Fernando Martín

Universidad Autónoma de Madrid, Madrid, Madrid, Spain

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Publications (87)358.02 Total impact

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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. · 38.60 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; · 6.73 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. · 2.03 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; · 3.71 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;
  • Carlos Marante, Luca Argenti, Fernando Martín
    Physical Review A 07/2014; 90. · 3.04 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. · 2.03 Impact Factor
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    ABSTRACT: This study presents a coupled experimental and theoretical work on the stability of singly charged glycine molecules (NH2CH2COOH + , the simplest amino acid) produced in the collision of neutral glycine with low-energy multiply charged ions in the gas phase. The main fragments observed exper-imentally result from Cα-C carboxyl bond cleavage. Additionally, some fragments are produced after an intramolecular rearrangement of the glycine cation, in particular the loss of a neutral water molecule fol-lowing a H-migration. Moreover, this work discusses qualitatively the energy transferred to the molecule in the collision with a comparative study of the fragmentation patterns for different projectile ion charge states.
    The European Physical Journal D 06/2014; 68. · 1.51 Impact Factor
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    ABSTRACT: This study presents a coupled experimental and theoretical work on the stability of singly charged glycine molecules (NH2CH2COOH+, the simplest amino acid) produced in the collision of neutral glycine with low-energy multiply charged ions in the gas phase. The main fragments observed experimentally result from C α -Ccarboxyl bond cleavage. Additionally, some fragments are produced after an intramolecular rearrangement of the glycine cation, in particular the loss of a neutral water molecule following a H-migration. Moreover, this work discusses qualitatively the energy transferred to the molecule in the collision with a comparative study of the fragmentation patterns for different projectile ion charge states.
    The European Physical Journal D 06/2014; 68(149). · 1.51 Impact Factor
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    ABSTRACT: In this work, we present a density functional theory study of the structure and stability of neutral and positively-charged coronene [Formula: see text]. In particular, we have investigated (i) adiabatic and vertical ionization potentials up to charge q = 9, (ii) the corresponding infrared spectra, and (iii) dissociation energies and potential energy surfaces for several hydrogen loss channels: sequential H+H, H+H(+), H(+)+H, H(+)+H(+), and direct H2 and [Formula: see text]. We have found that the stability of positively-charged coronene is extremely high as a consequence of the molecule's capability to redistribute the charge all over the structure. The computed dissociation energies and fragmentation barriers show that there is competition between different hydrogen loss channels and that the relative importance of these channels depends on the charge of the molecule. From a careful analysis of the potential energy surface we conclude that the channel with the lowest barrier corresponds to the loss of H2 from neutral, singly-, doubly-, and triply-charged coronene, [Formula: see text] from quadruply-charged coronene and H(+)+H(+) from quintuply-charged coronene.
    The Journal of chemical physics. 05/2014; 140(20):204307.
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    ABSTRACT: We present a molecular dynamics (MD) study on the structure of self-assembled monolayers (SAM) of alkylthiolates on various metal surfaces, with especial attention to Au(111) and Ag(111). Variations in the structure of these SAMs as a function of temperature and alkyl-chain length are systematically investigated. The MD simulations are performed by using a recently developed force field based on second-order Møller-Plesset perturbation theory calculations. Good agreement between the present results and the existing experimental data is found on Au(111). On Ag(111) the comparison between theory and experiment is also satisfactory for alkylthiolates with no more than 14 carbon atoms. The dependences of the average tilt angle of SAMs on temperature and chain length are easily understood by means of a simple single-chain model.
    The journal of physical chemistry. A. 05/2014;
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    Álvaro J. Galán, Luca Argenti, Fernando Martín
