Theory on polarization-averaged core-level molecular-frame photoelectron angular distributions: II. Extracting the X-ray induced fragmentation dynamics of carbon monoxide dication from forward and backward intensities

TL;DR

This paper proposes a new method using time-resolved polarization-averaged molecular-frame photoelectron angular distributions to visualize molecular dissociation dynamics, providing a simple bond length measurement tool with high accuracy.

Contribution

It introduces a novel EXAFS-type formula for analyzing PA-MFPADs to extract bond lengths during molecular dissociation, validated against ab-initio calculations.

Findings

The EXAFS formula accurately retrieves C-O bond lengths within 0.1 Å.

The method provides a simple, semi-empirical 'bond length ruler' for time-resolved molecular imaging.

Numerical validation confirms the formula's applicability to CO²⁺ dissociation dynamics.

Abstract

Recent developments of high-reputation-rate X-ray free electron lasers (XFELs) such as European XFEL and LSCS-II, combined with coincidence measurements at the COLTRIMS-Reaction Microscope, is now opening a door to realize a long-standing dream to create molecular movies of photo-induced chemical reactions of gas-phase molecules. In this paper, we theoretically propose a new method to experimentally visualize dissociation of diatomic molecules via time-resolved polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) measurements using the COLTRIMs--Reaction Microscope and two-color XFEL pump-probe set-up. The first and second order scattering theories with the Muffin-tin approximation give us simple EXAFS type formula for the forward and backward scattering peaks in the PA-MFPADs structure. This formula acts as an experimentally applicable "bond length ruler" by adjusting only three semi-empirical parameters from the time-resolved measurements. The accuracy and applicability of a new ruler equation are numerically examined against the PA-MFPADs of CO²⁺ calculated by Full-potential multiple scattering theory as a function of the C-O bond length reported in the preceding work. The bond lengths retrieved from the PA-MFPADs via the EXAFS formula well reproduce the original C-O bond lengths used in the reference <i>ab-initio</i> PA-MFPADs with accuracy of 0.1 {\AA}. We expect that time-resolved PA-MFPADs will be a new attractive tool to make molecular movies visualizing intramolecular reactions.