1,721,112 research outputs found
The Local Coherent-State Approximation to System-Bath quantum dynamics
No full textThe recently introduced Local Coherent-State Approximation (LCSA)
method (R. Martinazzo, M. Nest, P. Saalfrank and G.F. Tantardini,
\emph{J. Chem. Phys.} \textbf{125}, 194102 (2006)) is a time-dependent,
multiconfiguration method of the general G-MCTDH type which has been
specifically tailored to deal with system-bath dynamical problems.
In the usual system-bath picture, it uses discrete variable representation
(DVR) states for the subsystem and Hartree products of coherent-states
(CSs) for local bath states, in such a way to capture the important
system-bath correlations and to scale linearly with respect to the
number of bath degrees of freedom. Some generalizations of the method
are introduced, ranging from locally multiconfigurational variants
to generalized mixed quantum-classical schemes. In the multiconfiguration
variants the Hartree approximation is relaxed while preserving the
linear scaling property, and local bath states are described by superposition
of CSs products. In the generalized mixed quantum-classical approaches,
locality is reduced and each bath configuration is used for groups
of subsystem DVR states. These extensions include a DVR-based, full
G-MCTDH method and the traditional mixed quantum-classical approach
as limiting cases. Applications to model system-bath problems at
K (e.g. vibrational relaxation, tunneling and surface sticking) are
discussed and results are compared with exact MCTDH ones in systems
with bath degrees of freedom
The Local Coherent-State Approach to System-Bath Quantum Dynamics and Its Extensions
No full textSo called system-bath problems arise naturally in chemistry and physics, e.g. in gas-surface processes or reactions in condensed phase. Solution of these problems is enormously difficult in quantum mechanics. The most elegant and promising approaches relying on reduced equations of motion for the system density operator are presently limited to severe approximations, e.g. weak system-bath coupling and short bath correlation time. Here we consider the possibility of approximately following the unitary evolution of the whole system-bath state(s), in the spirit of the surrogate hamiltonian approach (R. Baer and R. Kosloff, J. Chem. Phys. 106, 8862 (1997)), and discuss a number of approximations specifically tailored for this problem. These approximations are mainly in the bath description, whose dynamics is not of direct relevance for our purposes. Starting with the recently introduced Local Coherent-State Approximation (LCSA) (R. Martinazzo, M. Nest, P. Saalfrank and G.F. Tantardini, J. Chem. Phys. 125, 194102 (2006)) for the T=0 K dynamics, we introduce a number of its generalizations. These include (i) a non-local variant using time-dependent system states, which more closely resembles the Gaussian MultiConfuration Time-Dependent Hartree (G-MCDTH) method (I. Burghardt, H.-D. Meyer and L.S. Cederbaum, J. Chem. Phys. 111, 2927 (1999)) with selected configurations; (ii) locally multiconfigurational variants which relaxe the Hartree approximation for the local bath states; (iii) generalized mixed quantum-classical approaches, in which locality is reduced and each bath configuration is used for groups of system DVR states. All these approaches are designed to scale linearly with respect to the bath dimensions. Applications to model system-bath problems at T=0 K (e.g. vibrational relaxation, tunneling and surface sticking) are discussed and results are compared with exact MCTDH ones in systems with 50-100 bath degrees of freedom. Futher extensions to T > 0 K dynamics are introduced
Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case
No full textFollowing previous investigation of collision induced (CI) processes involving hydrogen atoms chemisorbed on graphite [R. Martinazzo and G. F. Tantardini, J. Chem. Phys. 124, 124702 (2006)], the case in which the target hydrogen atom is initially physisorbed on the surface is considered here. Several adsorbate-substrate initial states of the target H atom in the physisorption well are considered, and CI processes are studied for projectile energies up to 1 eV. Results show that (i) Eley-Rideal cross sections at low collision energies may be larger than those found in the H-chemisorbed case but they rapidly decrease as the collision energy increases; (ii) product hydrogen molecules are vibrationally very excited; (iii) collision induced desorption cross sections rapidly increase, reaching saturation values greater than 10 Å2; (iv) trapping of the incident atoms is found to be as efficient as the Eley-Rideal reaction at low energies and remains sizable (3–4 Å2) at high energies. The latter adsorbate-induced trapping results mainly in formation of metastable hot hydrogen atoms, i.e., atoms with an excess energy channeled in the motion parallel to the surface. These atoms might contribute in explaining hydrogen formation on graphite
