Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer.

M Dell'Angela,A Föhlisch,H Öström,J J Turner,W F Schlotter,T Anniyev,M Beye,J A Sellberg,M Wolf,F Sorgenfrei,H Ogasawara,R Coffee,O Krupin,A Nilsson,W Wurth,T Katayama,S Kaya,D Nordlund,J Gladh

doi:10.1063/1.4914892

Abstract

Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.

Highlights

Possibility to study simple catalytically relevant reactions at surfaces
We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons
disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge

Summary

INTRODUCTION

Very recent developments have shown the possibility to achieve even higher photon energies at HHG, but this set-ups are still under development and are far from reaching the number of photons per pulse that are readily available at FELs. FELs, in particular, with high repetition rate such as FLASH, European XFEL and LCLS II, offer unique sources when it comes to the use of core level PES for femtochemistry and in general to the study of the non-equilibrium dynamics in complex materials such as correlated systems.. When performing a pump-probe experiment, a short laser pump pulse in the visible or UV is needed to trigger the dynamics under study Such an optical pulse causes additional emission of low energy electrons that introduce pump-induced vacuum space charge effects (i.e., peak shift and broadening of the core level lines in PES spectra) that need to be considered in addition to X-ray induced vacuum space charge. We were able to distinguish between a contribution to the spectra due to the vacuum space charge and a contribution that we associate with the lattice heating of the sample after excitation by the optical pump pulse

EXPERIMENT

RESULTS AND DISCUSSION

CONCLUSIONS