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Document Type:Latin Dissertation
Language of Document:English
Record Number:52897
Doc. No:TL22851
Call number:‭3315489‬
Main Entry:Zuleika Medina Torres
Title & Author:Theoretical study of gas-surface interactions: Temperature and diffraction effects on trapping and sticking processesZuleika Medina Torres
College:University of Massachusetts Amherst
Date:2008
Degree:Ph.D.
student score:2008
Page No:195
Abstract:The focus of our study is on the effect of the surface's corrugation and vibration on diffraction mediated selective adsorption (DMSA). In DMSA the normal translational energy of the atom is converted into motion parallel to the surface. For a rigid surface, this occurs at specific values of incident energy and angle due to energy quantization. However, on a real surface, an additional source of energy loss or gain can be provided by the coupling to the vibrational and electronic excitations of the substrate, and will determine whether or not the trapped particle will desorb or stick. Trapping and sticking processes in addition to being interesting from a fundamental science viewpoint are of utmost importance because sticking of particles on surfaces is the first step in many processes such as heterogeneous catalysis. In this work we developed a fully quantum formalism that describes the scattering, trapping, and sticking of an atom from a moving corrugated surface. We used a density matrix formalism that assumes that the particle-bath interaction does not significantly perturb the lattice. The dissipative terms can be directly derived from the full Hamiltonian. Both single and multi-phonon excitations are possible, and while the particle is trapped on the surface it is in continuous contact with the solid and can continue to exchange energy with the bath for a long time. Our results showed that for reasonable values of the corrugation (∼0.10), diffraction can make a significant contribution to the trapping of He on Cu(100). Our model also revealed how diffraction into the DMSA resonances is modified due to coupling to the phonons. Our model can describe the interaction of a particle with a moving corrugated, static corrugated, and moving flat surface, and the long time evolution of these trapped particles at any temperature, until they either thermalize and stick, or desorb. In addition, to benchmark our code in the static surface limit, we wrote a close coupling wave packet scattering code, with absorbing potentials and full energy resolution of the scattered states. Unlike standard models for trapping and sticking we can accurately model the system behavior over long times (200 ps). We have found that during the long term evolution of a particle with a moving corrugated surface at very low temperatures, the trapped particles fully relax to the lowest bound state before they desorb. For a given diffraction event, if the well depth is smaller than the amount of parallel kinetic energy transferred back into normal translational energy, the particle will desorb. On the other hand, if the well depth is larger than this energy, the particle will remain trapped, and eventually thermalize and stick. The rates for these processes, as well as for competing desorption and thermal relaxation pathways, are all temperature dependent.
Subject:Pure sciences; Gas-surface interactions; Diffraction; Trapping; Sticking; Corrugated surfaces; Chemistry; 0494:Chemistry
Added Entry:B. E. Jackson
Added Entry:University of Massachusetts Amherst