Maria Rutigliano

Non-equilibrium plasma kinetics: a state-to-state approach

State-to-state approaches are used to shed light on (a) thermodynamic and transport properties of LTE plasmas, (b) atomic and molecular plasmas for aerospace applications and (c) RF sustained parallel plate reactors. The efforts made by the group of Bari in the kinetics and dynamics of electrons and molecular species are discussed from the point of view of either the master equation approach or the molecular dynamics of elementary processes. Recent experimental results are finally rationalized with a state-to-state kinetics based on the coupling of vibrational kinetics with the Boltzmann equation for the electron energy distribution function.

Vibrational kinetics, electron dynamics and elementary processes in H2 and D2 plasmas for negative ion production: modelling aspects

We report current and past efforts made by our group in the ab initio modelling of different negative ion sources. In particular, we discuss the cross sections of elementary processes relevant to negative ion kinetics, including electron?molecule, atom?molecule and atom/molecule gas surface interactions, particularly emphasizing the role of vibrational excitation in affecting the cross sections. Attention is also paid to the elementary processes involving caesium in both volume and surface sources.Self-consistent models, which couple the Boltzmann equation and the vibrational kinetics, are used for describing multipole and rf discharges, while a PIC-MC (particle in cell) with Monte Carlo collisions is used to study electron and ion dynamics in a parallel plate reactor in the post-discharge regime. The present theoretical results should encourage further dedicated experimental work in the field.

Oxygen Adsorption on β-Cristobalite Polymorph: Ab Initio Modeling and Semiclassical Time-Dependent Dynamics

The adsorption dynamics of atomic oxygen on a model beta-cristobalite silica surface has been studied by combining ab initio electronic structure calculations with a molecular dynamics semiclassical approach. We have evaluated the interaction potential of atomic and molecular oxygen interacting with an active Si site of a model beta-cristobalite surface by performing DFT electronic structure calculations. As expected, O is strongly chemisorbed, E(b) = 5.57 eV, whereas molecular oxygen can be weakly adsorbed with a high-energy barrier to the adsorption state of approximately 2 eV. The binding energies calculated for silica clusters of different sizes have revealed the local nature of the O,O(2)-silica interaction. Semiclassical collision dynamic calculations show that O is mainly adsorbed in single-bounce collisions, with a smaller probability for adsorption via a multicollision mechanism. The probability for adsorption/desorption (reflected) collisions at the three impact energies is small but not negligible at the higher energy considered in the trajectory calculations, about P(r) = 0.2 at E(kin) = 0.8 eV. The calculations give evidence of a complex multiphonon excitation-deexcitation mechanism underlying the dynamics of stable adsorption and inelastic reflection collisions.

Nonequilibrium Vibrational Kinetics of an O/O Mixture Hitting a Catalytic Surface

The importance of a silica surface in affecting both the state-to-state kinetics and the macroscopic properties of an O2/O mixture hitting a blunt body at hypersonic speed has been investigated. First, the surface has been considered catalytic only for recombination: three different assumptions are examined and compared with the resultsfrom a noncatalytic surface model. Second, both deactivation and dissociation aretreated in theframework of a catalytic surface. Nomenclature Ca = q a/q CM = q M/q Cv = q v/q Cwa = q wa/q w D(M/a)m = diffusion coefe cient of the species M/a in the mixture,m 2 s i 1

Atomic and molecular data for spacecraft re-entry 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 (N2, O2, NO), Mars (CO2, CO, N2) and Jupiter (H2, He) atmospheres are considered.

Oxygen Adsorption on β-Quartz Model Surfaces: Some Insights from Density Functional Theory Calculations and Semiclassical Time-Dependent Dynamics

The O/β-quartz interaction is described by combining our time-dependent semiclassical approach to atom-molecule/surface scattering with first-principles electronic structure calculations at the DFT (PBE0) level of accuracy. In particular, the O, O(2) interaction potentials with an on-top Si atom and its nearest O atom both localized over three different silica clusters have been calculated as a function of the oxygen-silica approaching distance. The calculated DFT potential energy surface has been used in semiclassical trajectory calculations to investigate the sticking and inelastic reflection of oxygen atoms from a model β-quartz surface. The collisional mechanism, including the role played by the phonon dynamics, is brought to light and accurate sticking probabilities are calculated at five impact energies in the range [0.05-0.8] eV and T(S) = 1000 K. The different catalytic response of β-quartz and β-cristobobalite to the atomic oxygen flux is also discussed and highlighted.

A molecular dynamics simulation of hydrogen atoms collisions on an H-preadsorbed silica surface

The interaction of hydrogen atoms and molecules with a silica surface is relevant for many research and technological areas. Here, the dynamics of hydrogen atoms colliding with an H-preadsorbed β-cristobalite (0 0 1) surface has been studied using a semiclassical collisional method in conjunction with a recently developed analytical potential energy surface based on density functional theory (DFT) calculations. The atomic recombination probability via an Eley–Rideal (E–R) mechanism, as well as the probabilities for other competitive surface processes, have been determined in a broad range of collision energies (0.04–3.0 eV) for off-normal (θv = 45°) and normal (θv = 0°) incidence and for two different surface temperatures (TS = 300 and 1000 K). H2,gas molecules form in roto-vibrational excited levels while the energy transferred to the solid surface is below 10% for all simulated conditions. Finally, the global atomic recombination coefficient (γE–R) and vibrational state resolved recombination coefficients (γ(v)) were calculated and compared with the available experimental values. The calculated collisional data are of interest in chemical kinetics studies and fluid dynamics simulations of silica surface processes in H-based low-temperature, low-pressure plasmas.

From DFT Cluster Calculations to Molecular Dynamics Simulation of N2 Formation on a Silica Model Surface

B3LYP-DFT electronic structure cluster calculations have been performed to evaluate the adsorption properties of N and N2 interacting with Si x O y clusters in a given adsorption site. To check the convergence of the calculated binding energy, clusters of different size were used in the calculations. As expected, the N atom is chemisorbed, E b ≅ 2.75eV, while N2 is weakly physisorbed. The ab initio results were used to build three PES of the LEPS-type having different activation barrier. The obtained PES have been used in the semiclassical scattering equations and the dynamics of the N2 formation after atom recombination on a model silica surface was studied in great detail.