This thesis explores the further development and application of ultrafast soft Xray absorption spectroscopy (XAS) at the nitrogen K-edge to resolve fundamental molecular dynamics in both the gas and liquid phases. A laboratory-based high-order harmonic generation (HHG) source, driven by infrared pulses (centered around 1800 nm) produced by a Ti:Sa laser amplifier coupled with an optical parametric amplifier, was used to produce broadband soft X-ray pulses covering the energy range between 250 eV and 450 eV. The capabilities of this table-top setup for accessing ultrafast structural changes in molecules were first demonstrated by investigating the photodissociation and early conical intersection dynamics of gaseous nitrogen dioxide following single-photon excitation. The result highlights the sensitivity of transient X-ray absorption spectroscopy in probing the wave packet motion near the conical intersection. The research was further extended to the condensed phase using a novel liquid flat-jet system to study the charge transfer process of the metal-organic complex iron[II] 2,2’-bipyridine in the solution phase. By probing at the nitrogen K-edge, this work provides the first femtosecond-resolved observation of the ligand’s local electronic environment during metal-to-ligand charge transfer (MLCT). The results uniquely benchmark the MLCT state signature both spectrally and temporally, offering new insights into the involved intermediate states that govern the efficiency of charge separation in transition metal complexes.
