means of a classical binning method. Our results have been found to be in qualitative good agreement with recent experimental measurements.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 11/2014; 354. DOI:10.1016/j.nimb.2014.11.009 · 1.12 Impact Factor
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    ABSTRACT: This article reports on the temperature-controlled irreversible transition between the two isomeric forms of the strong electron acceptor dicyano-p-quinonediimine (DCNQI) on the Cu(100) surface. A combination of experiment (time-resolved, variable-temperature scanning tunneling microscopy, STM) and theory (density functional theory, DFT) shows that the isomerization barrier is lower than in the gas phase or solution due to the fact that charge transfer from the substrate modifies the bond configuration of the molecule, aromatizing the quinoid ring of DCNQI and enabling a more free rotation of the cyano groups with respect to the molecular axis.
    The Journal of Physical Chemistry C 11/2014; 118(47):27388. DOI:10.1021/jp508458y · 4.77 Impact Factor
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    ABSTRACT: Achieving control over the self-organization of functional molecules on graphene is critical for the development of graphene technology in organic electronic and spintronic. Here, by using a scanning tunneling microscope (STM), we show that the electron acceptor molecule 7,7′,8,8′-tetracyano-p-quinodimethane (TCNQ) and its fluorinated derivative 2,3,5,6-tetrafluoro-7,7′,8,8′- tetracyano-p-quinodimethane (F4-TCNQ), co-deposited on the surface of epitaxial graphene on Ru(0001), transform spontaneously into their corresponding magnetic anions and self-organize in two remarkably different structures. TCNQ forms densely packed linear magnetic arrays, while F4-TCNQ molecules remain as isolated non interacting magnets. With the help of density functional theory (DFT) calculations, we trace back the origin of this behavior in the competition between the intermolecular repulsion experienced by the individual charged anions, which tends to separate the molecules, and the delocalization of the electrons transferred from the surface to the molecules, which promotes the formation of molecular oligomers. Our results demonstrate that it is possible to control the spatial arrangement of organic magnetic anions co-adsorbed on a surface by means of chemical substitution, paving the way for the design of two-dimensional fully organic magnetic structures on graphene and on other surfaces.
    Nanoscale 10/2014; 6(24). DOI:10.1039/C4NR02917H · 7.39 Impact Factor
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    ABSTRACT: Interaction of a laser pulse with a centrally symmetric medium, such as an isotropic gas of atoms, leads to the generation of harmonic emission which contains exclusively odd harmonics of the incident field. This result is the consequence of both the central symmetry of the medium and the temporal symmetry of the oscillating electric field, , where ωl is the laser frequency. In the case of oriented heteronuclear molecules, the spatial symmetry no longer holds and both odd and even harmonics become allowed. Here we show, by solving the time-dependent Schrödinger equation for H, D, and T, that even-order harmonic generation is also possible for sufficiently long infrared (IR) laser pulses in homonuclear molecules. The appearance of even harmonics is a signature of the coupled electron-nuclear dynamics and reflects field-induced electron localization initiated by the strong laser field, which breaks the spatial symmetry in the system. The analysis of even harmonics generated by pulses of different durations might therefore provide information on correlated electron-nuclear dynamics and charge migration in more complex un-oriented molecular ensembles.
    Journal of Physics B Atomic Molecular and Optical Physics 10/2014; 47(20):204015. DOI:10.1088/0953-4075/47/20/204015 · 1.92 Impact Factor
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    ABSTRACT: We present a systematic theoretical study of the exohedral interaction between singly positively charged metal (M) atoms and the C60 fullerene in [M − C60 ]+ complexes. Calculations have been carried out by means of the density functional theory. We have considered alkali (Li, Na and K), alkaline-earth (Be, Mg and Ca) and first period transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) interacting with C60 in different positions: on top of a ring (pentagon or hexagon), a bond (hexagon-hexagon or hexagon-pentagon) and a C atom. A detailed topological analysis of the electronic density reveals metal - C60 exohedral interaction of the ion-induced dipole type, with the positive charge localized on the metal atom and with increasing covalent character for the heavier transition metals. An energy descomposition analysis allows us to quantify the different contributions to the bonding in each complex, being dominant the polarization one. A simple ion-induced dipole model explains the main features of the interaction.
