This study reports the measurement of attosecond photoemission delays in core-level photoionization of nitric oxide (NO), revealing unexpectedly large delays up to 700 attoseconds near the oxygen K-shell threshold. Using attosecond x-ray pulses from a free-electron laser (XFEL), the researchers scanned across the K-shell region to measure the time delay between photoemission from oxygen and nitrogen K-shells. The results show that the delay spectrum is richly modulated, indicating contributions from transient trapping of photoelectrons due to shape resonances, collisions with Auger-Meitner electrons, and multi-electron scattering effects. The findings demonstrate how x-ray attosecond experiments, supported by comprehensive theoretical modeling, can unravel the complex correlated dynamics of core-level photoionization. The photoemission delay was measured using attosecond angular streaking, where an ionizing x-ray pulse is overlapped with a circularly polarized infrared laser field. This technique maps the temporal profile of the x-ray photoemission to the final momentum of the photoelectrons. The delay is determined by the difference in momentum shift between photoelectron features produced by the same x-ray pulse. The measurement was performed at the Linac Coherent Light Source (LCLS) using attosecond x-ray pulses produced by enhanced self-amplified spontaneous emission (ESASE). The momentum distribution of the emitted photoelectrons was recorded using a coaxial velocity map imaging (c-VMI) spectrometer. The results show that the photoemission delay is highly sensitive to electron correlation effects, with the delay increasing due to the transient trapping of photoelectrons by the molecular potential near the oxygen K-shell threshold, a shape resonance. The delay is also affected by post-collision interactions between the outgoing photoelectron and the subsequent Auger-Meitner electron from the decay of the core-ionized state. Theoretical calculations using the Hartree-Fock (HF) method and the multichannel Schwinger configuration interaction (MCSCI) method were used to model the photoionization cross-section and delay. The MCSCI results showed good agreement with the measurement, indicating the importance of molecular structure in the attosecond delay dispersion. The study provides new insights into the dynamics of core-level photoionization and demonstrates the sensitivity of x-ray photoemission delay to electron correlation effects. The results highlight the importance of molecular structure in attosecond delay dispersion and offer a new experimental probe of electron correlation effects in quantum systems. The findings have implications for understanding the behavior of electrons in the attosecond regime and the properties of matter.This study reports the measurement of attosecond photoemission delays in core-level photoionization of nitric oxide (NO), revealing unexpectedly large delays up to 700 attoseconds near the oxygen K-shell threshold. Using attosecond x-ray pulses from a free-electron laser (XFEL), the researchers scanned across the K-shell region to measure the time delay between photoemission from oxygen and nitrogen K-shells. The results show that the delay spectrum is richly modulated, indicating contributions from transient trapping of photoelectrons due to shape resonances, collisions with Auger-Meitner electrons, and multi-electron scattering effects. The findings demonstrate how x-ray attosecond experiments, supported by comprehensive theoretical modeling, can unravel the complex correlated dynamics of core-level photoionization. The photoemission delay was measured using attosecond angular streaking, where an ionizing x-ray pulse is overlapped with a circularly polarized infrared laser field. This technique maps the temporal profile of the x-ray photoemission to the final momentum of the photoelectrons. The delay is determined by the difference in momentum shift between photoelectron features produced by the same x-ray pulse. The measurement was performed at the Linac Coherent Light Source (LCLS) using attosecond x-ray pulses produced by enhanced self-amplified spontaneous emission (ESASE). The momentum distribution of the emitted photoelectrons was recorded using a coaxial velocity map imaging (c-VMI) spectrometer. The results show that the photoemission delay is highly sensitive to electron correlation effects, with the delay increasing due to the transient trapping of photoelectrons by the molecular potential near the oxygen K-shell threshold, a shape resonance. The delay is also affected by post-collision interactions between the outgoing photoelectron and the subsequent Auger-Meitner electron from the decay of the core-ionized state. Theoretical calculations using the Hartree-Fock (HF) method and the multichannel Schwinger configuration interaction (MCSCI) method were used to model the photoionization cross-section and delay. The MCSCI results showed good agreement with the measurement, indicating the importance of molecular structure in the attosecond delay dispersion. The study provides new insights into the dynamics of core-level photoionization and demonstrates the sensitivity of x-ray photoemission delay to electron correlation effects. The results highlight the importance of molecular structure in attosecond delay dispersion and offer a new experimental probe of electron correlation effects in quantum systems. The findings have implications for understanding the behavior of electrons in the attosecond regime and the properties of matter.