The interaction of hydrogen atoms and molecules with a silica surface is relevant for manyresearch and technological areas. Here, the dynamics of hydrogen atoms colliding with anH-preadsorbed ?-cristobalite (0 0 1) surface has been studied using a semiclassical collisionalmethod in conjunction with a recently developed analytical potential energy surface based ondensity functional theory (DFT) calculations. The atomic recombination probability via anEley-Rideal (E-R) mechanism, as well as the probabilities for other competitive surfaceprocesses, have been determined in a broad range of collision energies (0.04-3.0 eV) foroff-normal (?v = 45o) and normal (?v = 0o) incidence and for two different surfacetemperatures (TS = 300 and 1000 K). H2,gas molecules form in roto-vibrational excited levelswhile the energy transferred to the solid surface is below 10% for all simulated conditions.Finally, the global atomic recombination coefficient (?E-R) and vibrational state resolvedrecombination coefficients (? (v)) were calculated and compared with the availableexperimental values. The calculated collisional data are of interest in chemical kinetics studiesand fluid dynamics simulations of silica surface processes in H-based low-temperature,low-pressure plasmas.

The modeling of atmospheric gas, interacting with the space vehicles in re-entry conditions in planetary exploration missions, requires a large set of scattering data for all those elementary processes occurring in the system. A fundamental aspect of re-entry problems is represented by the strong non-equilibrium conditions met in the atmospheric plasma close to the surface of the thermal shield, where numerous interconnected relaxation processes determine the evolution of the gaseous system towards equilibrium conditions. A central role is played by the vibrational exchanges of energy, so that collisional processes involving vibrationally excited molecules assume a particular importance. In the present paper, theoretical calculations of complete sets of vibrationally state-resolved cross sections and rate coefficients are reviewed, focusing on the relevant classes of collisional processes: resonant and non-resonant electron-impact excitation of molecules, atom-diatom and molecule-molecule collisions as well as gas-surface interaction. In particular, collisional processes involving atomic and molecular species, relevant to Earth (N-2, O-2, NO), Mars (CO2, CO, N-2) and Jupiter (H-2, He) atmospheres are considered.

In this contribution we focus on the dynamics of a variety of physico-chemical processes due to the interaction of oxygen atoms and molecules with silica and silica-based materials: 1) O atoms inelatic adsorption and adsorption/desorption processes of atomic oxygen; 2) Eley-Rideal O atoms recombination process followed by O2 formation in specific roto-vibrational levels; 3) Molecular oxygen dissociation and deactivation. We performed Molecular Dynamics calculations using an accurate potential energy surface for the interaction of O, O2 with the silica substrate determined by DFT electronic structure calculations for the different interactions Ogas-Obulk and Ogas-Sibulk [1]. The semiclassical collisional method [2] was then applied to describe the dynamics of the nuclear motions of the atoms/molecule over the calculated "effective" potential energy surface coupled to the dynamics of the phonon excitation/de-excitation processes in the substrate.

In this contribution we focus on the dynamics of a variety of physico-chemical processes due to the interaction of oxygen atoms and molecules with silica and silica-based materials: 1) O atoms inelatic adsorption and adsorption/desorption processes of atomic oxygen; 2) Eley-Rideal O atoms recombination process followed by O2 formation in specific roto-vibrational levels; 3) Molecular oxygen dissociation and deactivation. We performed Molecular Dynamics calculations using an accurate potential energy surface for the interaction of O, O2 with the silica substrate determined by DFT electronic structure calculations for the different interactions Ogas-Obulk and Ogas-Sibulk [1]. The semiclassical collisional method [2] was then applied to describe the dynamics of the nuclear motions of the atoms/molecule over the calculated "effective" potential energy surface coupled to the dynamics of the phonon excitation/de-excitation processes in the substrate.

