Peer-reviewed Publications |
Bergeron, H., Gazeau, J. - P., Siegl, P., & Youssef, A. (2010). Semi-classical behavior of Poschl-Teller coherent states. EPL, 92(6), 60003.
Résumé: We present a construction of semi-classical states for Poschl-Teller potentials based on a supersymmetric quantum mechanics approach. The parameters of these “coherent” states are points in the classical phase space of these systems. They minimize a special uncertainty relation. Like standard coherent states they resolve the identity with a uniform measure. They permit to establish the correspondence (quantization) between classical and quantum quantities. Finally, their time evolution is localized on the classical phase space trajectory. Copyright (C) EPLA, 2010
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Gauyacq, J. - P., Novaes, F. D., & Lorente, N. (2010). Magnetic transitions induced by tunneling electrons in individual adsorbed M-phthalocyanine molecules (M=Fe and Co). Phys. Rev. B, 81(16), 165423.
Résumé: We report on a theoretical study of magnetic transitions induced by tunneling electrons in individual adsorbed M-Phthalocyanine (M-Pc) molecules where M is a metal atom: Fe-Pc on a Cu(110)(2 x 1)-O surface and Co-Pc layers on Pb(111) islands. The magnetic transitions correspond to the change in orientation of the spin angular momentum of the metal ion with respect to the surroundings and possibly an applied magnetic field. The adsorbed Fe-Pc system is studied with a density-functional-theory-transport approach showing that (i) the magnetic structure of the Fe atom in the adsorbed Fe-Pc is quite different from that of the free Fe atom or of other adsorbed Fe systems and (ii) that injection of electrons (holes) into the Fe atom in the adsorbed Fe-Pc molecule dominantly involves the Fe 3d(z)2 orbital. These results fully specify the magnetic structure of the system and the process responsible for magnetic transitions. The dynamics of the magnetic transitions induced by tunneling electrons is treated in a strong-coupling approach. The Fe-Pc treatment is extended to the Co-Pc case. The present calculations accurately reproduce the strength of the magnetic transitions as observed by magnetic inelastic electron tunneling spectroscopy experiments; in particular, the dominance of the inelastic current in the conduction of the adsorbed M-Pc molecule is accounted for.
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Ivanovskaya, V. V., Zobelli, A., Teillet-Billy, D., Rougeau, N., Sidis, V., & Briddon, P. R. (2010). Enhanced H-2 catalytic formation on specific topological defects in interstellar graphenic dust grain models. PHYSICAL REVIEW B, 82(24), 245407.
Résumé: First-principles models of the formation of H-2 on interstellar media carbonaceous grains are usually concerned with processes occurring on ideal graphenic surfaces. Until now these models are unable to explain the formation of molecular hydrogen due to the presence of absorption barriers that cannot be overcome at the low temperatures of the interstellar media. We propose an approach emphasizing the role of specific topological defects for molecular hydrogen catalysis at interstellar dust grain models. Using the nudged elastic band method combined with density-functional techniques, we obtain the full catalytic cycle for the formation of the H-2 molecule on complex carbon topologies involving the presence of pentagonal rings and C adatoms. Depending on structures, reaction paths can be barrierless or have adsorption barriers as low as 10(-3)-10(-2) eV, which might be easily overcome at the temperatures of the interstellar medium. Such low adsorption barriers indicate that specific carbon grains topological defects are preferential sites for the molecular hydrogen formation in the interstellar medium.
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Ivanovskaya, V. V., Zobelli, A., Teillet-Billy, D., Rougeau, N., Sidis, V., & Briddon, P. R. (2010). Hydrogen adsorption on graphene: a first principles study. EUROPEAN PHYSICAL JOURNAL B, 76(3), 481–486.
