Space-charge aberrations in the photoelectron microscope

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Abstract
The contributions of space charge to chromatic and trajectory aberrations of accelerating electrons in a laser-illuminated photoelectron microscope are calculated. Average and fluctuating components of the electrostatic forces on the particles are considered spearately. The average forces produce a magnification change but no chromatic spread. Fluctuating effects include relaxation of potential to kinetic energy in a beam obeying Poisson emission statistics, and transverse-to-longitudinal velocity relaxation by multiple scattering. The potential energy relaxation is treated in a new way by modifying the Poisson distribution with position along the beam axis. The field fluctuations produce both chromatic and trajectory spreading effects, the latter being much larger at high currents. The resolution limit is about 500 Å for 30-kV electrons in a beam of 1-A/cm2 current density.