title: Defocus in cathode lens instruments.
abstract: Accurately measuring defocus in cathode lens instruments (Low Energy Electron Microscopy - LEEM, and Photo Electron Emission Microscopy - PEEM) is a pre-requisite for quantitative image analysis using Fourier Optics (FO) or Contrast Transfer Function (CTF) image simulations. In particular, one must establish a quantitative relation between lens excitation and image defocus. One way to accomplish this is the Real-Space Microspot LEED method, making use of the accurately known angles of diffracted electron beams, and the defocus-dependent shifts of their corresponding real-space images. However, this only works if a sufficiently large number of diffracted beams is available for the sample under investigation. An alternative is to shift the sample along the optical axis by a known distance, and measure the change in objective lens excitation required to re-focus the image. We analytically derive the relation between sample shift and defocus, and apply our results to the measurement and analysis of achromats in an aberration-corrected LEEM instrument.
authors.affiliation: IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA; Leiden University, Kamerlingh Onnes Laboratorium, P.O. Box 9504, NL-2300 RA Leiden, Netherlands. Electronic address: rtromp@us.ibm.com.
authors.forename: R M
authors.initials: RM
authors.lastname: Tromp
authors.affiliation: Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: M S
authors.initials: MS
authors.lastname: Altman