Peer-reviewed Publications |
Cazaux, S., Morisset, S., Spaans, M., & Allouche, A. (2011). When sticking influences H-2 formation. ASTRONOMY & ASTROPHYSICS, 535, A27.
Résumé: Aims. Because of their catalytic properties, interstellar dust grains are crucial to the formation of H-2, the most abundant molecule in the Universe. The formation of molecular hydrogen strongly depends on the ability of H atoms to stick on dust grains. In this study we determine the sticking coefficient of H atoms chemisorbed on graphitic surfaces, and estimate its impact on the formation of H-2. Methods. The sticking probability of H atoms chemisorbed onto graphitic surfaces is obtained using a mixed classical-quantum dynamics method. In this, the H atom is treated quantum-mechanically and the vibrational modes of the surface are treated classically. The implications of sticking for the formation of H-2 are addressed by using kinetic Monte Carlo simulations that follow how atoms stick, move and associate with each other on dust surfaces of different temperature. Results. In our model, molecular hydrogen forms very efficiently for dust temperatures lower than 15 K through the involvement of physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures lower than 2000 K, H-2 formation differs strongly if the H atoms coming from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by density functional theory (DFT) calculations to become chemisorbed. The product of the sticking times efficiency can be increased by many orders of magnitude when realistic barriers are considered. If graphite phonons are taken into account in the dynamics calculations, then H atoms stick better on the surface at high energies, but the overall H-2 formation efficiency is only slightly affected. Our results suggest that H-2 formation can proceed efficiently in photon-dominated regions, X-ray dominated regions, hot cores and in the early Universe when the first dust is available.
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Diaz-Tendero, S., Borisov, A. G., & Gauyacq, J. - P. (2011). Theoretical study of the electronic excited states in ultrathin ionic layers supported on metal surfaces: NaCl/Cu(111). Phys. Rev. B, 83(11), 115453.
Résumé: We present a theoretical study of the electronic excited states in ultrathin ionic layers supported on metal surfaces. We have studied 1, 2, 3, and 4 monolayers of NaCl on a Cu(111) surface. Energies, lifetimes, and associated wave functions of the excited states have been obtained with a joint, model potential-wave packet propagation approach. The excited state with the lowest energy has the character of an image potential state repelled from the surface by the NaCl layer. The next two states present a mixed character of image potential states and NaCl layer states corresponding to the quantization of the conduction band in the finite-size layer. We discuss the role of the layer thickness in decoupling these states from the metal surface and how it affects their lifetime.
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Gauyacq, J. P., & Lorente, N. (2011). Excitation of spin waves by tunneling electrons in ferromagnetic and antiferromagnetic spin-1/2 Heisenberg chains. PHYSICAL REVIEW B, 83(3), 035418.
Résumé: Excitation of finite chains of magnetic atoms adsorbed on a surface by tunneling electrons from a scanning tunneling microscope tip is studied using a Heisenberg Hamiltonian description of the magnetic couplings along the chain and a strong coupling approach to inelastic tunneling. The excitation probability of the magnetic levels is very high and the excitation spectra in chains of different lengths are very similar. The excitations in finite chains can be considered as spin waves quantized in the finite object. The energy and momentum spectra of the spin waves excited in the idealized infinite chain by tunneling electrons are determined from the results on the finite chains. Both ferromagnetic and antiferromagnetic couplings are considered, leading to very different results. In particular, in the antiferromagnetic case, excitations linked to the entanglement of the chain ground state are evidenced.
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Gauyacq, J. P., & Lorente, N. (2011). Magnetic excitation by tunneling electrons of frustrated ferromagnetic spin-1/2 chains and rings. PHYSICAL REVIEW B, 84(8), 085415.
Résumé: Excitation of finite size chains of magnetic spin-1/2 atoms adsorbed on a surface by tunneling electrons from a scanning tunneling microscope is studied theoretically in the case of a frustrated ferromagnetic structure of the chain. The magnetic excitation processes are described using the strong coupling approach from Lorente and Gauyacq [Phys. Rev. Lett. 103, 256802 (2009)]. Varying the exchange-coupling parameters, the chain length and the strength of an applied magnetic field generates a broad variety of magnetic structures in the chain. The links between these various structures and the excitation processes by tunneling electrons are presented, together with a discussion on how the magnetic structure of a chain could be inferred from an inelastic electron tunneling experiment. Extrapolation of the finite size calculations to infinite chains leads to a discussion of the characteristics of the spin waves that can be excited by tunneling electrons in these frustrated magnetic systems.
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Jackson, B., & Nave, S. (2011). The dissociative chemisorption of methane on Ni(100): Reaction path description of mode-selective chemistry. JOURNAL OF CHEMICAL PHYSICS, 135(11), 114701.
Résumé: We derive a model for the dissociative chemisorption of methane on a Ni(100) surface, based on the reaction path Hamiltonian, that includes all 15 molecular degrees of freedom within the harmonic approximation. The total wavefunction is expanded in the adiabatic vibrational states of the molecule, and close-coupled equations are derived for wave packets propagating on vibrationally adiabatic potential energy surfaces, with non-adiabatic couplings linking these states to each other. Vibrational excitation of an incident molecule is shown to significantly enhance the reactivity, if the molecule can undergo transitions to states of lower vibrational energy, with the excess energy converted into motion along the reaction path. Sudden models are used to average over surface impact site and lattice vibrations. Computed dissociative sticking probabilities are in good agreement with experiment, with respect to both magnitude and variation with energy. The v(1) vibration is shown to have the largest efficacy for promoting reaction, due to its strong non-adiabatic coupling to the ground state, and a significant softening of the vibration at the transition state. Most of the reactivity at 475 K is shown to result from thermally assisted over-the-barrier processes, and not tunneling. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3634073]
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Rougeau, N., Teillet-Billy, D., & Sidis, V. (2011). On the PES for the interaction of an H atom with an H chemisorbate on a graphenic platelet. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 13(39), 17579–17587.
