Final Report Summary - MAGNETIC BEAMS (Magnetically manipulated molecular beams; a novel ultra-sensitive approach for studying the structure and dynamics of water surfaces.)
One goal was developing a new instrument, the magnetic beam surface NMR spectrometer (MBSN). NMR which is so widely used in science, is not sensitive enough to be applied to a single layer of surface atoms/molecules, due to the low spin-polarization. The idea of the MBSN apparatus is to use magnetic fields to align the magnetic moments of gas phase molecules and then deposit them unto a cold surface where NMR experiments can be performed on the deposited surface layer. The approximately 100% spin-polarization of the molecules which can be achieved with this method, has the potential to increase the NMR sensitivity by 5 orders of magnitude and make single surface layer NMR experiments possible. During the project we developed a MBSN setup characterized by a relatively large flux of high spin-purity molecules combined with a sensitive NMR spectrometer. The performance of our setup is sufficient to detect a NMR signal from a single layer of hyper-polarized molecules, if the molecules maintain their spin-polarization for long enough (approx. 10 minutes). Initial MBSN measurement attempts on a layer of amorphous solid water (ASW) revealed surprisingly fast spin relaxation of the molecules after adsorption. We performed the first NMR spin-relaxation study of bulk ASW samples obtaining insight into the origin of this fast relaxation process. Future studies will exploit the MBSN setup we constructed to search for surfaces / conditions where spin relaxation rates are slower and NMR signal from single layers can be detected.
A second instrument development goal was designing, building and using a second-generation helium spin echo (HeSE) spectrometer. This instrument is a helium diffractometer which uses magnetic fields to separate the quantum mechanical wave functions of different nuclear spin states in time and in space, enabling measurements of atomic-scale motion on surfaces on a pico–nano second time scale, a time-scale that is beyond the reach of conventional surface science tools. After succesfully constructing the instrument, we used it to study water structures on a gold and copper surfaces, study the interaction of helium atoms with a NaCl surface, measure the non-isotropic collective motion of atoms on a stepped surface and other surface systems. Finally, an additional unplanned outcome of the project was the development of a novel method which uses the magnetic fields of the HeSE setup to control and measure molecular rotational orientation states. Using this method we coherently controlled the rotational orientation of a hydrogen molecules before and after they collided with a copper surface. This results paves the way for a wide range of new quantum state-to-state scattering measurements on ground state molecules which can now be performed.