Vacuum space-charge effects in nano-ARPES

Abstract

Angle-resolved photoemission spectroscopy (ARPES) provides a powerful tool for probing the spatially averaged electronic structure of condensed matter at high energy and momentum resolution. Current efforts to add spatial resolution by reducing the focal spot size of the incident radiation to the nanometer regime may result in an effective loss of resolution due to the increased Coulomb interaction between the photoelectrons emitted into vacuum. Here, the potential limitations of ``nano-ARPES'' at pulsed light sources are determined by molecular-dynamics simulations over a wide range of the most relevant parameters: the pulse duration and focal spot size of the incident light as well as the number of emitted electrons per pulse and their mean kinetic energy. The dependence of the space-charge-induced spectral shift and broadening on these parameters is found to be describable by empirical formulas. The simulation results particularly reveal a saturation of vacuum space-charge effects in the limit of small focal spot sizes and suggest that ``nano-ARPES'' at an effective energy resolution of 5 meV is possible and practical, employing typical synchrotron radiation in the extreme ultraviolet spectral range.