Helium Atom Scattering from Chiral Structures

Abstract

Helium Atom Scattering (HAS) is the only diffraction technique that combines absolute sur- face sensitivity with non-destructiveness and universality, but has been seldom applied to the study of chiral surfaces. The current thesis focuses on advancing the available theoretical and experimental tools to motivate the study of chiral surfaces using atomic beam techniques. The importance of chirality is discussed in chapter 1, along with the significant role played by surfaces in the creation of chiral media required for biologically and industrially relevant enantioselective reactions. The key aspects of the HAS technique is introduced and a candidate system, namely D-alaninol adsorbed on Cu(100), is proposed for the exploration of chiral expression on a metal surface. The research involves a two-fold approach: First is instrumental considerations to im- prove data acquisition and analysis; the second is the development of a theoretical basis to help quantify the interaction of He with a chiral surface as well as assess its sensitivity as a probe of surface chirality. Chapter 2 introduces a 3D scattering simulation that models the scattering apparatus, to com- pute diffraction peak profiles likely to arise from a periodically arranged plane of point scatterers representing the sample. A structural analysis of the multi-layer adsorption of D-alaninol on the Cu(100) surface has been provided in chapter 3. The experimental data was acquired using the MiniScat spectrometer. In addition to the 1D and 2D diffraction spectra arising from the chiral system, the uptake and desorption behaviour of the chiral D-alaninol molecules have been inves- tigated. In general, the experimental data was found to be in good agreement with the published data acquired with other surface techniques such as LEED, STM and XPS. To overcome the relative complexity of the chiral organic/ metal interface, the He-D-alaninol interaction was first modelled on another methylated system that was simpler. Therefore, an in- teraction potential function originally applied to the differential cross-section analysis of crossed atomic and molecular beams has been proposed and tested on the CH3-Si(111) surface, as de- scribed in chapter 4. Through the close-coupled analysis performed on the system, it became possible to assess the level of transferability between an interaction model describing the scat- tering of thermal He atoms by a crossed atomic-molecular beam and a second model describing He scattering by a gas-phase adsorbate. The close-coupled analysis was repeated for the D-alaninol/Cu(100) system in chapter 5, using both an asymmetrically corrugated and a pairwise version of the interaction potential previ- ously employed. The level of agreement between the experimental diffraction spectra and the close-couple computed diffraction data was assessed and a superior hybrid potential model was introduced. Considering the two-element adsorbates typically studied, the relatively large ad- sorbate unit cell size of the D-alaninol/Cu(100) system makes it one the most complex organic systems where the close-coupled approach has been successfully applied. A new ion-source design has been proposed and characterised in chapter 6. The upgrade resulted in 3 orders of magnitude increase in the detector sensitivity relative to the commercial quadrupole analyser previously installed. The ion-source and the subsequent ion-optics elements of the new detector assembly has been modelled using an existing Boris algorithm, as described in chapter 7. Based on the simulation data, practical improvements offering another order of magnitude increase in the detector efficiency has been identified. Finally, in chapter 8, a future direction for research on chiral surfaces using atomic beam techniques has been proposed.