Optical control of the XUV absorption spectra for spin-orbit states in Argon ion

The coherent evolution of the electron-hole evolution in ionic systems is of high interest in attosecond community. We conducted multi-pulse attosecond transient absorption spectroscopy to study the impact of laser dressing on the evolution of the transition dipole in Argon ion. Ionic states are prepared using a multi-photon ionization by the infrared (IR) pump pulse, which are subsequently excited with a delayed extreme ultraviolet (XUV) pulse, leading to transitions from the two spin-orbit split ionization thresholds to the doublet of 4d spin-orbit split states around 23.85 eV. The transient XUV photoabsorption spectra show 100fs quantum beats around the region where the XUV and the ionizing beam overlap temporally. To study the role of laser fields in time-dependent evolution of the transition dipole, we use a second dressing IR pulse, which is two orders of magnitude weaker in intensity than the pump IR pulse. The dressing pulse is fixed in delay and arrives 250 fs after the pump pulse. We observe that two main absorption lines between lowest ionic states and 4d doublet merge into one and split again, thereafter splits again into two as the action of dressing IR pulse ceases, hence demonstrating an efficient ultrafast optical switching technique with moderately strong laser fields.