Centennial Honors College
Western Illinois University
Undergraduate Research Day 2012
Poster Presentation
Manipulation of an Electron Beam
Xianxi Li
Faculty Mentor: Pengqian Wang
Physics
Electron impact dissociative ionization of molecules is a useful method in exploring the
structure and energy levels of molecules and their interaction with electrons. In the
experiment the electron beam is provided by an electron gun, and the ions are
measured by a mass spectrometer. One problem in the experiment is that the spatial
and temporal spread of the electron beam always obscures the mass spectrum of the
ion fragments and increases the difficulty in interpreting the experimental results. In
order to obtain a reliable mass spectrum, the spread of the electron beam must be
minimized. In this poster we present our work on manipulating an electron beam by
introducing ion optical elements in the electron gun. The electrons are generated from a
heated tungsten filament in a high vacuum. They are accelerated to a few hundreds of
electron volts. The electron beam can be pulsed to a duration of as short as 40
nanosecond with a high voltage electric pulse. The electron beam is focused to the
molecular target by an ion optical system. Simulation of the trajectory of the electrons
shows that the electron beam can be focused to a diameter of less than 1 mm. The
electron gun has been used in our lab to impact and ionize some atoms and molecules,
including argon, nitrogen and oxygen, and the resulted mass spectra have been
measured.