Time-dependent Strong-Field Effects in Atoms and Molecules observed by Attosecond Transient Absorption Spectroscopy

In this work, the absorption spectra of an extreme ultraviolet (XUV) attosecond light pulse in argon and molecular nitrogen are investigated as a function of a time delay between the XUV pulse and a near-infrared (NIR) femtosecond light pulse and the NIR peak intensity. The XUV pulses are produced by high-harmonic generation of the NIR pulse yielding attosecond time resolution for the time-delay-dependent absorption spectra. In this all-optical approach, the electronic and vibrational quantum states of the target systems are excited by the weak XUV pulse and dressed by the strong NIR pulse. The strong-field effects, which are observed in the time- and intensity-dependent optical density, are compared between argon and nitrogen. These are line-broadening, line-shape changes, sub-cycle and slower modulations. Emphasis is placed on to the modulations, which originate from two-NIR-photon transitions between different excited states. Furthermore, a focus lies on the intensity-dependent line-shape changes of both species. A measure for these changes is the laser-imposed phase of a wave packet’s dipole moment, which is excited by the XUV pulse. This phase is introduced in a dipole-control model and investigated for both species