Résumé: We present a systematic ab initio study of atomic hydrogen adsorption on graphene. The characteristics of the adsorption process are discussed in relation with the hydrogenation coverage. For systems with high coverage, the resultant strain due to substrate relaxation strongly affects H atom chemisorption. This leads to local structural changes that have not been pointed out to date, namely localized surface curvature. We demonstrate that the hydrogen chemisorption energy barrier is independent of the optimization technique and system size, being associated with the relaxation and rehybridization of the sole adsorbent carbon atom. On the other hand, the H desorption barrier is very sensitive to a correct structural relaxation and is also dependent on the degree of system hydrogenation.
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Matar, E., Bergeron, H., Dulieu, F., Chaabouni, H., Accolla, M., & Lemaire, J. L. (2010). Gas temperature dependent sticking of hydrogen on cold amorphous water ice surfaces of interstellar interest. JOURNAL OF CHEMICAL PHYSICS, 133(10), 104507.
Résumé: Using the King and Wells method, we present experimental data on the dependence of the sticking of molecular hydrogen and deuterium on the beam temperature onto nonporous amorphous solid water ice surfaces of interstellar interest. A statistical model that explains the isotopic effect and the beam temperature behavior of our data is proposed. This model gives an understanding of the discrepancy between all known experimental results on the sticking of molecular hydrogen. Moreover, it is able to fit the theoretical results of Buch et al. [Astrophys. J. 379, 647 (1991)] on atomic hydrogen and deuterium. For astrophysical applications, an analytical formula for the sticking coefficients of H, D, H(2), D(2), and HD in the case of a gas phase at thermal equilibrium is also provided at the end of the article. (C) 2010 American Institute of Physics. [doi:10.1063/1.3484867]
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Morisset, S., Ferro, Y., & Allouche, A. (2010). Study of the sticking of a hydrogen atom on a graphite surface using a mixed classical-quantum dynamics method. JOURNAL OF CHEMICAL PHYSICS, 133(4), 044508.
Résumé: The sticking of one hydrogen atom chemisorbed on the (0001) graphite surface is investigated using a mixed classical-quantum method. The phonon modes of the system in the collinear scattering approach are included in the dynamics calculations. The vibrational degrees of freedom of the surface (phonons) are treated classically, while the H-surface motion is treated using a one-dimensional quantum wave packet propagation method. The sticking probabilities are calculated and the individual contributions of the phonon bands to the collision dynamics are analyzed for surface temperatures of 10, 150, and 300 K and hydrogen kinetic energies ranging from 0.13 to 1.08 eV. An analytical form of the sticking probability as a function of the surface temperature is also proposed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3463001]
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Nave, S., & Jackson, B. (2010). Vibrational mode-selective chemistry: Methane dissociation on Ni(100). PHYSICAL REVIEW B, 81(23), 233408.
Résumé: A first-principles full-dimensional model for CH(4) dissociation on Ni(100) is derived using a reaction path formulation. Vibrational excitation of the methane is found to significantly enhance reactivity when the molecule undergoes transitions to the ground or lower-energy vibrational states with the excess energy converted into motion along the reaction path. The nu(1) vibration has the largest efficacy for promoting reaction, with the nu(3) efficacy smaller, but significant.
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Nave, S., Tiwari, A. K., & Jackson, B. (2010). Methane dissociation and adsorption on Ni(111), Pt(111), Ni(100), Pt(100), and Pt(110)-(1x2): Energetic study. JOURNAL OF CHEMICAL PHYSICS, 132(5).
Résumé: We use density functional theory to examine 24 transition states for methane dissociation on five different metal surfaces. In our calculations, the nonlocal exchange-correlation effects are treated within the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional. In all cases, the minimum energy path for dissociation is over a top site. The barriers are large, 0.66-1.12 eV, and relatively insensitive to the rotational orientation of the (nonreacting) methyl group and the azimuthal orientation of the reactive C-H bond. There is a strong preference on the Pt surfaces for the methyl fragment to bond on the top site, while on the Ni surfaces there is a preference for the hollow or bridge sites. Thus, during the dissociation on Pt, only the low mass H atom needs to significantly move or tunnel, while on Ni, both the dissociating H and the methyl fragment move away from the top site. For all 24 configurations there is a strong force at the transition state to pucker the metal atom over which the reaction occurs. The resulting magnitude of the variation in the barrier height with the motion of this atom varies a bit from surface-to-surface, but is of the order of 1 eV/angstrom. We derive a model for the effective reaction barrier height that includes the effects of lattice motion and substrate temperature and compare with recent experiments and other theoretical studies. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3297885]
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Novaes, F. D., Lorente, N., & Gauyacq, J. - P. (2010). Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100). Phys. Rev. B, 82(15), 155401.
