Following the flow of excitation inside a material with attosecond core-level soft X-ray spectroscopy

We show that core-level x-ray absorption near edge structure (XANES) spectroscopy with attosecond soft x-ray (SXR) pulses can image the flow of energy inside a material in real time. Our attosecond-resolved measurement with a pump-probe delay step size of 0.6 fs reveals the buildup of coherent charge oscillations, i.e., polarization of the material. These oscillations occur at occupied states below and unoccupied states above the Fermi level predominantly at the pump carrier frequency. We identify the incoherent background due to the dephasing of coherent charge oscillation. This background rises within a few oscillations of the light field, signifying the ultrafast transfer of energy from the light field into the electron and hole excitation of the material. We find that ultrafast dephasing of the coherent carrier dynamics is governed by impact excitation (IE) for electrons, while holes exhibit a switch-over from impact excitation to Auger heating (AH) already during the 11-fs duration of the infrared light field. We further analyze the coherent phonon signal and find that the non-Raman active strongly coupled optical phonon contributes to 90% of the total dephasing, originating from very strong electron-SCOPs coupling, thus acting almost impulsively. In conclusion, we demonstrate the ability to track energy flow upon light absorption between electrons, holes, and phonons in real-time.