Photoemission-time-delay measurements and calculations close to the 3s-ionization-cross-section minimum in Ar

Abstract

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 attosecond 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. 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 $3s\ensuremath{\rightarrow}kp$ channel lead to an induced ``Cooper'' minimum in the 3s ionization cross section.