    RSC Advances 10/2014; 4(95). DOI:10.1039/C4RA10776D · 3.84 Impact Factor
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    ABSTRACT: We present experimental and theoretical results on single- and multiple-electron capture, and fragmentation, in C6+ + C-60 collisions at velocities in the v(col) = 0.05 - 0.4 a.u. range. We use time-of-flight mass spectrometry and coincidence detection of charged fragments to separate pure target ionization from processes in which the C-60 target is both ionized and fragmented. The coincidence technique allows us to identify different types of fragmentation processes such as C-60(q+) -> C-58(q+) + C-2 and C-60(q+) -> C-58((q-1)+) + C-2(+). A quasimolecular approach is employed to calculate charge transfer and target excitation cross sections. First-order time-dependent perturbation and statistical methods are used to treat the postcollisional processes: the calculated rate constants for C-2 and C-2(+) emission from the excited and charged fullerene are then used to evaluate the fragmentation dynamics. We show that the target ionization cross section decreases with the induced target charge state and the impact energy. C-2 emission from C-60(q+) is found to dominate when q <= 2 while C-2(+) emission dominates when q >= 5, in agreement with the present and previous experimental results.
    Physical Review A 09/2014; 90(3). DOI:10.1103/PhysRevA.90.032701 · 2.99 Impact Factor
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    ABSTRACT: ETCNQ molecules are used as a sensitive probe for the Kondo response of the electron gas of a nanostructured graphene grown on Ru(0001) presenting a moiré pattern. All adsorbed molecules acquired an extra electron by charge transfer from the substrate, but only those adsorbed in the FCC-Top areas of the moiré show magnetic moment and Kondo resonance in the STS spectra. DFT calculations trace back this behavior to the existence of a surface resonance in the low areas of the graphene moiré, whose density distribution strongly depends on the stacking sequence of the moiré area and effectively quenches the magnetic moment for HCP-Top sites.
    Nano Letters 07/2014; 14(8). DOI:10.1021/nl501584v · 13.59 Impact Factor
  • Carlos Marante · Luca Argenti · Fernando Martín
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    ABSTRACT: As a first step towards meeting the recent demand for new computational tools capable of reproducing molecular-ionization continua in a wide energy range, we introduce a hybrid Gaussian-B-spline basis (GABS) that combines short-range Gaussian functions, compatible with standard quantum-chemistry computational codes, with B splines, a basis appropriate to represent electronic continua. We illustrate the performance of the GABS hybrid basis for the hydrogen atom by solving both the time-independent and the time-dependent Schrodinger equation for a few representative cases. The results are in excellent agreement with those obtained with a purely B-spline basis, with analytical results, when available, and with recent above-threshold ionization spectra from the literature. In the latter case, we report fully differential photoelectron distributions which offer further insight into the process of above-threshold ionization at different wavelengths.
    Physical Review A 07/2014; 90(1). DOI:10.1103/PhysRevA.90.012506 · 2.99 Impact Factor
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    ABSTRACT: The differential photoionization cross section ratio (ν = 1)/(ν = 0) for the symmetric stretching mode in the C 1s photoionization of CF4 was studied both theoretically and experimentally. We observed this ratio to differ from the Franck–Condon ratio and to be strongly dependent on the photon energy, even far from the photoionization threshold. The density-functional theory computations show that the ratio is significantly modulated by the diffraction of the photoelectrons by the neighbouring atoms at high photon energies. At lower energies, the interpretation of the first very strong maximum observed about 60 eV above the photoionization threshold required detailed calculations of the absolute partial cross sections, which revealed that the absolute cross section has two maxima at lower energies, which turn into one maximum in the cross section ratio because the maxima appear at slightly different energies in ν = 1 and ν = 0 cross sections. These two strong, low-energy continuum resonances originate from the trapping of the continuum wavefunction in the molecular potential of the surrounding fluorine atoms and from the outgoing electron scattering by them.
    Journal of Physics B Atomic Molecular and Optical Physics 06/2014; 47(12):124032. DOI:10.1088/0953-4075/47/12/124032 · 1.92 Impact Factor
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    ABSTRACT: We report ab initio calculations on laser-assisted photoionization of the hydrogen molecule in the energy region where autoionization from doubly excited states is expected to occur. We use a UV-pump/IR-probe scheme in which an isolated attosecond UV pulse and a 750 nm IR pulse are combined. The IR pulse has a relatively low intensity (1012 W cm−2), which allows us to perform a perturbative analysis of the calculated ionization probabilities differential in either electron or nuclear energy or both. We show that, for dissociative ionization, the electron energy distributions as a function of time delay exhibit unusual streaking patterns that are due to the presence of autoionizing states. These patterns significantly differ from the standard ones observed in direct single ionization of atoms and molecules. We also show that, by using such a pump–probe scheme, one can suppress autoionization from doubly excited states for time delays between 0 and 4 fs.
    Journal of Physics B Atomic Molecular and Optical Physics 06/2014; 47(12):124013. DOI:10.1088/0953-4075/47/12/124013 · 1.92 Impact Factor