On space charge effects in laboratory-based photoemission electron microscopy using compact gas discharge extreme ultraviolet sources

Daniel Wilson, Christoph Schmitz, Denis Rudolf, Carsten Wiemann, Claus M. Schneider, Larissa Juschkin
2020 New Journal of Physics  
The analysis of electronic and structural properties of surfaces has been greatly advanced by photoemission electron microscopy and spectroscopy techniques. To further improve lateral and energy resolution of the instruments, it is necessary to optimize parameters of the radiation sources employed for photoemission studies (e.g. photon flux, pulse duration, spot size etc). We studied space charge effects observed in an energy-filtering photoemission electron microscope operated with a compact
more » ... ed with a compact laboratory-scale gas-discharge extreme ultraviolet light source. In this system, we found limits of spatial-and energy-resolution controlled by the source radiation parameters. The pulse repetition rate can be varied in the kHz range and the duration of the EUV emission was measured to be several tens of nanoseconds long, and thereby very different from the standard synchrotron sources typically used for similar experiments. The spatial resolution could be improved by a factor of 5, but only on the expense of the photon density per pulse, which had to be decreased by a factor of 17 in order to reduce the image blur due to space charge effects. Furthermore, we found broadening of the x-ray photoelectron spectroscopy peaks for high photon fluxes. We have also performed a n-body Monte Carlo simulation to evaluate the difference between core-level photoelectrons and secondary electrons with respect to space charge. Photoemission electron microscopy (PEEM) and spectroscopy (PES) are well established and commonly used methods to study and characterize electronic and structural properties of solids, molecules and thin films with high energy or/and spatial resolution [1]. Spectroscopic microscopy became feasible with energy-filtering instruments (EF-PEEM) and has contributed substantially to the understanding of the electronic structure and chemical states in condensed matter. To further extend the capabilities of this technique in a laboratory environment, and to improve the instrument's lateral and energy resolution, substantial efforts have been invested especially in the development of new light sources optimized for the EF-PEEM. One particular challenge for lateral and energy resolution in photoelectron microscopy and spectroscopy is overcoming parasitic space charge effects, most prominent when EF-PEEM operates with x-ray free electron laser (XFEL) or fs-laser high harmonic generation light sources. The space charge effects are associated with the ultra-short duration of the radiation pulses resulting in high densities of photoelectrons [2] [3] [4] [5] [6] . The space charge effect is a phenomenon originating from the collective Coulomb interaction (repulsion) of free charges in space or mirror charges on a metallic sample surface, limiting the performance of charged particle optics, in particular, in the (energy-filtered) photoelectron emission microscopes [4] [5] [6] [7] [8] [9] , where crossovers of the electron trajectories may lead to high charge densities. For PEEM and PES, space-charge results in a deterioration of the spatial resolution and also in a broadening
doi:10.18154/rwth-2020-11208 fatcat:auohgc3mpvd4xb3c5jwc5ak6te