Fig. 1: Schematic picture of TR-XPS and ESCS in 2-thiouracil in a molecular orbital representation.
From: Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy
a Molecular valence (π, n, π*) orbitals and a core (sulphur 2p) orbital. b Probe of the S 2p core level with a binding energy Ebind by means of a soft x-ray (SXR) light pulse, leading to a photoelectron with a kinetic energy, Ekin. A UV pump pulse (cyan arrow) excites the 2-thiouracil from its electronic ground state (S0) to a ππ* state (S2) which then relaxes further, for instance into the S1 (nπ*) state shown here. The difference in Ebind with respect to the ground state is the excited-state chemical shift \(({{{{{\rm{ESCS}}}}}}={E}_{{{{{{\rm{bind}}}}}}}^{{{{{{\rm{excited}}}}}}\; {{{{\rm{{state}}}}}}}-{E}_{{{{{{\rm{bind}}}}}}}^{{{{{{\rm{ground}}}}}}\; {{{{\rm{{state}}}}}}})\). The molecular structures in the lower part of the panel represent the difference in charge density between the ground state and the respective excited states (red: decreased electron density, blue: increased electron density). Increase in positive charge at the sulphur site (marked with X) increases Ebind and the ESCS.