The interaction of methane with an aluminium-free zeolite (ZSM-5) porous substrate has been investigatedby means of DFT and DFT-D calculations. We observe no charge transfer between host-guest speciesand, most interestingly, the energetic balance appears to be reasonably linked to the volume size ofindividual internal cavities. In fact, the gaseous-molecule is loosely bound only in larger 10MR poreswhile, inside the narrow 6MR ring, on because of the proximity of individual electronic clouds, the chemicalinteraction is repulsive. From a comparison with DFT approach it is evident that dispersion energiesare crucial for a correct energetics and that long range forces drive the adsorption processes. Similarresults are obtained for other small species, like hydrogen (atom and molecule) and CH3 radical species,considered in our current, although not exhaustive, investigation as products of hypothetical methanedissociative adsorptions.

We report cross-sections and rate coefficients for excited states colliding with electrons, heavy particles and walls useful for the description of H-2/He plasma kinetics under different conditions. In particular, the role of the rotational states in resonant vibrational excitations of the H-2 molecule by electron impact and the calculation of the related cross-sections are illustrated. The theoretical determination of the cross-section for the rovibrational energy exchange and dissociation of H-2 molecule, induced by He atom impact, by using the quasi-classical trajectory method is discussed. Recombination probabilities of H atoms on tungsten and graphite, relevant for the determination of the nascent vibrational distribution, are also presented. An example of a state-to-state plasma kinetic model for the description of shock waves operating in H-2 and He-H-2 mixtures is presented, emphasizing also the role of electronically-excited states in affecting the electron energy distribution function of free electrons. Finally, the thermodynamic properties and the electrical conductivity of non-ideal, high-density hydrogen plasma are finally discussed, in particular focusing on the pressure ionization phenomenon in high-pressure high-temperature plasmas.

The Eley-Rideal recombination reaction of H chemisorbed on the four-fold site of W(001) at asurface temperature TS = 500 K is studied using the fully three-dimensional semiclassicalcollisional model and an accurate potential energy surface for the H-W(001) system. Therecombination probability, calculated at different collisional energies in the range (0.05-5) eV,shows a broad maximum around 0.4 for energies between 0.1 eV and 2.5 eV. The exothermicenergy partitioning in the final states of the desorbing H2 molecules shows that, at low impactenergies, only the first three vibrational levels of the hydrogen molecule are energeticallyaccessible, while at the higher impact energies vibrational levels up to v = 7 can be populated.The energy exchanged with the phonons surface is small but not negligible.

The Eley-Rideal recombination reaction of H chemisorbed on the four-fold site of W(001) at a surface temperature TS=500K was studied using the fully-dimensioned semiclassical collisional model and an accurate potential energy surface for the H-W(001) system. The recombination probabilities calculated at impact energies in the range (0.05-5)eV, the energy distributions in the final states as well as other relevant collisional data are presented and discussed.

The interaction of methane with an extra-framework oxygen atom in acidic Zeolite (H-ZSM5) porous substrate has been investigated by means of Density Functional Theory plus Dispersion energy calculations and reaction path has been obtained exploiting Climbing Image Nudged Elastic Band method (c-NEB). Zeolite was modelled by its crystallographic structure subject to periodic boundary condition. Relaxation of the zeolite cage is fundamental for a correct energetics. The reaction path for the H - abstraction reaction of methane, in presence of an open shell oxygen atom within the zeolite along (010) straight channel leads to the formation of a slightly distorted H2O water molecule and CH3 radical. These in bulk calculations support the interesting idea that open shell systems, involving an extra cage Oxygen atom, favour the H-abstraction from small hydrocarbons.

Zeolites are crystalline microporous alumino-silicates characterized by internal cavities and channels of molecular dimensions which allow the use of these materials in different catalytic or separation processes. On the other hand, O(3P) species is well known for its excellent performance in oxidation processes. In this contribution, by first principle Density Functional Theory corrected by semi-empirical dispersion forces (D-DFT)[1,2] calculations, using plane waves expansions and ultrasoft pseudopotentials, we show that triplet oxygen atom, anchored by acidic site in H-ZSM-5 zeolite, plays a catalytic role in the H-abstraction reaction from CH4 to produce (OH)-and CH3 radical. Open shell systems were treated using unrestricted formalism and zeolite was modeled by its crystallographic structure subject to periodic boundary conditions [3]. The reaction is found to be strongly exothermic and a reaction mechanism, with a relatively small energy barrier, is proposed.

