Attosecond Transient Absorption Study of Coherent Hole Oscillation in Ar+

We report on the observation, characterization, and control of the electron dynamics in ionized argon atoms. We utilized an intense mid-infrared (MIR) pulse to create a coherent superposition of the spin-orbit split ground state of the ion. A weak extreme ultraviolet (XUV) pulse then probes the hole oscillation through time-resolved transient absorption spectroscopy. We investigated several 3p to nd transitions accessible with our XUV high harmonics which show a 23fs beat corresponding to the energy separation between the initially populated states. The experimental attosecond transient absorption signals for different pathways were simulated using detailed TDSE simulations and perturbative analytic calculations. The analysis of phase relations between the oscillations reveals important information about transition dipole moments in the system. In addition, we employed another strong MIR pulse to achieve transient control over the absorption by inducing Stark shifts of the states without affecting the electronic coherences.