Capturing transient core-to-core resonances in Kr in intense extreme-ultraviolet laser fields by electron-ion coincidence spectroscopy

Resonances between core-hole states have been long hypothesized to understand nonlinear responses of atoms and molecules exposed to intense x-ray free-electron laser (XFEL) pulses, but evaded clear identification in previous studies due to their transient nature. Here we report clear evidence of core-to-core resonances by state-of-the-art electron-ion coincidence spectroscopy on Kr atoms. The observed $3p\text{−}3d$ resonance manifested the significance in multiple ionization to highly charged states against the ultrafast core-hole decay. The present study, elucidating the role of the core-to-core resonance in electronic responses of matter to high-frequency laser fields, will advance our understanding of nonlinear spectroscopy and ultrafast imaging by ultrashort intense EUV and x-ray pulses.