MD simulations of atomic hydrogen scattering from zero band-gap materials

by Marvin Kammler
Doctoral thesis
Date of Examination:2019-07-05
Date of issue:2019-07-26
Advisor:Prof. Dr. Alec Wodtke
Referee:Prof. Dr. Ricardo Mata
Referee:Prof. Dr. Peter Saalfrank
Persistent Address: http://dx.doi.org/10.53846/goediss-7580

 

 

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Abstract

English

This work deals with simulations of atomic hydrogen beam scattering from various surfaces. The studied surfaces are several (post-)transition metals, an insulator and graphene. Simulations were run with a self-written program that implements numerous potential energy surfaces (PESs) to describe the different systems. All PESs used in this work were reparametrized with different approaches to accurately reproduce higher level reference data obtained from density functional theory. These PESs were subsequently used to run classical Newtonian dynamics simulations of atomic beam scattering. Nuclear quantum effects (NQEs) that might arise in this process due to the small mass of the projectile can be accounted for by means of ring polymer molecular dynamics. With the help of the simulations, one can gain valuable insight into scattering angle distributions, energy loss during the collision or sticking probabilities. These quantities can then be related to the incidence conditions and surface temperature. Isotope substitution can reveal the magnitude of NQEs and allows to estimate the kinetic isotope effect. The investigated surfaces exhibit a range of different properties. A metal surface with its unbound electrons causes the impinging particle to lose kinetic energy mostly due to electron hole pair excitation. Different residence times at the surface lead to a very broad energy loss spectrum. The H atom loses barely any energy when colliding with an insulator. Here, the energy transfer during the elastic scattering process is completely determined by the mass of the surface atoms. When scattering from graphene, the particles can experience either a small or a large energy loss depending on the normal component of the incidence energy. In some situations, both energy loss channels are even apparent at the same time. This system is shown to rapidly accept much energy from a light collision partner without the need for nonadiabatic dynamics. Due to a simultaneous involvement of many degrees of freedom in the collision process, kinetic energy can quickly be transported away from the impact site. The characteristics of orbital (re-)hybridization paired with covalent bond formation opens up a novel energy dissipation pathway.
Keywords: potential energy surface; genetic algorithm; surface scattering; graphene; atomic beam; molecular dynamics; ring polymer; nuclear quantum effect; nonadiabatic energy loss
 
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