Research

Understanding elementary processes of complex chemical reactions is of paramount importance for the development of chemical rate theories and for modeling physical environments which can hardly be reproduced in the lab. Recent years have shown a tremendous progess in theoretical and computational chemistry which led to an ever increasing interesting in dynamical processes as opposite to the early period of the quantum chemistry, when ttention was focused on molecular structure. Correspondingly, a transition from thermodynamics to kinetics has been observed in macroscopic chemistry. Dynamical processes in the gas-phase and at the gas-surface interphase are important in several scientific areas, ranging from fundamental physical (e.g. the chemistry of the interstellar medium) to technological applications (e.g. catalysis), but have one common denominator: the dynamics of atoms and molecules. Thus, although different problems in the gas-phase and at the gas-surface interphase often require specific expertises in such different fields, one common language is basic to all of them. Activated processes exist among gas-phase reactions as well as gas-surface reactions and transition state theories are useful in their understanding. Resonances are observed in molecular collisions as well as in molecule-surface collisions and their existence allows the use of statistical theories in both cases. Thermal baths are needed to correctly model gas-surface phenomena, where the existence of a macroscopic system (the surface) cannot be neglected; the electromagnetic field in the gas-phase plays a similar role in some gas-phase processes.

Because of these similarities we work on gas-phase and gas-surface dynamical problems with the hope of treating(solving) them on the same footing. We are interested in a number of technologically relevant gas-surface systems and, at the same time, we are studying some gas-surface and gas-phase systems of relevance for the interstellar medium. Basic to these problems is the understanding of the interaction forces between atoms and molecules, and molecules and surfaces. Thus, electronic structure calculations are crucial to predict the dynamical behavior of the system under investigation and, in some cases, allow us to get potential energy surfaces to be used in dynamical calculations. We use accurate wavefunction methods (e.g. Multi Reference Configuration Interaction methods, MRCI) coded in standard ab-initio packages like GAMESS in order to study small gas-phase systems and we apply plane-wave Density Functional Theory (DFT) methods to gas-surface problems. We use and develop classical and quantum methods in order to study the dynamics. Classical and quasi-classical trajectory calculations for gas-surface and gas-phase problems are performed with our own trajectory code (TRAJ). Time-Dependent Wave Packet methods are developed and/or applied to simple dynamical problems, e.g. atom-diatom or atom-adatom reactions.

We are currently interested in the following topics.

Catalysis

Hydrogen recombination on metal surfaces. Hydrogen formation on metallic surfaces at low temperature could be of interest for the hydrogen economy. Recent experimental investigations have revealed that under these conditions the traditional Eley-Rideal mechanism is not able to explain the reaction dynamics and they suggested the existence of 'hot' atomic species, the so-called Hot-Atoms, which diffuse at hyperthermal energies above the surface plane. The relative importance of the above two direct mechanisms is still not well understood, as well as the role of the adsorbate coverage and of the surface structure.

Water adsorption on metal surfaces. Water adsorption on metal surfaces is of paramount importance in heterogeneous catalysis, electrochemistry and corrosion. The first few molecular layers can trigger important physical changes on the surface substrate on which water adsorb. The nature of the very first layers (the so-called bilayer) on well studied substrates such as Ru(0001) is still under debate and it is not clear whether or not water dissociation occurs upon adsorption.

Oxygen adsorption on metal surfaces. Oxygen adsorption upon metal surfaces is in some cases, e.g. on Al surfaces, the first step toward surface passivation. The study of the nature of the adsorption sites and of the possible reconstruction geometries is a key-step in understanding the dynamics of atomic and molecular adsorption processes.

Chemistry of the interstellar medium

Hydrogen formation on graphite surfaces

Molecular hydrogen is the most abundant molecule in interstellar clouds and forms on the surface of 'dust' grains. Although the nature of the interstellar dust is still unknown, it is now commonly accepted that it is of carbonaceous origin. Thus, the study of hydrogen formation on Highly Oriented Pyrrolitic Graphite surfaces is of relevance to understand this recombination process in interstellar space. Hydrogen formation is important for the chemistry of clouds and their thermodynamics.

Radiative association reactions

CH₂⁺formation via radiative association. The abundance and ubiquity of the CH⁺ molecular ion in low density interstellar clouds present a long-standing puzzle to astrochemistry. The ion is thought to be formed from CH₂⁺, which in turn is obtained via radiative association of C⁺ and H₂. Radiative association reactions are among the most important processes in several astrophysical environments.

Lithium ionic chemistry

The lithium chemistry in the Early Universe, i.e. before the formation of the first stars produced 'metals' and grains, is of interest for the LiH abundance at the end of the post-recombination era. This highly polar molecule could have given its imprinting on the Cosmic Background Radiation. The importance of the ionic chemistry is due to the fact that, thanks to the low ionization potentials of Li-bearing molecules, ionic lithium species are as abundant as neutral ones.