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Novel contrast mechanisms in photoelectron microscopy


Grunze,  M.
Cellular Biophysics, Max Planck Institute for Medical Research, Max Planck Society;

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Zharnikov, M., Neuber, M., & Grunze, M. (1999). Novel contrast mechanisms in photoelectron microscopy. Journal of Electron Spectroscopy and Related Phenomena, 98-99, 25-40. doi:10.1016/S0368-2048(98)00273-4.

Two previously undiscussed contrast mechanisms in Auger and photoelectron microscopy, namely photoelectron diffraction contrast and molecular orbital orientation contrast, are presented. The former contrast mechanism is based on the phenomenon of photoelectron diffraction and forward focusing of Auger and photoelectrons, the latter is based on near-edge absorption fine structure (NEXAFS) spectroscopy and stems from the dependence of the NEXAFS resonance intensity on the orientation of the electric field vector of the incoming linearly polarized light with respect to the molecular orbital associated with this resonance. The applicability of both contrast mechanisms was demonstrated using a nickel polycrystal and a monolayer of benzoic acid (BA) on this polycrystal as test systems. Within the photoelectron diffraction approach well-resolved images of the individual microcrystallites on the surface of Ni polycrystal were obtained by using the photoelectrons from both the localized core level (Ni 2p3/2) and the free-electron-like valence band. Within the molecular orbital orientation approach the well-resolved images of the azimuthally aligned BA molecules on the surface of the (110) microcrystallites incorporated into a Ni polycrystal were acquired. In these experiments the photon energy was tuned to the excitation energy of the π* orbital of the phenyl ring, which is a constituent of a BA molecule, and the C1s →π* excitation was monitored by the carbon KLL Auger electrons. The distinction between photoelectron diffraction and molecular orbital orientation contrast mechanisms within an imaging experiment is discussed.