Résumé: The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are computed from first principles. The theory is based on a strong-coupling approach that evaluates the decay of a spin excitation due to electron-hole pair creation. Using a previously developed theory [Phys. Rev. Lett. 103, 176601 (2009) and Phys. Rev. B 81, 165423 (2010)], we compute the excitation rates by a tunneling current for all the Mn spin states. A rate equation approach permits us to simulate the experimental results by Loth and co-workers (Nat. Phys. 6, 340 (2010)] for large tunneling currents, taking into account the finite population of excited states. Our simulations give us insight into the spin dynamics, in particular, in the way polarized electrons can reveal the existence of an excited-state population. In addition, it reveals that the excitation process occurs in a way very different from the deexcitation one. Indeed, while excitation by tunneling electrons proceeds via the s and p electrons of the adsorbate, deexcitation mainly involves the d electrons.
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Perez-Gonzalez, O., Zabala, N., Borisov, A. G., Halas, N. J., Nordlander, P., & Aizpurua, J. (2010). Optical Spectroscopy of Conductive Junctions in Plasmonic Cavities. Nano Lett., 10(8), 3090–3095.
Résumé: The optical properties of a nanoparticle dimer bridged by a conductive junction depend strongly on the junction conductivity. As the conductivity increases, the bonding dimer plasmon blueshifts and broadens. For large conductance, a low energy charge transfer plasmon also appears in the spectra with a line width that decreases with increasing conductance. A simple physical model for the understanding of the spectral feature is presented. Our finding of a strong influence of junction conductivity on the optical spectrum suggests that plasmonic cavities might serve as probes of molecular conductance at elevated frequencies not accessible through electrical measurements.
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Quijada, M., Diez Muino, R., Borisov, A. G., Alonso, J. A., & Echenique, P. M. (2010). Lifetime of electronic excitations in metal nanoparticles. New J. Phys., 12, 053023.
Résumé: Electronic excitations in metal particles with sizes up to a few nanometers are shown to have a one-electron character when a laser pulse is applied off the plasmon resonance. The calculated lifetimes of these excitations are in the femtosecond timescale but their values are substantially different from those in bulk. This deviation can be explained from the large weight of the excitation wave function in the nanoparticle surface region, where dynamic screening is significantly reduced. The well-known quadratic dependence of the lifetime with the excitation energy in bulk breaks down in these finite-size systems.
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Riedel, D., Delattre, R., Borisov, A. G., & Teperik, T. V. (2010). A Scanning Tunneling Microscope as a Tunable Nanoantenna for Atomic Scale Control of Optical-Field Enhancement. Nano Lett., 10(10), 3857–3862.
Résumé: The high stability of a low temperature (9 K) scanning tunneling microscope junction is used to precisely adjust the enhancement of an external pulsed vacuum ultraviolet (VUV) laser The ensuing VUV optical-field strength is mapped on an hydrogenated Si(100) surface by imprinting locally one-photon atomic scale hydrogen desorption Subsequent to irradiation, topography of the Si(100) H surface at the reacted area revealed a desorption spot with unprecedented atomic precision Our results show that the shapes. positions. and sizes of the desorption spots are correlated to the calculated optical-field structure, offering real control of the optical-held distribution at molecular scale
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Sizun, M., Bachellerie, D., Aguillon, F., & Sidis, V. (2010). Investigation of ZPE and temperature effects on the Eley-Rideal recombination of hydrogen atoms on graphene using a multidimensional graphene-H-H potential. Chem. Phys. Lett., 498(1-3), 32–37.
