State-Resolved Probing of Attosecond Timescale Molecular Dipoles.

We report an experimental study of iodomethane attosecond transient absorption spectroscopy (ATAS) in the region of iodine 4d core-to-valence/Rydberg excitation. Similar to previous atomic experiments, extreme ultraviolet near-infrared (XUV-NIR) delay-dependent absorbance changes reflect a light-induced phase due to an NIR-field driven AC Stark shift of the excited states, as well as pathway interferences arising from couplings between neighboring states. As a novel aspect of molecular ATAS, we observe pronounced differences between the ATAS signatures of valence and Rydberg states. While the core-to-valence transitions carry the majority of the XUV oscillator strength, the core-to-Rydberg transitions are dominantly affected by a moderately strong, nonionizing NIR field. Our experimental findings are corroborated by ab initio calculations and ATAS simulations.