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

D T Mayer ,S Düsterer,M S Robinson,M Wallner,Tommaso Mazza ,David Picconi ,Bastian Manschwetus ,Jan Metje ,Christopher Ehlert ,Peter Saalfrank ,Andrea Trabattoni ,Thomas Wolf ,Richard J Squibb ,Skirmantas Ališauskas ,Fabiano Lever ,Raimund Feifel ,Francesca Calegari ,Marion Kuhlmann ,Mario Niebuhr ,Markus Gühr

doi:10.1038/s41467-021-27908-y

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 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.

Highlights

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales
We find a direct relation between the excited-state chemical shift (ESCS) and the local charge at the probe site in analogy to the potential model for the chemical shift (CS) in static XPS6
We show that the largest effect on the ESCS is due to electronic relaxation, especially if the local charge at the probed atom is grossly changed in the process

Summary

Introduction

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. We find a direct relation between the ESCS and the local charge at the probe site in analogy to the potential model for the CS in static XPS6. This allows one to circumvent complex calculations of IPs, while allowing for an interpretation based on chemical intuition.

Methods

Results

Conclusion