Studying Complex Surface Dynamical Systems Using Helium-3 Spin-Echo Spectroscopy

Abstract

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 di↵erence 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 coe cient of 2.5 ± 0.5 ps 1. 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 ps 1 .

Disentangling theorem and scattering functions for long-range coherent tunneling

“…The width of the lowest band for H/Ni(111) diffusion has been computed as 5 × 10 −4 meV [24], whereas the plotted value δ = 0.01 is a more optimistic estimate based on the hypothetical close-packed surface with degenerate fcc and hcp sites. The pessimistic bandwidth is comparable to the present energy resolution of the Cambridge HeSE instrument in the sense of complete spectral reconstruction of S( K, ω) [20,77,78], but there is ample scope for improving the energy resolution with independently proven instrumentation upgrades, for example, by increasing the maximum magnetic field integral for the 3 He nuclear spin manipulations [79]. Additionally, since the spin-echo method operates in a quasitime domain, full spectral reconstruction is not compulsory to interpret the partial line shape in the time domain, which leads to a substantially higher effective resolution given a suitable physical model for the ISF [19,77,80].…”

Amplitude of jump motion signatures in classical vibration-jump dynamics

“…At any given ∆K, both A and α can be determined from simulated ISFs via the best exponential fit to the long-time tail. Comparison of α(∆K) with the Chudley-Elliott model defines the jump distribution, and is a standard data reduction method used to compare molecular dynamics simulations and HeSE data 22,[43][44][45] . Predicting α(∆K) is an application of rate theory 46 , 47 , whereas the present work focuses on developing an approximate theory for A(∆K).…”

Vibrational lifetimes and friction in adsorbate motion determined from quasi-elastic scattering

“…12,13 Here, we present the first HeSE investigation of the vibrational lifetimes of a molecule in an adsorption well. This system is ideally suited to such a study as we can observe different modes of motion simultaneously without the added complication of strong lateral interactions which would alter the experimental lineshapes 14,15 whilst the molecule is also heavy enough that we expect its behaviour to be predominantly classical. This system is ideally suited to such a study as we can observe different modes of motion simultaneously without the added complication of strong lateral interactions which would alter the experimental lineshapes 14,15 whilst the molecule is also heavy enough that we expect its behaviour to be predominantly classical.…”

“…We use the model system cyclopentadienyl (Cp), C 5 H 5 , on Cu(111) to demonstrate the method, studying its vibrational lifetimes and the associated friction. This system is ideally suited to such a study as we can observe different modes of motion simultaneously without the added complication of strong lateral interactions which would alter the experimental lineshapes 14,15 whilst the molecule is also heavy enough that we expect its behaviour to be predominantly classical. In addition, the diffusive motion of Cp/Cu(111) is well understood thanks to our previous experiments, 16,17 allowing us to check for consistency within the measurements.…”