The realization of future fusion reactors has revealed the need of new data on hydrogen recombination on several kinds of materials at high temperature level. In the literature, one can found only data at low temperature for different materials (silica, stainless steel, some pure metals and carbon). Today tungsten appears one of the most promising materials with respect to graphite and beryllium as plasma-facing material to use in magnetic fusion energy devices, both on the divertor and first-wall in tokamaks. Therefore it seems necessary to perform measurement of the recombination coefficient of atomic hydrogen at higher temperature levels. For this reason, in the last years, a large interest both experimental and theoretical was focused on this material and its interaction with hydrogen and its isotopes [1, 2].In this contribution, we propose a joint experimental and theoretical investigation on hydrogen atoms interaction with a W(110) surface following a previous study we have performed on atomic oxygen interaction on quartz [3].The experimental part is performed using the MESOX experimental set-up to evaluate the recombination coefficient of atomic hydrogen, based on the measurement of the relative concentration profiles H?/H2 or H?/He by optical emission spectroscopy [4, 5]. Experimental results obtained for the recombination coefficient of hydrogen atoms (?H) are obtained for tungsten from 700 to 1350 K.These latter are compared with the results obtained in Molecular Dynamics (MD) simulations based on the semi-classical collisional method [6] for the recombination reaction between an atom adsorbed on the surface and an atom impinging from the gas phase. The simulation was done for the W(110) surface at two temperatures of 700 and 1000 K, for normal incidence of H atom hitting the surface and for collisional energies in the range 0.05-6 eV. The effect of adsorption site on the reaction dynamics was also evaluated.A fairly good agreement between the experimental and calculated data was obtained.[1] M. Rutigliano and M. Cacciatore 2011 Phys. Chem. Chem. Phys. 13 7475-7484[2] S. Markelj and I. C?de? 2011 J. Chem. Phys. 134 124707[3] L. Bedra, M. Rutigliano, M. Balat-Pichelin, M. Cacciatore 2006 Langmuir 22 7208-7216[4] M. Balat-Pichelin, J.M. Badie, R. Berjoan, P. Boubert 2003 Chem. Phys. 291 181-194[5] J. W. Coburn and M. Chen 1980 J. Appl. Phys. 51 3134-3136[6] G. D. Billing 2000 Dynamics of Molecule Surface Interactions John Wiley & Sons, New-York

Atom recombination at wall is a phenomenon involved in many plasma experiments and also in present tokamaksand future fusion plasma reactors like ITER. This exothermic surface reaction is catalyzed by the materialand depends on its composition and temperature. In the MESOX experimental set-up, several methods weredeveloped for the measurement of the recombination parameters. In this paper, a method developed for the experimentalevaluation of the recombination coefficient of atomic hydrogen ?H on tungsten at high temperature ispresented using two series of atomic lines (H? and He or H? and H2) and the results obtained for surface temperatureup to 1350 K are given. A Molecular Dynamics Simulation has been done for the recombination of hydrogenatoms on tungsten in conditions close to the experimental ones using a semi-classical collisionalmethod.Modeling results are compared to the experimental data for two surface temperature values and a fairlygood agreement was obtained.

A cesiated surface model was considered to study the dynamics of hydrogen atom scattering using a semiclassical collisional method. Using dipole correction method, the work function of the considered surface, is calculated to be 1.81eV (± 0.02) eV. The Potential Energy Surface for the interaction of H atoms with the surface was determined via first principle electronic structure calculations including the interaction with both Cs and Mo atoms of the surface. We found the scattered H atoms to have a negative partial charge of nearly 0.4 with the backscattered flux arising mainly from H atoms impinging directly (or very close) to Cs atoms on the surface. On the contrary, H atoms impinging in the voids between the Cs atoms propagate through the first Cs layer and remain adsorbed. The propagation occurs mainly in the vertical direction. The scattering probability after a very quick increase remains almost constant around an average value of 0.35.

