# Single‐ and Three‐Photon Ionization of N2 in Presence of Fano Resonances

## TL;DR

The paper studies how nitrogen molecules ionize when exposed to single and three photons, revealing changes in resonance shapes and validating new computational methods.

## Contribution

A novel computational approach using hybrid basis states and open-source code tRecX is demonstrated for multi-photon ionization.

## Key findings

- Single-photon ionization results match synchrotron data.
- Three-photon ionization shows significant changes in resonance line shapes.
- Good agreement with experimental results is achieved using new methods.

## Abstract

We report single‐ and three‐photon ionization cross‐sections of the N2 molecule in the region of the Hopfield series of doubly excited states. Results are obtained by solving the time‐dependent Schrödinger equation in a hybrid basis combining neutral and ionic CI states with a fully numerical basis. Contributions to the spectrum during and after the interaction are obtained using the tSurff and iSurf methods. Calculations at arbitrary molecular orientation and details of the spectral calculation are presented. For single‐photon ionization synchrotron data is reproduced. For three‐photon ionization we find a pronounced change of resonance line shape when going from single‐ to three‐photon transitions.

With upcoming high‐intensity attosecond light sources new theoretical approaches for multi‐photon interactions with molecular systems in the non‐perturbative and multi‐photon regime are needed. Solutions of the time‐dependent Schrödinger equation provide all this, but the massive involvement of continuum states necessitates a combination of Gaussian with purely numerical bases. A range of new methods and their implementation in the open source code package tRecX is demonstrated for multi‐photon absorption at the Hopfield series of N2 at arbitrary polarization direction. Good agreement with experiment is found and pronounced dependence of line‐shape on multi‐photon order is observed.