Electron Scattering from Fundamental Gaseous Targets

Csu Fullerton Auxiliary Services Corporation, Fullerton CA

Investigators

Abstract

Electrons scattering from matter are responsible for a host of phenomena, including the light emitted from planetary atmospheres (including that of the Earth), the sparks in gasoline engines, and the fragmentation of DNA in biological tissues exposed to radiation. These processes can be understood by the theory of quantum mechanics. The present experimental project involves setting up controlled collisions between a well-defined beam of electrons and gas atoms or molecules placed in their path. The aim is to carefully investigate how these electrons are deflected by the target gas atoms or molecules and how they change the physical and chemical state of these targets for a given energy of the electrons. The experiments are conducted for a wide range of electron energies and will look at the dynamic interaction between the electrons and the targets and delve into the physics which controls how the electrons are scattered in various directions at these controlled energies. The targets include simple atoms such as neon, argon, krypton, and xenon, molecular hydrogen and nitrogen, and more complex molecules such as water, alcohols and benzene-type aromatic molecules. The experimental results are used to test detailed quantum scattering models to promote our understanding of the interaction of these electrons with targets. The project continues prior work in the same lab which has advanced the modeling of industrial processes. A benefit from this project will be the exposure of undergraduates to world-wide PhD programs as a result of their engagement in this project. A new electron time-of-flight spectrometer will be able to handle very slow electrons emerging from scattering events. The electron beam is pulsed and the scattered electrons are separated in velocity by the amount of time they take to reach the detector. The scattering of low energy electrons with kinetic energies ranging from 0.5 eV to 100 eV is studied using electron energy loss spectroscopy where the incident energy of a 1 mm collimated beam of high energy resolution crosses a tenuous beam of pure atoms or molecules in a vacuum chamber. The energy separation of electrons (produced from a tungsten filament source) is made at high resolution (30-50 meV full-with at half maximum) using electrostatic lenses combined with hemispherical analyzers. The measurements consist of differential scattering cross sections and polarization correlations for electron scattering from rare gas atoms and simple diatomic molecules (H2 and N2). The data provide tests for models of electron scattering and shed light on the quantum dynamics of the scattering process which involves details of Coulomb interactions, electron spin processes (spin-exchange, spin-orbit), and resonant interactions. Models to date have been evolving to handle more complex targets as computational power is rapidly increasing. The present project will look at the polarization of emitted vacuum ultraviolet radiation and, perpendicular to the scattering plane, in coincidence with differentially scattered electrons whose energy loss coincides with the excitation energy of the radiation. Importantly, it will measure the circular polarization of the radiation in the vacuum ultraviolet, a parameter which is related to the angular momentum imparted to the target by the scattered electron, and provides valuable physical insights to the collision process. In addition, the development of a new time-of-flight spectrometer (using a fast 1 nanosecond pulsed electron beam) will enable absolute calibration of scattering cross sections as well as be able to handle slow emergent electrons in the energy range of 0.5eV to 20eV, and add to the range and accuracy of the overall ongoing measurements. This project aims to continue its productive supply of accurate collision data involving undergraduates in laboratory research.