FULLY QUANTUM DYNAMICAL STUDIES OF ELEY-RIDEAL H2 RECOMBINATION ON GRAPHITE
Creators: name: PASQUINI, MARTA
Publisher: name: Universit� degli Studi di Milano
Year: 2018

description: The formation of molecular hydrogen in the InterStellar Medium (ISM) is an important topic for astrochemistry and astrophysics: H2 is the most abundant molecular species in the entire universe, but the mechanism leading to its formation is not completely understood yet. The scientific community now agrees that the reaction occurs on the graphitic surface of the interstellar dust grains, which act as catalysts facilitating the collision between the reactants and dissipating the excess of energy. Moreover, the substrate is crucial in determining the ro-vibrational excitation of the newly formed molecule, whose energy can be exploited in subsequent reactive processes involving H2. In this PhD project, the H2 formation on graphite through the Eley-Rideal mechanism has been investigated with quantum dynamical methods, using specific implementations and models, that overcome the limitations imposed by traditional approaches. In the first part of the project, we addressed the problem of the isotope effect in the Eley-Rideal H2 formation on graphite; in particular, we focused on hydrogen-deuterium substitutions and on the collision energy regime relevant for the chemistry of the ISM. We employed a reduced dimensionality dynamical model, where the surface is considered as rigid and flat, and the calculations were performed using a Time-Dependent WavePacket (TDWP) method. We employed a specific implementation of the TDWP approach, developed within our research group, that allows us to obtain reliable results at collision energies down to 10-4 eV, which would be very challenging with standard methodologies. Our results show a strong isotope effect in the reaction probabilities obtained from simulations of collinear collisions and almost no isotope effect when the full dimensionality of the problem is considered. This suggests that the process is not as direct as previously believed and that the mechanism proceeds through glancing collisions. On the other side, the trapping process is strongly influenced by the involved masses and, in particular, by the mass of the projectile atom. Indeed, when the reactant coming from the gas phase is heavier, more physisorption bound states are available, so that the trapping is favored. We also investigated the ro-vibrational populations of the products, which are found to be vibrationally "hot" and rotationally "cold". In the second part of the project, we investigated the role of the graphitic surface in both reaction dynamics and energy partitioning of the Eley-Rideal H2 formation on graphite within a fully quantum approach. In order to include the coupling to phonons in our quantum dynamical simulations, we resorted to a system-bath model, in which the two hydrogen atoms and the binding carbon are the main reactive system, while the rest of the graphitic lattice is represented as a ensemble of independent harmonic oscillators (HOs), the so-called "bath". Each HO is bilinearly coupled to the main system through the C atom and the bath is characterized so that it reproduces the fluctuative-dissipative properties of graphite. The overall dynamics is described by a Independent Oscillator Hamiltonian, which is perfectly suited to perform quantum dynamical calculations with the Multi Configuration Time-Dependent Hartree (MCTDH) method and its recent variant Multi-Layer MCTDH (ML-MTDH). We focused on the collinear configuration and the collision energy interval 0.2-1.0 eV. Our results show that the reactive dynamics is close to the diabatic limit, in which the formation of H2 is much faster than the substrate relaxation. The latter, indeed, starts only after the reactive event is completed, so that the amount of energy transferred to the surface is about the puckering energy (~ 0.8 eV), which implies a considerable heating of the substrate. The energy partitioning favors translational over vibrationally excitation of the product molecule.