Dennis Mayer, Fabiano Lever, David Picconi, Jan Metje, Skirmantas Ališauskas, Francesca Calegari, Stefan Düsterer, Christopher Ehlert, Raimund Feifel, Mario Niebuhr, Bastian Manschwetus, Marion Kuhlmann, Tommaso Mazza, Matthew Scott Robinson, Richard James Squibb, Andrea Trabattoni, Måns Wallner, Peter Saalfrank, Thomas J. A. Wolf, Markus Gühr

• The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.