Hydrogen bond effects in multimode nuclear dynamics of acetic acid observed via resonant x-ray scattering.

Viktoriia Savchenko,Minjie Dong,Pavel Krasnov,Michael Odelius,Sergey P Polyutov,Jan-Erik Rubensson,Daniel Mcnally,Patrick Norman,Conny Såthe,Annette Pietzsch,Xingye Lu,Alexander Föhlisch,Thorsten Schmitt,Victor Ekholm,Johan Gråsjö,Iulia Emilia Brumboiu,Faris Gel’Mukhanov,Victor Kimberg,Olle Björneholm

doi:10.1063/5.0049966

Abstract

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS.

Highlights

Understanding and modeling quantum nuclear effects in disordered systems, such as liquids, is one of the major challenges of modern condensed matter physics
In the processes studied here, related to the core-excitation of the O K-edge, not all vibrational modes contribute to the x-ray absorption spectroscopy (XAS) and RIXS spectra
We have performed a detailed study of the RIXS from acetic acid via the two lowest spectral features at the oxygen K-edge, related to the OC1s and OH1s electronic excitations

Summary

Introduction

Understanding and modeling quantum nuclear effects in disordered systems, such as liquids, is one of the major challenges of modern condensed matter physics. Modern x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) are well suited for studies of systems with various types of inter- and intra-molecular interactions.. The main advantage of XAS and RIXS is the intrinsic site and element selectivity even in complex systems due to the high localization of core-electrons on a particular atomic center. In the case of RIXS, this phenomenon ( known as ultrafast dissociation) results in a long vibrational progression and brings a unique opportunity to probe the intra- and inter-molecular interactions for large displacements from the equilibrium geometry and to map the potential energy curves (PECs) up to the dissociation limit.. Since the PEC shape is affected by the local environment of the target molecule, RIXS, due to its spatial sensitivity, becomes a powerful tool to study inter-molecular interactions, local properties, and the structure of disordered systems with controlled magnitude of disorder. Tuning the excitation energy to various core-excited states allows for the localization of the vibrational excitation on a particular bond, for the extraction of the PECs along specific bonds, and, for obtaining more precise information about the HB networks in liquids

Methods

Results

Conclusion