Polarized Electron Physics

Electrons have the fundamental property of “spin,” which is analogous to that of a spinning top, and is associated with their angular momentum. This project studies collisions between polarized electrons, which have their spins aligned in one direction, and chiral, or “handed” molecules. Such molecules, of which DNA is an example, are characterized by a spiral, or helical geometry.

These experiments address physics questions about the dynamics of electron-chiral molecule scattering, particularly with regard to the magnetic effects caused by the electron spins. They will also provide important clues about the origins of biological homochirality – the fact that all naturally-occurring DNA spirals in the same direction. A recently developed source of polarized electrons called a rubidium spin filter is being used in this work; it has the advantage that it is insensitive to the contamination by chiral target molecules that has plagued earlier experiments using polarized electron sources based on photoemission from gallium arsenide.

Molecules of hydrogen (that are not chiral) are also being studied. Hydrogen is the simplest molecule, but its interaction with very slow electrons is poorly understood. This project will use very slow electrons that have very well defined energy and that are spin polarized to provide stringent tests of theory calculations that consider spin-dependent effects in such fundamental collisions.

Improved sources of polarized electrons are also being developed that use multiphoton ionization of very sharp semiconductor tips to give the photoemitted electrons a preferential spin direction. This research done to develop polarized electron technology holds the promise of providing new, high resolution analytical tools that can be used for biological and materials research, and for industry.

Graduate and undergradute students will develop their scientific abilities by their involvement in the research as will the general public though education outreach by the scientific team.

The experiments involving collisions between polarized electrons and chiral molecules will extend previous work that showed chiral effects with halocamphor targets. The goal is to now demonstrate such effects in molecules that have biological significance, such as cysteine, cystine, and selenocysteine, and to study the effect of the maximum target nuclear charge and location of the target’s chiral center on the chiral asymmetries we observe.

The scattering of electrons by simple atoms is well understood, but the theory for electron scattering by even the simplest molecules such as H2 is in its infancy, especially when one considers processes involving electronic excitation. The H2 experiments will focus on the energy region just above the excitation threshold for specific processes, where the theory is particularly difficult, because the excited molecule and the receding electron, which has almost no energy, interact strongly for a long time.

These studies will rigorously test new theories being developed for such collisions. Ideas for novel sources of polarized electrons based on multiphoton ionization of semiconducting nanotips and metallic nanostructures will be studied. The goal of this work is to make pulses of electrons that are “fast,” i.e., that have a duration comparable to that of the light pulses producing them, so they can time-resolve processes such as chemical reactions and magnetic wave motion in solids.

Experiments will be done to learn if current-carrying nanostructures of tungsten can also produce polarized electrons when struck by short light pulses. This work, based on the spin Hall effect, will provide a bridge between the field of spintronics and the production of free polarized electron beams.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.