Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

TL;DR

This paper introduces a novel time-resolved x-ray photoelectron spectroscopy method to observe and quantify ultrafast charge dynamics in excited molecules, revealing relaxation and coherence phenomena within hundreds of femtoseconds.

Contribution

It extends static chemical shifts to excited states, enabling direct measurement of atomic charge dynamics in molecules using TR-XPS.

Findings

Major relaxation to ground state within 220-250 fs

Observation of 250-fs electronic state oscillation

Demonstration of charge motion tracking in excited molecules

Abstract

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 new 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.