Observation of molecular resonant double-core excitation driven by intense X-ray pulses

The ultrashort and intense pulses of X-rays produced at X-ray free electron lasers (XFELS) have enabled unique experiments on the atomic level structure and dynamics of matter, with time-resolved studies permitted in the femto- and attosecond regimes. To fully exploit them, it is paramount to obtain a comprehensive understanding of the complex nonlinear interactions that can occur at such extreme X-ray intensities. Herein, we report on the experimental observation of a resonant double-core excitation scheme in N₂, where two 1σ core-level electrons are resonantly promoted to unoccupied 1πg molecular orbitals by a single few-femtosecond broad-bandwidth XFEL pulse. The production of these neutral two-site double core hole states is evidenced through their characteristic decay channels, which are observed in good agreement with high-level theoretical calculations. Such multi-core excitation schemes, benefiting from the high interaction cross sections and state- and site-selective nature of resonant X-ray interactions, should be generally accessible in XFEL irradiated molecules, and provide interesting opportunities for chemical analysis and for monitoring ultrafast dynamic processes.

Eetu Pelimanni, Adam E. A. Fouda, Phay J. Ho, Thomas M. Baumann, Sergey I. Bokarev, Alberto De Fanis, Simon Dold, Gilbert Grell, Iyas Ismail, Dimitrios Koulentianos, Tommaso Mazza, Michael Meyer, Maria-Novella Piancastelli, Ralph Püttner, Daniel E. Rivas, Björn Senfftleben, Marc Simon, Linda Young, Gilles Doumy