REACTIONS AT SURFACES: BEYOND THE STATIC SURFACE APPROACH IN QUANTUM DYNAMICS
Creators: name: BONFANTI, MATTEO
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: Thanks to the peculiar electronic properties of gas-solid interfaces, surfaces play an important role in many chemical processes. In my thesis, I considered few different reactions at surfaces and addressed the problem of their description by means of quantum dynamical methods. In particular, the focus of the work is on the inclusion of surface motion in the dynamical models. This problem is very challenging for state-of-art quantum methods, due to the unfavorable scaling with the number of degrees of freedom. To avoid this computational limit a variety of methods were adopted, ranging from a static approach in a low dimensional Time Dependent Wave Packet (TDWP) calculations to a full dynamical description of dissipation in the framework of Multi-Configuration Time-Dependent Hartree method (MCTDH). 

I considered three different physical problems. The first one is the exothermic, collinearly-dominated Eley-Rideal H2 formation on graphite. In particular, I focused on the importance of the model used to describe the graphitic substrate, in light of the marked discrepancies present in available literature results. To this end, I considered the collinear reaction and computed the Potential Energy Surface (PES) for a number of different graphitic surface models using Density Functional Theory (DFT) for different dynamical regimes. I performed quantum dynamics with wave-packet techniques down to the cold collision energies relevant for the chemistry of the interstellar medium. Results show that the reactivity at moderate-to-high collision energies sensitively depends on the shape of the PES in the entrance channel, which in turn is related to the adopted surface model. At low energies I ruled out the presence of any barrier to reaction, thereby highlighting the importance of quantum reflection in limiting the reaction efficiency.

In a second part of my work, I studied the effect of lattice displacement on the interaction of H2 with the Cu(111) surface using the Specific Reaction Parameter (SRP) approach to DFT. I systematically investigated how the motion of the surface atoms affects some features of the PES, such as the dissociation barrier height and the barrier geometry corresponding to some representative reaction pathways, and the anisotropy of the potential at these geometries. This analysis allowed the identification of the surface degrees of freedom that are likely to be most relevant for H2 dissociation. In particular, I found that the lattice coordinate displacements that have the largest effect on the H2/Cu(111) DFT-SRP barrier heights and locations concern the motion of the 1st layer and 2nd layer Cu atoms in the Z direction, and motion of the 1st layer atoms in the directions parallel to the surface. Whereas the first degree of freedom mostly affects the barrier geometry, the second and third motions can lower or raise the barrier height. The latter effect cannot be described with the usual surface oscillator dynamical models employed in the past to include surface motion, and its dynamical influence on the dissociative adsorption needs to be further investigated. 

In the third part of the thesis I addressed the problem of including dissipative effects in the reaction dynamics of hydrogen sticking and scattering on surfaces. I considered dissipative baths with different spectral properties and represented them with a linear chain of coupled harmonic oscillators, exploiting an equivalent effective-mode representation that has recently been developed. I studied the system dynamics with MCTDH, aiming on one hand to an accurate description of dissipation at a short time scale, and on the other hand to a simplified but qualitatively correct behavior of the long time dynamics. In this framework, I found a very useful scheme to represent the long time dynamics of the system without incurring in unwanted Poincar�'s recurrences. I used this method to obtain the sticking probability of one hydrogen atom scattered by a simple one dimensional Morse potential. The methodology developed in this work is going to be extended to the more realistic problem of hydrogen sticking on graphitic surfaces.