Chemistry and Chemical Biology ETDs

Publication Date

Fall 11-15-2022

Abstract

Gas-surface scattering involves the conversion of energy in different forms in the impinging molecule, possible bond breaking and forming, and the energy transfer across gas-solid interface. Possible energy dissipation channels include adiabatic energy transfer to the motion of surface phonons and non-adiabatic interactions with surface electron-hole pairs. A complete understanding of the interplay of energy dissipation and physical/chemical changes in the molecule is vital for many important applications such as materials fabrication and heterogenous catalysis.

Gas-surface encounters may occur where non-adiabatic effects are negligible. Two such systems were investigated, both with ample experimental data. One is concerned with the scattering of small molecules, i.e., H2O, CO2 and glycine, from a highly ordered pyrolytic graphite (HOPG) surface. Molecular dynamics (MD) simulations revealed that each of the three molecules scattered from the surface via three mechanisms: impulsive scattering, extended impulsive scattering, and trapping. The results showed that the scattering dynamics are heavily dependent on the strength of molecule−surface interaction. Molecules with a stronger attraction tend to have longer residence times on the surface and consequently experience more translational energy dissipation and vibrational excitation. The other work investigated the interaction of the N atom with the HOPG surface, including the adsorption, diffusion of the N atom and the formation of N2 through different mechanisms. While N2 recombination does not have a barrier with Eley–Rideal (ER) pathway, the Langmuir–Hinshelwood (LH) pathway is limited by the diffusion barrier of the adsorbed N atom. The N2 molecule formed by recombinative desorption is found to be bother translationally and internally hot. These possible pathways and mechanisms are helpful for understanding the hyperthermal collision experiment of atomic nitrogen from HOPG.

We also studied adiabatic and non-adiabatic mechanisms in O atom scattering from HOPG. DFT results suggest that the excited O(1D) binds stronger with HOPG than its ground state counterpart O(3P). As a result, the impinging O(3P) could either stay on the triplet state or crosses over to the singlet state via spin-orbit coupling, leading to different scattering outcomes. To understand the adiabatic and nonadiabatic pathways, two spin fixed potential energy surfaces (PESs) were developed for interaction of the triplet and singlet O with HOPG. The experimental results on O(3P) scattering agree well with MD calculations performed on the triplet PES, which implies that spin conversion is not likely to happen. However, experimental data indicated that the incoming O(1D) beam scatters as O(3P), implying facile spin flip. Our theoretical simulations suggest that O(1D) needs to dissipate enough kinetic energy before it reaches the crossing seam and scatters. The comparisons with experiment help us to gain insight into the nature of interaction of the atomic oxygen with graphene.

In another investigation, we explored the scattering of atomic hydrogen from a semi-conductor surface. Recent experiments on H scattering from the reconstructed Ge(111)-c(2x8) surface, which is a semiconductor with a band gap of 0.49 eV, found that there are two kinds of scattered H atom in terms of final kinetic energy. The fast channel originates from scattering of H which loses a small amount of energy during the collision, while the slow channel experiences much greater energy loss. We attributed the fast peak to the adiabatic scattering of H atom with surface Ge atoms and MD calculations indeed reproduce the experimental distribution quantitatively. As for the origin of the slow peak, its origin is likely to be related to electronic excitations across the band gap. Indeed, the large energy dissipation only appears when the incidence kinetic energy is larger than the band gap of the semiconductor (~0.49 eV). Hence, it is speculated that this channel is due to nonadiabatic creation of surface electron-hope pairs across the band gap. For such nonadiabatic transitions, the electronic friction model fails to capture the dynamics, as shown by our simulations. These results pose a challenge to the current theoretical models to describe energy transfer from fast nuclear motion to electronic motion.

Project Sponsors

Hua Guo

Language

English

Keywords

gas-surface dynamics, non-adiabatic effect, scattering

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Hua Guo

Second Committee Member

Martin L. Kirk

Third Committee Member

Abhaya K. Datye

Fourth Committee Member

Yi He

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