The interaction of deuterium atoms on a cesiated surface and the formation of hydrogen isotopologues molecules via the Eley-Rideal mechanism is studied using the computational setup recently adopted in the simulations of the same reactions for H atoms. The probability for scattering and adsorption processes on the surface as well as the mechanism underlying the reaction is pointed out for D atoms impinging on the surface in the same dynamical conditions previously used for H atoms.The isotopic effect in molecule formation is highlighted by considering the formation of D2 and HD molecules, this latter obtained by exchanging either the incoming or pre-adsorbed H atom with D isotope. Collisional data have been determined for two different adsorption sites on the surface. The recombination probabilities and coefficients for D2 and HD and the probabilities for other competitive surface processes have been determined. Internal states of isotopic molecules were solved and compared with those of H2 molecule formed on the same surface showing that the heteronuclear molecules are vibrationally more excited. A strong isotope mass effect emerged in the collision of D atom enhancing the probabilities for adsorption/desorption processes. Interestingly, deuterium atom impinging on the pre-adsorbed H atom on the surface increases the recombination probability, which anyway remains low.

The interaction of atomic and molecular oxygen with silica and silica-based materials under high temperature conditions can lead to various chemico-physical processes of great importance in many research areas of fundamental and technological interest, in aerothermodynamics as well as in the chemistry of oxygen-based laboratory plasmas. Among the many possible chemical processes going on at the oxygen-silica interlayer, a primary role is played by the atom recombination process because of the large exothermic energy released in the reaction that can be transferred to the surface causing damage both thermal and chemical. Oxygen atom recombination may also be a very effective heterogeneous channel for the formation of roto-vibrationally excited O2 molecules.Besides atom recombination, other collisional surface processes include inelastic scattering, adsorption and adsorption-desorption processes. In this study the aforementioned processes are simulated using Molecular Dynamics (MD) technique. We follow a fully ab intio approach carried out through two main steps: firstly DFT electronic structure calculations at the GGA level of accuracy are performed to calculate the interaction forces exerted between the gas-phase O,O2 species and the model silica surfaces, then the semiclassical approach1 is applied to describe the dynamics of the nuclear motions of the gas-phase atoms over the calculated potential energy surfaces. In the MD simulations the surface temperature is kept constant to 1000K, while the impact energy of the O atoms colliding with the silica surface is varied in a wide range.The results of the simulations will be presented and discussed in relation to the computed probabilities for the various surface processes, global and state-to-state recombination probabilities and coefficients, reaction energetics, roto-vibrational distributions for the formed O2 molecule and the energy exchanged between the gas-phase oxygen atoms and the silica phonons due to the multi-phonon excitation-deexcitation processes. Correlations between the dynamics of the surface processes and the structural behaviours of the silica surface will be also highlighted.[1] G. D. Billing, Dynamics of Molecule Surface Interactions, John Wiley&Sons, NY, 2000

