Coincidence measurement of two-photon double ionization of argon through an autoionizing resonance

We present coincidence measurements of two-photon double ionization (TPDI) of argon driven by femtosecond pulses tunable around 26.5eV photon energy, which are obtained from a high-harmonic generation source. The measured photoelectron spectra are interpreted with regard to three TPDI mechanisms. Theoretical predictions are obtained by an approximate model for direct TPDI and atomic structure calculations, which are implemented into a Monte Carlo simulation. The prevailing mechanism involves the excitation and prompt photoionization of an autoionizing resonance in neutral argon. We provide evidence for pronounced electron-electron interaction in this ultrafast ionization process. Furthermore, we show that the dominant TPDI mechanism can be altered by slight tuning of the photon energy. The present work paves the way for scrutinizing and controlling nonlinear photoionization in the extreme ultraviolet using table-top sources.