Speaker: Assoc. Prof. Gil Alexandrowicz (Department of Chemistry Technion-Israel Institute of Technology)

Time: March 08, 2016 10:00

Place: 理科楼三楼报告厅（C302）

In our group we use magnetically manipulated beams of atoms and molecules for studying surface structure and dynamics. One technique we use is called helium spin echo spectroscopy, the setup is essentially a time resolved atom interferometer, which is capable of measuring atomic scale motion which takes place on a time scale of pico to nanoseconds[1], a range which is inaccessible using conventional surface science techniques. I will explain the basic principles of the technique and the experimental apparatus, and present as an example a recent study where we used this apparatus to study the fast and highly correlated diffusive motion of sodium atoms on a nano-structured surface[2]. Using molecular dynamics simulations for the interpretation of the data, we extract the energy barrier for diffusion, a lower limit for the Schwoebel barrier and also observe a non-isotropic repulsive interaction between the Na atoms which is attributed to a screening effect of the atomic step[2]. A second different research project, I will briefly describe, is an attempt to use nuclear magnetic resonance (NMR) to study surface structure and dynamics. NMR is a particularly sensitive technique which is often used to determine atomic-scale structure and dynamics in the bulk, however, it is hardly used in surface science due to its low detection sensitivity. We have demonstrated that a high purity ortho-H2O beam can be obtained using simple magnetic manipulations[3-4]. I will describe our ongoing attempts to use this molecular beam to grow thin ice films which are ~ 100% spin polarised and perform the first NMR experiments from a deposited monolayer of hyper-polarized water molecules. >[1] Science. 304, 1790 (2004) >[2] Journal of Physical Chemistry Letters., 2015, 6, 4165 (2015). >[3] Science, 331, 319 (2011) >[4] Physical Review A, 86, 062710 (2012)