# Ab-initio Theory of Photoionization via Resonances

## TL;DR

This paper introduces a non-Hermitian ab-initio method for accurately computing photoionization spectra near autoionization resonances using only bound and metastable states, simplifying traditional continuum calculations.

## Contribution

The authors develop a novel non-Hermitian theoretical framework that relates spectral line asymmetries to complex transition dipoles, applicable to multi-electron systems and laser-driven ionization spectra.

## Key findings

- Successfully computed helium autoionization spectrum
- Derived formulas for Fano resonance lineshape and Autler-Townes splitting
- Applicable to complex atoms and molecules

## Abstract

We present an \emph{ab-initio} approach for computing the photoionization spectrum near autoionization resonances in multi-electron systems. While traditional (Hermitian) theories typically require computing the continuum states, which are difficult to obtain with high accuracy, our non-Hermitian approach requires only discrete bound and metastable states, which are accurately computed with advanced quantum chemistry tools. We derive a simple formula for the absorption lineshape near Fano resonances, which relates the asymmetry of the spectral peaks to the phase of the complex transition dipole moment. Additionally, we present a formula for the ionization spectrum of laser-driven targets and relate the `Autler-Townes' splitting of spectral lines to the existence of exceptional points in the Hamiltonian. We apply our formulas to compute the autoionization spectrum of helium, but our theory is also applicable for non-trivial multi-electron atoms and molecules.