Ph.D Thesis

Ph.D StudentCorem Gefen
SubjectStudying Surface Interactions using Scattering Methods
DepartmentDepartment of Chemistry
Supervisor PROF. Aharon Blank
Full Thesis textFull thesis text - English Version


The interaction of atoms and molecules with solid surfaces govern many surface related processes such as heterogeneous catalysis, diffusion, growing surface structures and others. Studying these interactions is important for many fields such as microelectronics, atmospherical chemistry, and astrochemistry.  In this dissertation, we use elastic helium scattering methods to study such interactions for several systems using a unique system called helium spin echo spectrometer.

 The Water/solid interfaces are relevant to a broad range of physical and chemical phenomena and processes; however, the basic principles that govern water interactions with solids are not fully understood. In this document, I present several helium atom scattering measurements of the structure of the first water layer on solids.  We have studied the Au(111) surface which is known to have a particularly weak interaction with water molecules. Conflicting evidence from several indirect studies has suggested that water either grows as 3D ice crystals or as an amorphous wetting layer.  In contrast, our measurements showed that between 110K and 130K, H2O grows as highly commensurate well-ordered islands which only partially wet the gold surface. The islands produce a clear  diffraction pattern and are characterized by a well-defined height of ~ 5Å with respect to the surface gold atoms. A second study case was the Cu(511), this surface is an ideal stepped surface.  We present our observations of the behavior of a complex single water layer, and the investigations of how it reconstructed into a structure in such a way that it aims to stabilize the interface between the investigated corrugated solid surface and the water ice layer. Our third system is of the NaCl(100). For this insulating surface system we designed and constructed a new sample holder which allowed performing helium scattering experiments on ionic crystals. Our experiments using a 3He probe showed a 1x1 diffraction pattern, supporting previous 4He measurements on this system.

Finally, we present our investigation of the He-NaCl interaction potential by exploiting the phenomena of bound state resonances which occurs when the incoming angle and the energy of an incidence gas atom are such that the atom becomes, for a short time, bound in one of the states of the potential well of the atom-surface when it would have normally been scattered by the surface. This is done using our spectrometer's ability to accurately measure the energy distribution of the scattered helium beam which changes due to these bound state resonance.