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Cover Image, Volume 116, Issue 21
No full textOn page 1575, Matteo Bonfanti and Rocco Martinazzo examine reactions at surfaces under a magnifying glass. Fruitful combination of theory, modelling, and simulations provides a powerful tool to understand the dynamics of atoms and molecules at the gas‐solid interface. This is pictured on the cover with a simple illustration of two prototypical recombination processes, the Eley‐Rideal (left) and the Langmuir‐Hinshelwood (right) reactions. Image credit goes to Matteo Bonfanti. (DOI: 10.1002/qua.25192
Classical and quantum dynamics at surfaces : basic concepts from simple models
Full text linkElementary processes involving atomic and molecular species at surfaces are reviewed. The emphasis is on simple classical and quantum models that help to single out unifying dynamical themes and to identify the basic physical mechanisms that underlie the rich variety of phenomena of surface chemistry. Starting from an elementary description of the energy transfer between a gas-phase species and a surface—for both classical and quantum lattices—the key processes establishing the formation of an adsorbed phase (sticking, diffusion and vibrational relaxation) are discussed. This is instrumental for introducing the simplest chemical transformations involving adsorbed species and/or scattering of gas-phase molecules: Langmuir–Hinshelwood, Hot-Atom, and Eley–Rideal reactions forming complex molecules from elementary constituents, and dissociative chemisorption of molecules into smaller fragments. Applications are also provided illustrating the ideas developed along the way at work in real-world gas-surface problems
Quantum dynamics of hydrogen atoms on graphene. II. Sticking
Full text linkFollowing our recent system-bath modeling of the interaction between a hydrogen atom and a graphene surface (M Bonfanti et al., XXX), we present the results of converged quantum scattering calculations on the activated sticking dynamics. The focus of this study is the collinear scattering on a surface at zero temperature, which is treated with high-dimensional wavepacket propagations with the multi-configuration time-dependent Hartree method. At low collision energies barrier-crossing dominates the sticking and any projectile that overcomes the barrier gets trapped in the chemisorption well. However, at high collision energies energy transfer to the surface is a limiting factor, and fast H atoms hardly dissipate their excess energy and stick on the surface. As a consequence, the sticking coefficient is maximum ($0:65) at an energy which is about one and half larger than the barrier height. Comparison of the results with classical and quasi-classical calculations shows that quantum fluctuations of the lattice play a primary role in the dynamics. A simple impulsive model describing the collision of a classical projectile with a quantum surface is developed which reproduces the quantum results remarkably well for all but the lowest energies, thereby capturing the essential physics of the activated sticking dynamics investigate
REACTIONS AT SURFACES: BEYOND THE STATIC SURFACE APPROACH IN QUANTUM DYNAMICS
Full text linkThanks to the peculiar electronic properties of gas-solid interfaces, surfaces play an important role in many chemical processes. In my thesis, I considered few different reactions at surfaces and addressed the problem of their description by means of quantum dynamical methods. In particular, the focus of the work is on the inclusion of surface motion in the dynamical models. This problem is very challenging for state-of-art quantum methods, due to the unfavorable scaling with the number of degrees of freedom. To avoid this computational limit a variety of methods were adopted, ranging from a static approach in a low dimensional Time Dependent Wave Packet (TDWP) calculations to a full dynamical description of dissipation in the framework of Multi-Configuration Time-Dependent Hartree method (MCTDH).
I considered three different physical problems. The first one is the exothermic, collinearly-dominated Eley-Rideal H2 formation on graphite. In particular, I focused on the importance of the model used to describe the graphitic substrate, in light of the marked discrepancies present in available literature results. To this end, I considered the collinear reaction and computed the Potential Energy Surface (PES) for a number of different graphitic surface models using Density Functional Theory (DFT) for different dynamical regimes. I performed quantum dynamics with wave-packet techniques down to the cold collision energies relevant for the chemistry of the interstellar medium. Results show that the reactivity at moderate-to-high collision energies sensitively depends on the shape of the PES in the entrance channel, which in turn is related to the adopted surface model. At low energies I ruled out the presence of any barrier to reaction, thereby highlighting the importance of quantum reflection in limiting the reaction efficiency.
In a second part of my work, I studied the effect of lattice displacement on the interaction of H2 with the Cu(111) surface using the Specific Reaction Parameter (SRP) approach to DFT. I systematically investigated how the motion of the surface atoms affects some features of the PES, such as the dissociation barrier height and the barrier geometry corresponding to some representative reaction pathways, and the anisotropy of the potential at these geometries. This analysis allowed the identification of the surface degrees of freedom that are likely to be most relevant for H2 dissociation. In particular, I found that the lattice coordinate displacements that have the largest effect on the H2/Cu(111) DFT-SRP barrier heights and locations concern the motion of the 1st layer and 2nd layer Cu atoms in the Z direction, and motion of the 1st layer atoms in the directions parallel to the surface. Whereas the first degree of freedom mostly affects the barrier geometry, the second and third motions can lower or raise the barrier height. The latter effect cannot be described with the usual surface oscillator dynamical models employed in the past to include surface motion, and its dynamical influence on the dissociative adsorption needs to be further investigated.
In the third part of the thesis I addressed the problem of including dissipative effects in the reaction dynamics of hydrogen sticking and scattering on surfaces. I considered dissipative baths with different spectral properties and represented them with a linear chain of coupled harmonic oscillators, exploiting an equivalent effective-mode representation that has recently been developed. I studied the system dynamics with MCTDH, aiming on one hand to an accurate description of dissipation at a short time scale, and on the other hand to a simplified but qualitatively correct behavior of the long time dynamics. In this framework, I found a very useful scheme to represent the long time dynamics of the system without incurring in unwanted Poincaré's recurrences. I used this method to obtain the sticking probability of one hydrogen atom scattered by a simple one dimensional Morse potential. The methodology developed in this work is going to be extended to the more realistic problem of hydrogen sticking on graphitic surfaces
Quantum dynamical investigation of the isotope effect in H-2formation on graphite at cold collision energies
Full text linkThe Eley-Rideal abstraction of hydrogen atoms on graphitic surfaces at cold collision energies was investigated using a time-dependent wave packet method within the rigid-flat surface approximation, with a focus on hydrogen-deuterium isotopic substitutions. It is found that the marked isotope effect of collinear collisions disappears when the full dimensionality of the problem is taken into account, thereby suggesting that abstraction is less direct than commonly believed and proceeds through glancing rather than head-on collisions. In contrast, a clear isotope effect is observed for "hot-atom'' formation, which appears to be strongly favored for heavy projectiles because of their higher density of physisorbed states. Overall, the dynamics is essentially classical and reasonably well described by quasi-classical trajectory methods at all but the lowest energies (less than or similar to 10 meV). A comparison of the results obtained in the (substrate) adiabatic and diabatic limits suggests that the reaction is only marginally affected by the lattice dynamics, but highlights the importance of including energy dissipation processes in order to accurately describe the internal excitation of the product molecules
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Unitary Approaches to Dissipative Quantum Dynamics
Full text linkWe describe in detail a general system–bath strategy for investigating the quantum behavior of small systems interacting with complex environments. In this approach, a simplified heat bath is used as a surrogate for realistic environments, and explicit, unitary quantum simulations of the “universe” (the system plus the bath) are performed by means of high-dimensional wave-packet techniques. In this chapter, we describe the underlying Hamiltonians and the related reduced dynamical descriptions, show how to recast real-world problems into this form, introduce some of the methods currently used to deal with high-dimensional quantum dynamics, and present the results of this strategy when applied to numerous problems of physicochemical interest
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