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    ABSTRACT: We present a theoretical study of the photoelectron attosecond beating at the basis of RABBIT (Reconstruction of Attosecond Beating By Interference of Two-photon transitions) in the presence of autoionizing states. We show that, as a harmonic traverses a resonance, its sidebands exhibit a peaked phase shift as well as a modulation of the beating frequency itself. Furthermore, the beating between two resonant paths persists even when the pump and the probe pulses do not overlap, thus providing a sensitive non-holographic interferometric means to reconstruct coherent metastable wave packets. We characterize these phenomena quantitatively with a general finite-pulse analytical model that accounts for the effect of both intermediate and final resonances on two-photon processes, at a negligible computational cost. The model predictions are in excellent agreement with those of accurate ab initio calculations for the helium atom in the region of the N=2 doubly excited states.
    05/2014;
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    ABSTRACT: Dynamics simulations were performed to study soft landing of SiNCS+ and (CH3)2SiNCS+ ions on a self-assembled monolayer of perfluorinated alkanethiols on gold (F-SAM). Using classical trajectories, the short-time collision dynamics (picosecond scale) were investigated to analyze trapping probabilities for these silyl ions. Thermal desorption of trapped ions was simulated by using “boxed molecular dynamics” (BXD). The simulations predict substantial ion trapping in the collisions of these ions with the F-SAM, especially when the projectile’s incident direction is normal to the surface. Desorption of the SiNCS+ ion occurs significantly faster than desorption of the methylated ion, which may explain why soft landing was experimentally observed for the latter ion only [Miller, S. A.; Luo, H.; Pachuta, S. J.; Cooks, R. G. Science 1997, 275, 1447–1450; Luo, H.; Miller, S. A.; Cooks, R. G.; Pachuta, S. J. Int. J. Mass. Spectrom. Ion Processes 1998, 174, 193–217]. The free energy profiles for desorption of these ions show minima at 15 Å above the gold slab, indicating that the silyl ion has a preference to reside on top of the monolayer. Deep penetration of the ion into the monolayer is prevented by a large free energy barrier. However, according to DFT calculations, if this process occurred, the SiNCS+ ion could strongly bind to the Au(111) surface that supports the perfluorinated alkanethiol chains.
    The Journal of Physical Chemistry C 05/2014; 118:10159. · 4.84 Impact Factor
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    ABSTRACT: We theoretically explore the use of UV pump – UV probe schemes to resolve in time the dynamics of nuclear wave packets in excited electronic states of the hydrogen molecule. The pump pulse ignites the dynamics in singly excited states, that will be probed after a given time delay by a second identical pulse that will ionize the molecule. The field-free molecular dynamics is first explored by analyizing the autocorrelation function for the pumped wave packet and the excitation probabilities. We investigate both energy and angle differential ionization probabilities and demonstrate that the asymmetry induced in the electron angular distributions gives a direct map of the time evolution of the pumped wave packet.
    Journal of Physics Conference Series 04/2014; 488(1):012017.
  • Etienne Plésiat, Piero Decleva, Fernando Martín
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    ABSTRACT: Vibrationally resolved molecular frame photoelectron angular distributions (MFPADs) of fixed-in-space molecules have been evaluated for diatomic and polyatomic molecules. Calculations have been performed by using an extension of the static-exchange density functional theory formerly developed by P. Decleva [1] and coworkers and extended in order to include the nuclear motion in the Born-Oppenheimer approximation. The method proved to be very accurate for diatomic molecules [2, 3, 5], particularly at high energy of the photoelectron. In this work, we present the results obtained for the inner shell photoionization of C2H2, NH3, CH4, CF4, BF3 and SF6.
    Journal of Physics Conference Series 04/2014; 488(2):022022.
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    ABSTRACT: Fragmentation of doubly charged biomolecules, uracil and amino acids, has been investigated using different ab inito Molecular Dynamics Methods. Time-Dependent Density Functional Theory Molecular Dynamics give a description of the non-adiabatic effects, the charge redistributions that occur in the first few femtoseconds and reveal the importance of the chemical environment. The combination of different techniques allow us to interpret the complex multicoincident spectra obtained experimentally when the molecules collides with ions or are excited with synchrotron radiation.