The interaction of plasmas with surfaces can lead to different surface processes active in different collisional energy regimesaccording to the behaviour of the gas-phase species and substrate involved. In particular, atom recombination at surfaces can be aneffective source of roto-vibrationally excited molecules and, at the same time, an effective process for surface atom abstractionand atom removal from the plasma region [1]. Such processes have a strong impact on the physical-chemistry of the bulk regionand at the plasma-surface interface. In fact, it is well established that the reactivity of molecular plasmas under low-pressure, lowtemperatureconditions depends on, and is often controlled by, the formation of energetically vibrationally activated molecules [2].Therefore, it is important to understand the processes by which one can store vibrational quanta in the vibrational manifold of theactive molecules in the plasma or gaseous media.This contribution is focused on the molecular surface processes due to the interaction of a flux of H and O atoms colliding with asilica surface at low-collisional energies (0.1-3 eV). Hydrogen and oxygen plasmas are currently of great interest in differentresearch areas of fundamental and technological interest such as microelectronics, nano-medicine, modern solar cells, fusionreactors, astrophysics and aerothermodynamics.We will present the results obtained during the last years by our group in the study of the interaction of atoms of H and O on asilica surface based on analytical DFT Potential Energy Surfaces [3, 4]. Molecular Dynamics calculations have been performed usinga semiclassical collisional method that provides a detailed knowledge of the multiphonon inelastic processes that assist thedynamics of the chemical and physical phenomena due to the chemi-/physic-sorption of atoms and molecules on substrate [5].In particular, results concerning the recombination of H [4] and O [6] atoms on the silica surface will be discussed, in a comparativemanner, respect to the recombination probabilities and coefficients, energy distribution in the final states and vibrationaldistributions of the formed molecules.[1] M. Cacciatore and M. Rutigliano, 2009 Plasma Sources Sci.Technol. 18 023002[2] J. Amorim , J. Loureiro and D. Schram , 2001 Chem. Phys. Lett., 346 443-8[3] M. Rutigliano ,C. Zazza , N. Sanna , A. Pieretti , G.Mancini , V. Barone and M. Cacciatore, 2009 J. Phys. Chem. A 113 15366-75[4] M. Rutigliano, P. Gamallo, R. Sayós, S. Orlandini and M. Cacciatore, Plasma Sources Sci. Technol. 2014 23 045016[5] G. D. Billing, Dynamics of Molecule Surface Interactions; Wiley: New York, 2000[6] M. Rutigliano and M. Cacciatore, J. Therm. Heat Transfer 2015 in press doi: http://arc.aiaa.org/doi/abs/10.2514/1.T4596

In this contribution, we give examples of results recently obtained in fundamental molecular dynamics studies and DFT electronic structure calculations performed on prototype surface processes due to the interaction of atomic and molecular oxygen, nitrogen, hydrogen, C on substrates of different nature. The discussed systems are known to be among the most important heterogeneous systems relevant to laboratory and natural gas-surface systems. The surface processes followed in the MD simulations include atom recombination reactions, dissociative chemisorption, adsorption and adsorption/desorption of atoms and molecules. Correlations between the dynamics of the surface processes and the molecular properties of the gas-phase particles and the structural behaviours of the surfaces will be highlighted.

In the present chapter some prototype gas and gas-surface processes occurring within the hypersonic flow layersurrounding spacecrafts at planetary entry are discussed. The discussion is based on microscopic dynamical calculationsof the detailed cross sections and rate coefficients performed using classical mechanics treatments for atoms, moleculesand surfaces. Such treatment allows the evaluation of the efficiency of thermal processes (both at equilibrium and nonequilibrium distributions) based on state-to-state and state specific calculations properly averaged over the population ofthe initial states. The dependence of the efficiency of the considered processes on the initial partitioning of energy amongthe various degrees of freedom is discussed.

The interaction of methane with an extra-framework oxygen atom in acidic Zeolite (H-ZSM5) porous substrate has been investigated by means of different state of the art calculations using two model systems in their triplet spin state: zeolite was modelled both by its crystallographic structure subject to periodic boundary condition and by cluster approach in gas-phase. We have evaluated the energetics of H-abstraction reaction from methane within the (010) straight channel of acidic zeolite by different computational approaches. First by in bulk plane waves Density Functional Theory (with and without Dispersion energy correction) and then, in gas-phase cluster, by typical quantum chemistry approaches based on molecular orbital theory, where electronic wave function is expanded in gaussian basis sets, within variational or perturbative schemes. Reaction paths have been determined either by Climbing Image Nudged Elastic Band method (c-NEB) in bulk or by Intrinsic Reaction Coordinate (IRC) method in gas phase. Transition State has been characterized in cluster models using Becke-3-Lee Yang Parr (B3LYP) and BeckeHalf&HalfLeeYangParr (BHLYP) hybrid functionals. Calculated energy barriers and all results allow an interesting vis-à-vis of commonly used, alternative, often exclusive models. This study indicates that open shell systems composed of O atoms and H-ZSM5 are worth to be considered in H-abstraction from small hydrocarbons in applicative processes.

