|Ph.D Student||Baranovsky Elina|
|Subject||Magnetically Manipulated Molecular Beams: Studying the|
Structure and Dynamics of Water Surfaces
|Department||Department of Chemistry||Supervisor||Professor Gil Alexandrowicz|
|Full Thesis text|
Studying the structure and the dynamics of different forms of ice and in particular the surface layers of ice, is important for a wide range of research fields and applications. Alongside many other experimental techniques, nuclear magnetic resonance (NMR) has been used to shed light on the structure and dynamics of ice. NMR provides the powerful combination of being a gentle and inert probe on the one hand, and being a probe which is sensitive to the details of the chemical environment on an atomic length scale on the other. NMR is also sensitive to the atomic scale dynamics which take place over a wide range of time scales, which makes it particularly suitable to follow phase transitions which are characterized by a change in the mobility.
On the other hand, a rather small number of NMR studies have been performed on amorphous ices and, to the best of our knowledge, NMR hasn’t been applied to study vapor deposited ices, and ASW in particular. Furthermore, it hasn’t been yet possible to apply NMR to study surface layers of ice, due to the lack of sensitivity of conventional NMR experiments. In our group we are pursuing this goal, using a unique ortho-water molecular beam approach.
This thesis describes three research projects which are all related in some way to the goal of studying ice and its surfaces using NMR. The first project was to quantify the ortho-water (o-H2O) component in a magnetic focused molecular beam, using Fourier transform infra-red (FTIR) spectroscopy. Results show that more than 4-fold enrichment in the o-H2O nuclear spin isomer can be achieved and an o-H2O purity of ∼93% can be accomplished using the magnetically focused beam. The results of this study stimulated the project of building a new and unique Molecular Beam Surface NMR (MBSN) spectrometer.
The second project is the design, assembling and testing of the MBSN apparatus. The instrument is aimed at measuring proton NMR signals from deposited mono-layers and even sub-mono-layers of adsorbed molecules. The potential for ground breaking sensitivity arises from the polarization of the adsorbed molecules before reaching the surface, which should lead to an approximately five orders of magnitude increase in signal strength.
In the last project relaxation times of ASW were studied using part of the new MBSN system. We found an interesting multi-timescale relaxation process of ASW which is strongly linked to the deposition conditions. This result is very different than what we got when measuring crystalline ice, which showed a single exponential recovery. Another unusual observation is that annealing the ASW samples at elevated temperatures (above crystallization temperature) doesn’t alter the fast relaxation component, and that NMR relaxation profiles obtained when measuring annealed ASW were different from crystalline ice curves. The results of this project are interesting from the aspect of understanding the morphology of ASW films. Furthermore, they provide clues for why we still didn’t manage to measure a NMR signal from mono-layer thick ASW films and possible routes for future attempts of MBSN measurements.