    Journal of Physics Conference Series 04/2014; 488(1):012037.
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    ABSTRACT: Understanding the coupled electronic and nuclear dynamics in molecules by using pump-probe schemes requires not only the use of short enough laser pulses but also wavelengths and intensities that do not modify the intrinsic behavior of the system. In this respect, extreme UV pulses of few-femtosecond and attosecond durations have been recognized as the ideal tool because their short wavelengths ensure a negligible distortion of the molecular potential. In this work, we propose the use of two twin extreme UV pulses to create a molecular interferometer from direct and sequential two-photon ionization processes that leave the molecule in the same final state. We theoretically demonstrate that such a scheme allows for a complete identification of both electronic and nuclear phases in the wave packet generated by the pump pulse. We also show that although total ionization yields reveal entangled electronic and nuclear dynamics in the bound states, doubly differential yields (differential in both electronic and nuclear energies) exhibit in addition the dynamics of autoionization, i.e., of electron correlation in the ionization continuum. Visualization of such dynamics is possible by varying the time delay between the pump and the probe pulses.
    Proceedings of the National Academy of Sciences 03/2014; · 9.81 Impact Factor
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    ABSTRACT: We use time-dependent density functional theory and Born-Oppenheimer molecular dynamics methods to investigate the fragmentation of doubly ionized uracil in gas phase. Different initial electronic excited states of the dication are obtained by removing electrons from different inner-shell orbitals of the neutral species. We show that shape-equivalent orbitals lead to very different fragmentation patterns revealing the importance of the intramolecular chemical environment. The results are in good agreement with ionion coincidence measurements of uracil collision with 100 keV protons.
    01/2014; 12(2).
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    ABSTRACT: High harmonic light sources make it possible to access attosecond timescales, thus opening up the prospect of manipulating electronic wave packets for steering molecular dynamics. However, two decades after the birth of attosecond physics, the concept of attosecond chemistry has not yet been realized; this is because excitation and manipulation of molecular orbitals requires precisely controlled attosecond waveforms in the deep UV, which have not yet been synthesized. Here, we present a unique approach using attosecond vacuum UV pulse-trains to coherently excite and control the outcome of a simple chemical reaction in a deuterium molecule in a non-Born-Oppenheimer regime. By controlling the interfering pathways of electron wave packets in the excited neutral and singly ionized molecule, we unambiguously show that we can switch the excited electronic state on attosecond timescales, coherently guide the nuclear wave packets to dictate the way a neutral molecule vibrates, and steer and manipulate the ionization and dissociation channels. Furthermore, through advanced theory, we succeed in rigorously modeling multiscale electron and nuclear quantum control in a molecule. The observed richness and complexity of the dynamics, even in this very simplest of molecules, is both remarkable and daunting, and presents intriguing new possibilities for bridging the gap between attosecond physics and attochemistry.
    Proceedings of the National Academy of Sciences 01/2014; · 9.81 Impact Factor
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    ABSTRACT: TCNQ derivatives adsorbed on a metal surface undergo a self-limited decyanation reaction that only affects two out of the four cyano groups in the molecule. Combined Scanning Tunneling Microscopy/X-ray Photoelectron Spectroscopy experiments and Density Functional Theory calculations relate the self-limiting behavior to the transfer of electrons from the metal to the molecule.
    Chemical Communications 12/2013; · 6.38 Impact Factor

Publication Stats

382 Citations
358.02 Total Impact Points

Institutions

  • 2003–2014
    • Universidad Autónoma de Madrid
      • • Department of Chemistry
      • • Department of Condensed Matter Physics
      Madrid, Madrid, Spain
  • 2013
    • Instituto de Estructura de la Materia
      Madrid, Madrid, Spain
    • Madrid Institute for Advanced Studies
      Madrid, Madrid, Spain
  • 2012
    • Max Planck Institute for Nuclear Physics
      Heidelburg, Baden-Württemberg, Germany
  • 2011
    • Lund University
      • MAX-Lab
      Lund, Skane, Sweden
  • 2010
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany