Correction of the deterministic part of space–charge interaction in momentum microscopy of charged particles

• Published in: Ultramicroscopy 2015-12, Vol.159, p.488-496
• Main Authors: Schönhense, G., Medjanik, K., Tusche, C., de Loos, M., van der Geer, B., Scholz, M., Hieke, F., Gerken, N., Kirschner, J., Wurth, W.
• Format: Article
• Language: English
• Subjects: Charged particles; Femtosecond; HAXPES; Illumination; Microscopy; Momentum microscopy; Phase shift; Photoemission; Reduction; Space charge; X-ray sources
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2015.05.015

Ultrahigh spectral brightness femtosecond XUV and X-ray sources like free electron lasers (FEL) and table-top high harmonics sources (HHG) offer fascinating experimental possibilities for analysis of transient states and ultrafast electron dynamics. For electron spectroscopy experiments using illumination from such sources, the ultrashort high-charge electron bunches experience strong space–charge interactions. The Coulomb interactions between emitted electrons results in large energy shifts and severe broadening of photoemission signals. We propose a method for a substantial reduction of the effect by exploiting the deterministic nature of space–charge interaction. The interaction of a given electron with the average charge density of all surrounding electrons leads to a rotation of the electron distribution in 6D phase space. Momentum microscopy gives direct access to the three momentum coordinates, opening a path for a correction of an essential part of space–charge interaction. In a first experiment with a time-of-flight momentum microscope using synchrotron radiation at BESSY, the rotation in phase space became directly visible. In a separate experiment conducted at FLASH (DESY), the energy shift and broadening of the photoemission signals were quantified. Finally, simulations of a realistic photoemission experiment including space–charge interaction reveals that a gain of an order of magnitude in resolution is possible using the correction technique presented here. •Photoemission spectromicroscopy with high-brightness pulsed sources is examined.•Deterministic interaction of an electron with the average charge density can be corrected.•Requires a cathode-lens type microscope optimized for best k-resolution in reciprocal plane.•Extractor field effectively separates pencil beam of secondary electrons from true signal.•Simulations reveal one order of magnitude gain in resolution.