Résumé: We study the Eley-Rideal recombination of H atoms on graphene under the physical conditions of the interstellar medium. Effects of the ZPE motions of the chemisorbed H atom and of the graphene thermal motions are investigated. Classical molecular dynamics calculations undertaken with the multidimensional potential of Bachellerie et al. [Phys. Chem. Chem. Phys. 11 (2009) 2715] are reported. The ZPE effects are the strongest. The closer the collision energy is to the classical reaction threshold the more sizeable the effects. The quantum reaction cross section is also estimated below and above the classical threshold using a capture model. (C) 2010 Elsevier B.V. All rights reserved.
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Steeb, F., Mathias, S., Wiesenmayer, M., Fischer, A., Aeschlimann, M., Bauer, M., & Gauyacq, J. P. (2010). Probing adsorbate dynamics with chirped laser pulses in a single-pulse scheme. PHYSICAL REVIEW B, 82(16), 165430.
Résumé: Femtosecond dynamics of the model-like adsorption system Cs/Cu(111) is probed by two-photon photoelectron spectroscopy (2PPE) using phase-modulated (chirped) laser pulses. The experimental data are quantitatively modeled within a wavepacket propagation approach under explicit consideration of the adsorbate motion. The results enable us to assign characteristic chirped-pulse 2PPE features to the ultrafast adsorbate dynamics associated with the excited state lifetime and the adsorbate motion, and to improve on the qualitative interpretation of experimental data as published in Petek et al. [J. Phys. Chem. A 104, 10234 (2000)]. Our results show that nonlinear photoemission with a chirped pulse in a single-pulse scheme can complement real-time studies based on pump-probe schemes to gain quantitative insights into the femtosecond dynamics of ultrafast surface processes.
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Teillet-Billy, D., Rougeau, N., Ivanovskaya, V. V., & Sidis, V. (2010). Interaction of Atoms with Graphenic-Type Surfaces for the Chemistry of the Interstellar Medium: New Properties of H Dimers on the Surface. INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, 110(12), 2231–2236.
Résumé: Following the works of Rougeau et al. (Chem Phys Lett 2006, 431,135) and Ferro et al. (Phys Rev B 2008, 78, 085417) on the one-sided double chemisorption of H atoms on graphenic platelets, we investigate the two-sided double chemisorption using DFT-GGA PW91 calculations. Equilibrium characteristics and potential energy curves for chemisorption are reported for the ortho, meta, para, and bottom positions. Contrary to the one-sided case, the two-sided ortho chemisorption, as well as the bottom position, is barrier-less, whereas the two-sided para chemisorption exhibits an activation barrier. The highest occupied Kohn-Sham orbital (HOKSO) of the H-graphene radical is shown to signal the privileged barrier-less double chemisorption sites. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2231-2236, 2010
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Tiwari, A. K., Nave, S., & Jackson, B. (2010). The temperature dependence of methane dissociation on Ni(111) and Pt(111): Mixed quantum-classical studies of the lattice response. JOURNAL OF CHEMICAL PHYSICS, 132(13), 134702.
Résumé: The barrier to the dissociative adsorption of methane on metal surfaces is generally large, and its height can vary with the motion of the lattice atoms. One fully quantum and three different mixed quantum-classical approaches are used to examine this reaction on Ni(111) and Pt(111) surfaces, using potential energy surfaces derived from density functional theory. The three approximate methods are benchmarked against the exact quantum studies, and two of them are shown to work reasonably well. The mixed models, which treat the lattice motion classically, are used to examine the lattice response during the reaction. It is found that the thermal motion of the lattice atoms strongly modifies the reactivity, but that their motion is not significantly perturbed. Based on these results, new models for methane reactions are proposed based on a sudden treatment of the lattice motion and shown to agree well with the exact results. In these new models, the reaction probability at different surface temperatures is computed from static surface reaction probabilities, allowing for a quantum calculation of the reaction probability without having to explicitly treat the motion of the heavy lattice atoms.
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