Theoretical analysis of the role of complex transition dipole phase in XUV transient-absorption probing of charge migration

TL;DR

This paper provides a theoretical framework showing how complex dipole phase information in XUV transient absorption spectra can reveal charge migration dynamics in molecules, demonstrated with ab-initio calculations on ICCBr+.

Contribution

It introduces an analytical expression linking complex dipole phase to site-specific charge migration signals and confirms a constant phase difference through quantum chemistry calculations.

Findings

Complex dipole phase encodes charge migration information.

Constant π phase difference between iodine and bromine spectral windows.

Simulated spectra accurately reconstruct charge-migration dynamics.

Abstract

We theoretically investigate the role of complex dipole phase in the attosecond probing of charge migration. The iodobromoacetylene ion (ICCBr$^+$) is considered as an example, in which one can probe charge migration by accessing both the iodine and bromine ends of the molecule with different spectral windows of an extreme-ultraviolet (XUV) pulse. The analytical expression for transient absorption shows that the site-specific information of charge migration is encoded in the complex phase of cross dipole products for XUV transitions between the I-$4d$ and Br-$3d$ spectral windows. Ab-initio quantum chemistry calculations on ICCBr$^+$ reveal that there is a constant $\pi$ phase difference between the I-$4d$ and Br-$3d$ transient-absorption spectral windows, irrespective of the fine-structure energy splittings. Transient absorption spectra are simulated with a multistate model including the complex dipole phase, and the results correctly reconstruct the charge-migration dynamics via the quantum beats in the two element spectral windows, exhibiting out-of-phase oscillations.