Polarized Electron Physics

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

Electrons have the fundamental property of "spin," which is analogous to that of a spinning toy top, and is associated with their angular momentum. This project studies collisions between polarized electrons, which have their spins aligned in one direction, and so-called "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 role played 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. Atoms of zinc are also being used as targets, to check the results of an experiment done recently by an Australian group in which zinc atoms excited by polarized electrons emitted light in a way that is forbidden by all known theories of atomic collisions. If the Australian result is reproduced, much of our basic theoretical knowledge on this topic will be shown to be in error. Improved sources of polarized electrons are also being developed, with the goal of "turnkey" ease of use. This project focuses on two particular ways to make polarized electrons: the first involves electron collisions with rubidium atoms in which spin is transferred from the rubidium to the free electrons. The second uses multiphoton ionization of semiconductors such as gallium arsenide to give the electrons a preferential spin direction. This research done to develop polarized electron technology holds the promise of providing new analytical tools that can be used for biological and materials research, and for industry. The experiments involving collisions between polarized electrons and chiral molecules will extend previous work that showed chiral sensitivity in both quasi-elastic and reactive scattering with halocamphor targets. Now that such sensitivity has been observed, our goal is to demonstrate such effects in molecules that have biological significance, such as cysteine, 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 goal in the zinc experiments is to check whether the rather large value of canted linear polarization (polarization fraction P2) observed in fluorescence from excited zinc atoms is reproducible. No extant theory of electron-atom scattering, including the state-of-the-art "R-matrix with pseudostates" approach, has been able to confirm this result, even though they make quantitatively accurate predictions of other collision parameters for this system. Source development work will be based on the successfully demonstrated "rubidium spin-filter" design by our group, in which optically-pumped rubidium undergoes spin-exchange collisions with an incident unpolarized beam of electrons. This project will focus on improving the vacuum system, the rubidium target reliability, and the optical pumping protocol. It will also study a variety of buffer gases to understand the complex physics of the interaction between the electron beam and the rubidium vapor. In the gallium arsenide experiment, unamplified pulses from a femtosecond titanium-sapphire oscillator are used to photo-emit electrons. First experiments will investigate the intensity and polarization of the electrons emitted from bulk gallium arsenide and tip-like gallium arsenide shards. Then targets of gallium arsenide cusp arrays will be considered.