Resonant Double-Inner-Shell Excitations in Nitrous Oxide with Ultrafast, Intense X-rays*

The unprecedented intensities at few-to-sub-femtosecond timescales provided by X-ray free electron laser (XFEL) pulses can drive multiphoton interactions in molecules to generate new phenomena such as localized wavepackets of valence excited states. Understanding the additional fundamental processes occurring under such conditions is crucial to make sense of new experimental results. Under high-intensity, low probability processes can greatly influence the measurements; in resonant excitation with few-to-sub-femtosecond X-rays, sequential resonant excitations occurring prior to core-hole decay could significantly impact experiments inducing excited state dynamics. Thus, we theoretically investigate double nitrogen K-edge excitations of nitrous oxide (N₂O) with multiconfigurational electronic structure calculations. Advanced modelling techniques predict the experimental conditions and observables, including time-dependent simulations producing single and double core-excited states under intense, few-to-sub-femtosecond X-ray's, on-the-fly dynamics of the respective nuclear motion and two-step double core-excited Auger-Meitner decay spectra.