We study the photoionization of argon atoms close to the 3s$^2$3p$^6$ $rightarrow$ 3s$^1$3p$^6$4p $leftrightarrow$ 3s$^2$3p$^5$ $varepsilon ell$, $ell$=s,d Fano window resonance. An interferometric technique using an attosecond pulse train, i.e. a fr
equency comb in the extreme ultraviolet range, and a weak infrared probe field allows us to study both amplitude and phase of the photoionization probability amplitude as a function of photon energy. A theoretical calculation of the ionization process accounting for several continuum channels and bandwidth effects reproduces well the experimental observations and shows that the phase variation of the resonant two-photon amplitude depends on the interaction between the channels involved in the autoionization process.
We present experimental measurements and theoretical calculations of photoionization time delays from the $3s$ and $3p$ shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecon
d pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random phase approximation with exchange (RPAE). The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results especially in the region where strong intershell correlations in the 3s to kp channel lead to an induced Cooper minimum in the 3s ionization cross-section.