Adsorption, clustering and reactions of H atoms on graphene
No full textRecent years have witnessed an ever increasing interest in studying the interaction of hydrogen
atoms with graphenic surfaces. Such interest comes from its importance in fusion reactors,
possible relevance in hydrogen storage, necessity in understanding hydrogen formation in
interstellar medium, and from the opportunities it offers in the rapidly exploding field of
graphene-related device fabrication. In this contribution we summarize the basic features of this
interaction and the dynamical behaviour it gives rise to, as they result from a combined
theoretical study using electronic structure (TB, DFT, MRPT) and quantum wave packet
calculations. We start considering adsorption of a single H atom on graphene, which has been
long known[1] to be an activated process requiring substantial lattice reconstruction. We then
show how, differently from metals, formation of the adsorbate-substrate bond strongly modifies
the electron properties of the substrate at the Fermi level, through the formation of so-called
midgap states (itenerant electrons). The latter, in turn, introduce a bias in the adsorption
properties[2] which is responsible for the observed, markedly non-random adsorption pattern,
i.e. clustering of H atoms on the surface[3]. We show how binding (barrier) energies increase
(decrease) linearly as a function of the site-integrated magnetization, which is a measure of the
site-occupation for the itinerant electron. Midgap states, and the related bias to adsorption, are
common to all graphenic substrates, and we show that they lead to analogous results in
Polycylic Aromatic Hydrocarbons (PAH). The latter further highlight the importance of edge
sites which, having a reduced hindrance, show better adsorption properties[4]. Next, we
consider dynamics of hydrogen atoms on graphene, focusing in particular on the Eley-Rideal
reaction out of a single, adsorbed H atom. We consider the rigid, flat surface model[5] and
provide exact, quantum results for this model in both high and low collision energy regimes,
showing evidence for unusual quantum effects at high energies[6] and quantum reflection at
low energies[7]. We conclude by discussing how controlled adsorption of hydrogen atoms can
be used to engineer graphene electronic structure, e.g. opening that band-gap which is of
paramount importance for graphene-based logic-devices. In particular, we show how a
symmetry-preserving band-gap opening can be achieved by arranging H atoms to form certain
honeycomb superlattices[8].
[1] Jeloaica L. and Sidis V., Chem. Phys. Lett., 1999, 300, 157; Sha X. and Jackson B, Surf. Sci., 2002, 496, 318
[2] Casolo S., LØvvik O. M., Martinazzo R. and Tantardini G.F., J. Chem. Phys., 2009, 130, 054704
[3] Hornekaer L., Rauls E., Xu W., Sljivancanin Z. Otero R., Stensgaard I., Laegsgaard E., Hammer B.,
Besenbacher F. , Phys. Rev. Lett., 2006, 97, 186102
[4] Bonfanti M. Casolo S., Tantardini G.F. and Martinazzo R., in preparation
[5] Persson M and Jackson B, J .Chem. Phys., 1995, 102, 1708; Lemoine D. and Jackson B., Comp. Phys.
Comm., 2001, 137, 415
[6] Martinazzo R. and Tantardini G.F., J. Phys. Chem. A, 2005, 109, 9379; J .Chem. Phys., 2006, 124, 124702;
J .Chem. Phys., 2006, 124, 124703.
[7] Casolo S., Bonfanti M., Martinazzo R. and Tantardini G.F., J. Phys. Chem. A, 2009, 113, 14545.
[8] Martinazzo R., Casolo S. and Tantardini G.F., Phys. Rev. B, 2010, 81, 245420
Hydrogen formation on graphitic surfaces: energetics and dynamics of elementary processes
No full textA summary is given of our current understanding of hydrogen formation on graphitic surfaces. In particular, the focus is on the energetics and dynamics of elementary processes, i.e. physi- and chemisorption of single H atoms, dimer and cluster formation, and Eley-Rideal reaction dynamics
Theoretical study of hydrogen adsorption and dynamics on graphitic surfaces
No full textInteraction of hydrogen atoms with graphitic surfaces is currently subject of intense research activity because of its relevance in fields as diverse as nuclear fusion, hydrogen storage and astrophysics. In particular, the adsorption of H atoms on the graphitic dust grains of the interstellar medium (ISM) and their subsequent recombination reactions are considered to be key steps for H2 formation and its anomalous large abundance especially in diffuse regions of the ISM.
