Attosecond Electron Correlation and Molecular Resonance in \textit{K}-shell Photoexcitation of Nitric Oxide*

Angular streaking is employed to resolve the attosecond electronic motion of nitric oxide (NO) irradiated with intense, sub-femtosecond, soft X-ray pulses from a free-electron laser.\footnote{Duris, Li \textit{et al.}, Nat. Phot. \textbf{14}, 30 (2020)} We scan the incident photon energy through the O \textit{K}-edge and measure the dynamics of the resultant photoemission. Below the edge we drive the O $1s \rightarrow 2p\pi ^\ast$ core excitation and time-resolve the subsequent electron correlation-driven Auger decay. Above the \textit{K}-edge we measure the photon energy-dependent dynamics determining the precise release time of the O \textit{K}-shell electron into the continuum. We observe the attosecond time-delay associated with the O $1s \rightarrow 2p\sigma ^\ast$ shape resonance previously identified in cross-section measurements.