Studying complex surface dynamical systems using helium-3 spin-echo spectroscopy
The aim of the present thesis is to extend the range of application of the helium-3 spin-echo (HeSE) technique to complex surface dynamical systems. A twofold approach is presented: the development of improved experimental equipment and the investigation of a series of prototypical aromatic adsorbate systems. Chapter 1 discusses the motivation for this work and compares HeSE with other techniques probing surface motion. Subsequently, an introduction to the HeSE method is given in Chapter 2, explaining the theoretical background and describing the main components of the Cambridge spectrometer as well as the principle data aquisition and interpretation methods. Chapters 3 and 4 describe the development and testing of two new pieces of equipment. MiniScat, a compact helium atom scattering apparatus, has been designed as a sample preparation facility to enable structural studies and increase the experimental throughput. A new supersonic helium beam source improves the resolution of the HeSE spectrometer by a factor of 5.5 and extends accessible timescales into the nanometre range. The dynamics of cyclopentadienyl (Cp) on Cu(111) are presented in Chapter 5. The ionically bound Cp is remarkably mobile, moving in single jumps between adjacent hollow sites over an energy barrier of 41 ± 1 meV. The data exhibit multicomponent lineshapes that allow the determination of the energy difference between fcc and hcp sites of 12.3±0.3 meV in a Bayesian method probing the probability space of all data combined. Molecular dynamics (MD) simulations provide a friction coecient of 2.5 ± 0.5 ps1. Chapter 6 shows that a physisorbed pyrrole hops between bridge sites on Cu(111). Strong lateral interactions alter the lineshapes from a predicted double exponential towards an apparent single exponential decay. First principles density functional theory calculations by Sacchi and Jenkins reveal that a large contribution to the experimentally determined apparent activation energy of 53 ± 4 meV arises from a site-dependence in the zero point energies, primarily of the vibrational C–H and N–H out-of-plane bending and ring torsion modes which are not directly involved in the di↵usion process. The surface dynamics of thiophene/Cu(111) are investigated in Chapter 7. Thiophene adsorbs on top sites and forms a covalent S–Cu bond. Two competing activated processes manifest themselves in a kink in the Arrhenius plot: jump di↵usion between adjacent top sites over a barrier of 59 ± 4 meV and rotation around the S–Cu anchor point over a barrier of 22±2 meV. In addition, vertical motion relative to the surface is observed. MD simulations of the di↵usive motion reveal an exceptionally high friction of 5 ± 2 ps1.
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