In this contribution a number of processes concerning adsorption of hydrogen atoms on the (0001) graphite surface and their recombinative desorption are addressed by means of electronic structure and quantum dynamical calculations. Firstly, accurate first-principles calculations with the plane-wave, periodic Density Functional Theory (DFT) approach and a large 5x5 cell are used to investigate single, double and multiple adsorption of hydrogen atoms on graphite. Binding and barrier energies for chemisorption of hydrogen atoms are computed for a number of different configurations, the spin densities are analysed, and the results are rationalized in terms of the p-resonance theory of aromatic compounds [1]. Secondly, several model Polycyclic Aromatic Hydrocarbons (PAHs) are used both in DFT and Multi-Reference Perturbation Theory (MRPT) calculations to assess the quality of the DFT data, to investigate the adsorption properties on (H-saturated) edges of graphitic surfaces, and to validate the above p-resonance chemical picture [2]. This extends recent work on physisorption and diffusion of hydrogen atoms on graphite [3]. Thirdly, time-dependent wave packet calculations in the rigid, flat surface approximation are used to investigate a number of processes induced by collisions of projectile hydrogen atoms on target hydrogens previously adsorbed on graphite. The collision energy ranges from values where collision induced desorption is possible [4] to values typical of interstellar cloud conditions, i.e. from some eV down to 10-5 eV. In particular, converged results obtained with a novel two-wave packet approach [5] show that hydrogen formation by Eley-Rideal reaction in the cold collision energy regime is largely prevented by quantum reflection [6]. This result is true both for initially chemisorbed and for initially physisorbed H atoms on graphite, and does not depend on the presence of a barrier in the reaction path, thereby reducing the role that Eley-Rideal hydrogen recombination processes play at interstellar cloud conditions.
We conclude by critically re-discussing the possibility of forming molecular hydrogen on graphite at low temperature in the light of the above results
Interaction of hydrogen atoms with carbon-sp2 structures: adsorption energetics and Eley-Rideal dynamics
No full textThe interaction and dynamics of hydrogen atoms on and with graphitic substrates is summarized in the light of their possible relevance for hydrogen formation in the interstellar medium. Firstly, accurate DFT results for adsorption of single hydrogen atoms, dimer and cluster formation and edge-related effects, are rationalized with the help of simple tight-binding ideas, in order to shed light about the general mechanisms governing such processes. Secondly, recent quantum scattering results for the Eley-Rideal formation reaction are shown in an astrophysically relevant cold collision energy regime
Quantum studies of Hydrogen dynamics on graphite surfaces
No full textA number of dynamical processes involving hydrogen atoms on graphite surfaces is addressed by means of time-dependent quantum dynamical calculations. These processes are relevant for hydrogen formation in interstellar clouds, where molecular formation has been long hypothesized to occur on the surface of dust grains. Firstly, hydrogen formation via Eley- Rideal recombination is studied within the rigid, flat surface approximation [1] starting from both chemisorbed and physisorbed species, with some emphasis on unexpected quantum effects in the reaction dynamics at high collision energies [2]. Competitive processes such as collision induced desorption of adsorbed species and adsorbate-induced trapping are also discussed. Secondly, the special behavior of the dynamics in the so-called cold collision energy regime (i.e. for collision energies down to 1 K) is considered with a zero-energy two- wavepackets approach. Thirdly, tunneling diffusion of physisorbed hydrogen atoms at T=0 K is modeled with wavepacket dynamics on a new ab-initio Potential Energy Surface based on correlated wavefunction calculations on a cluster model [3]. Extension of the model to chemisorption is also discussed with the aim of assessing the reliability of available Density- Functional-Theory data. Fourthly, our work in progress in dealing with energy dissipation to the surface is presented. Specifically, the Local Coherent-State Approximation (LCSA) to system-bath quantum dynamics [4] is introduced and results of model calculations are presented and compared to the exact Multi Configuration Time-Dependent Hartree ones. These model calculations comprise vibrational relaxation, tunneling dynamics and surface sticking in systems with 50-80 bath (surface) degrees of freedom. Extensions to realistic problems and T>0 K dynamics are discussed, with some emphasis on the good (linear) scaling properties of the LCSA method with respect to the bath size
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
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
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