Unraveling real-time chemical shifts in the ultrafast regime

TL;DR

This paper demonstrates the use of ultrafast multi-site XPS with femtosecond x-ray probes to monitor real-time chemical bond dissociation in molecules, extending the technique's applicability to out-of-equilibrium systems.

Contribution

It introduces a method combining femtosecond x-ray probes with partial charge models to track ultrafast chemical dynamics at multiple atomic sites.

Findings

Real-time observation of C-F and C-H bond cleavage in fluoromethane.

Validation of partial charges model with ab-initio calculations and experimental data.

Extension of XPS analysis to out-of-equilibrium, complex molecular systems.

Abstract

Traditional x-ray photoelectron spectroscopy (XPS) relies upon a direct mapping between the photoelectron binding energies and the local chemical environment, which is well-characterized by an electrostatic partial charges model for systems in equilibrium. However, the extension of this technique to out-of-equilibrium systems has been hampered by the lack of x-ray sources capable of accessing multiple atomic sites with high spectral and temporal resolution, as well as the lack of simple theoretical procedures to interpret the observed signals. In this work we employ multi-site XPS with a narrowband femtosecond x-ray probe to unravel different ultrafast dissociation processes of a polyatomic molecule, fluoromethane (CH$_{3}$F). We show that XPS can follow the cleavage of both the C-F and C-H bonds in real time, despite these channels lying close in binding energy. Additionally, we apply the partial charges model to describe these dynamics, and verify this extension with both advanced ab-initio calculations and experimental data. These results enable the application of this technique to out-of-equilibrium systems of higher complexity, by correlating real-time information from multiple atomic sites and interpreting the measurements through a viable theoretical modelling.