Particle-in-cell methodology is applied to study the simultaneous charging and coagulation of a nanoparticle, taking into account the self-consistent dynamics of surrounding plasma induced by laser ablation in liquid. The model uses, as an input, plasma temperature and electron number density which are experimentally obtained by high temporally resolved optical emission spectroscopy of the laser-induced plasma in water. Results show the important role of ions in the growth process and of the atom-induced evaporation process for the final nanoparticle size. The competition between different mechanisms of nanoparticle formation in the laser-induced plasma is finally discussed.

The O/beta-quartz interaction is described by combining our timedependentsemiclassical approach to atom-molecule/surface scattering with firstprincipleselectronic structure calculations at the DFT (PBE0) level of accuracy. Inparticular, the O, O2 interaction potentials with an on-top Si atom and its nearest O atomboth localized over three different silica clusters have been calculated as a function of theoxygen-silica approaching distance. The calculated DFT potential energy surface hasbeen used in semiclassical trajectory calculations to investigate the sticking and inelasticreflection of oxygen atoms from a model beta-quartz surface. The collisional mechanism,including the role played by the phonon dynamics, is brought to light and accuratesticking probabilities are calculated at five impact energies in the range [0.05-0.8] eV andTS = 1000 K. The different catalytic response of beta-quartz and beta-cristobobalite to theatomic oxygen flux is also discussed and highlighted.

The inelastic scattering of D2 and HD molecules impinging on a graphite surface in well-defined initial roto-vibrational states has been studied by using the computational setup recently developed to characterize important selectivities in the molecular dynamics occurring at the gas-surface interface. In order to make an immediate comparison of determined elastic and inelastic scattering probabilities, we considered for D2 and HD molecules the same initial states, as well as the same collision energy range, previously selected for the investigation of H2 behaviour. The analysis of the back-scattered molecules shows that, while low-lying initial vibrational states are preserved, the medium-high initial ones give rise to final states covering the complete ladder of vibrational levels, although with different probability for the various cases investigated. Moreover, propensities in the formation of the final rotational states are found to depend strongly on the initial ones, on the collision energy, and on the isotopologue species.

The inelastic scattering of hydrogen molecules in well-defined roto-vibrational states, impinging a graphitesurface from sub-thermal up to hyper-thermal collision energies, has been investigated by usinga new Potential Energy Surface, formulated in terms of a recently proposed Improved Lennard Jonesmodel, suitable to describe non-covalent interactions in the full space of the configurations. The collisiondynamics is studied by a semiclassical method. The focus has been on behaviour of molecules initially inlow-medium lying roto-vibrational states, for which, under the assumed conditions, initial vibrationalstate is in general preserved during the collision. For the rotational relaxation, some selectivities in thefinal state formation have been characterized. They are emerging especially at low collision energies,where the scattering is manly driven by the attractive forces controlling the physical adsorption. Therotational and vibrational accommodation coefficients have been evaluated and found to be in agreementwith those reported in literature.

Hydrogen atoms recombination via the Eley-Rideal mechanism on a cesiated surface was studied using a semiclassical collisional model. The Potential Energy Surface governing the reaction was evaluated by ab initio calculations in the framework of Density Functional Theory. The recent results obtained for H interaction potential with the same surface model were used together the first principle results obtained for H2 interaction potential with Cs and Mo surface atoms. Molecular Dynamics calculations have been performed for two different typical adsorption sites identified on the surface. The probabilities for recombination reaction and for the complete set of elementary surface processes arising from the assumed initial condition were evaluated. It appears that the recombination occurs only for one adsorption site on the surface and with low probability, while for the other site H atoms are mainly scattered from the surface with a partial negative charge. The vibrational distributions of formed H2 molecules exhibit a non-Boltzmann behaviour with peaks on medium-high lying vibrational levels. The considered surface appears promising for negative ion sources, contributing to the formation of ions by means of both usually considered mechanisms, surface production and volume production via dissociative attachment. The global and state-to-state recombination coefficients were also calculated to be used in kinetic modelling of negative ion sources.