טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAvidor Nadav
SubjectHelium Atom Scattering: Instrumentation Development and
Structural Studies of Thin-Films of Water
DepartmentDepartment of Chemistry
Supervisor Professor Gil Alexandrowicz
Full Thesis textFull thesis text - English Version


Abstract

Helium Atom Scattering (HAS) is a powerful general technique to explore the motion and structure of small adsorbates on surfaces. This thesis describes two research projects where the structure of water thin-films on surfaces was studied using HAS diffraction measurements. In addition, two instrumental design projects are presented, both of which are related to the construction of the 3Helium Spin Echo (3HeSE) spectrometer for ultra-high resolution surface-dynamics measurements.

Chapter 1 introduces the field of surface science, basic concepts of helium atom diffraction studies and a brief overview of the 3HeSE technique which is relevant to the instrumental development chapters.

Chapter 2 presents the work of designing and fabricating unique electromagnets (solenoids) used for spin manipulation in the 3HeSE spectrometer. In a way, the coils are the heart of the machine, allowing the energy analysis to be performed with an exceptionally high resolution. The ideas behind the improved magnetic performance are explained, followed by a description of the final design and fabrication process.

Chapter 3 presents the work of designing and fabricating a prototype for a new 3He detector for the 3HeSE technique, based on the concept of exciting the helium atoms to a metastabe state instead of the more standard ionization method. In principle, this approach may improve (reduce) the problematic background count rate which 3He detectors suffer from due to background HD molecules.

Chapter 4 and 5 present structure measurements of water ad-layers on top of oxygen precovered Ruthenium, and on a Nickel surface. In chapter 4 the sensitivity of the Helium atom to the water protons is used to determine the structure of H2O/O(2x1)/Ru{0001} at elevated temperatures . In chapter 5 HAS is used to characterize a 281/2x281/2R19o structure of H2O on Ni{111} at 110K which transforms to a powder like ring pattern diffraction at higher temperatures.