Résumé: Motivated by the problem of H(2) formation in diffuse clouds of the interstellar medium (ISM), we study the effect of including van der Waals-type corrections in DFT calculations on the entrance PES of the Eley-Rideal reaction H(b) + H(a)-GR -> H(b)-H(a) + GR for a graphenic surface GR. The present calculations make use of the PBE-D3 dispersion corrected functional of Grimme et al. (2010) and are carried out on cluster models of graphenic surfaces: C(24)H(12) and C(54)H(18). To assess the soundness of the chosen functional we start by revisiting the H-GR adsorption potential. We find a satisfactory on top physisorption well (43-48 meV) correctly located at an H-GR distance of 3 angstrom. We then revisit the H(b)-H(a)-GR system using both the PW91 and PBE functionals. Our calculations do not reproduce the tiny potential barrier reported earlier for large H(b) distances from the surface. The barrier in the calculations of Sidis et al. (2000) and Morisset et al. (2003, 2004) has been traced to their previous use of an LSDA + POSTSCF PW91 procedure rather than the genuine PW91 one. The new PBE-D3 PES for the H(b)-H(a)-GR system is reported as a function of the H(b) distance to the surface and its impact parameter relative to the H(a) chemisorbate for the so-called “fixed puckered” (“diabatic” or “sudden”) approach. The results are discussed in relation to recent experimental and theoretical work.
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Stepanow, S., Mugarza, A., Ceballos, G., Gambardella, P., Aldazabal, I., Borisov, A. G., & Arnau, A. (2011). Localization, splitting, and mixing of field emission resonances induced by alkali metal clusters on Cu(100). Phys. Rev. B, 83(11), 115101.
Résumé: We report on a joint scanning tunneling microscopy (STM) and theoretical wave packet propagation study of field emission resonances (FER's) of nanosized alkali metal clusters deposited on a Cu(100) surface. In addition to FER's of the pristine Cu(100) surface, we observe the appearance of island-induced resonances that are particularly well resolved for STM bias voltage values corresponding to electron energies inside the projected band gap of the substrate. The corresponding dI/dV maps reveal island-induced resonances of different nature. Their electronic densities are localized either inside the alkali cluster or on its boundaries. Our model calculations allow us to explain the experimental results as due to the coexistence and mixing of two kinds of island-induced states. On the one side, since the alkali work function is lower than that of the substrate, the nanosized alkali metal clusters introduce intrinsic localized electronic states pinned to the vacuum level above the cluster. These states can be seen as the FER's of the complete alkali overlayer quantized by the cluster boundaries. On the other side, the attractive potential well due to the alkali metal cluster leads to two-dimensional (2D) localization of the FER's of the Cu(100) surface, the corresponding split component of the resonances appearing below the bottom of the parent continuum. Our main conclusions are based on the attractive nature of the alkali ad-island potential. They are of general validity and, therefore, significant to understand electron confinement in 2D.
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Zugarramurdi, A., Borisov, A. G., Zabala, N., Chulkov, E. V., & Puska, M. J. (2011). Clustering and conductance in breakage of sodium nanowires. Phys. Rev. B, 83(3), 035402.
Résumé: We study the conductance during the elongation and breakage of Na nanowires described with the ultimate jellium model. A combined approach is used where the nanowire breakage is simulated self-consistently within the density-functional theory, and the wave packet propagation technique is applied for ballistic electron transport. For certain conditions the breakage of the nanowire is preceded by formation of clusters of magic size in the break junction. This affects the conductance G, in particular the shape of the G = 3G(0) to G = G(0) (=2e(2)/h) step upon elongation. The observed trends can be explained as due to the transient trapping of ballistic electrons inside the cluster, leading to a resonant character of the electron transport through the break junction. The cluster-derived resonances appear as peak structures in the differential conductance which may serve as an experimental signature of clustering.
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Zugarramurdi, A., Zabala, N., Borisov, A. G., & Chulkov, E. V. (2011). Comment on “Phase Contribution of Image Potential on Empty Quantum Well States in Pb Islands on the Cu(111) Surface”. Phys. Rev. Lett., 106(24), 249601.
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Zugarramurdi, A., Zabala, N., Borisov, A. G., & Chulkov, E. V. (2011). Theoretical study of constant current scanning tunneling spectroscopy in Pb overlayers. Phys. Rev. B, 84(11), 115422.
Résumé: We present a theoretical study of the constant current scanning tunneling spectroscopy of quantum well states localized in Pb(111) overlayers on Cu(111) surfaces. The distance-voltage characteristic of the tunneling junction is obtained with a mixed approach. The wave packet propagation technique is applied to describe electron tunneling from the tip into the sample, and the density functional calculations provide the necessary inputs for the one-dimensional model potential representation of the system. The excited-state population decay mechanisms via inelastic electron-electron and electron-phonon interactions are taken into account with a bias-dependent absorbing potential introduced in the metal. Our results are in good agreement with recent experimental studies [Phys. Rev. Lett. 102, 196102 (2009), Phys. Rev. B 81, 205438 (2010)] over the energy range where the free-electron description of the Pb overlayer used here applies. We find that at high bias the quantum well states experience a Stark energy shift and partially acquire a character of field emission resonances. The present model study also sheds light at the experimentally observed departure of the energies of the quantum well states from the particle-in-a-box prediction for the bias above 4 eV. The measured trend can be consistently explained as due to the departure of the realistic Pb band structure in the Gamma-L direction from free-electron parabola when the electron momentum approaches the Gamma point.
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