The energy of Auger electrons, i.e. secondary electrons emitted after a non-radiative decay of a deeply bound electronic core-hole, is an intrinsic property of the electronic structure of the excited atom and does not directly rely on the energy of the exciting photon, electron or ion. The electron emission is not mono-energetic, though; the fast - attosecond to femtosecond - decay times imply corresponding spectral widths in a range of a few tens of meV to a few eV. This time-energy correspondence suggests a uniform distribution of all energy components over time. In this work, we demonstrate that under spectroscopically relevant conditions, this presumption is wrong. Instead, our time-resolved experiments show evidence of an energetic chirp, i.e. a pronounced time-dependent variation of the Auger-electrons´ kinetic energy. It appears as a consequence of the correlated motion of both photo- and Auger electrons in the Coulomb field of the remaining ion. While the underlying mechanism - also known as ‘post-collision interaction’ (PCI) - has extensively been discussed in the literature, no attention has so far been paid to the consequence of the effect for the temporal properties of the escaping electron wave packets. We visualize this temporal energy variation by superimposing 13.5nm XUV pulses from FLASH with the oscillating electric field of a strong terahertz (THz) wave from the FLASH THz undulator in a xenon gas-target. Significantly, modified widths of kinetic energy spectra for opposite field gradients clearly indicate a chirp (see Fig. 1).
Figure 1: Kinetic energy spectra of xenon Auger lines formed after absorption of XUV photons in the presence of a THz field. The modified widths of the Auger lines for opposite field gradients indicate an energy chirp.
The experiments have been confirmed in the laboratory where a high harmonics XUV source and a laser based THz source were employed. The observed spectral modulations are reproduced with semi-classical as well as quantum simulations, and are explained by an analytical model, which includes PCI in the presence of a time-dependent streaking field.