My thesis relates on the study of ultrafast molecular dynamics through the development of a soft X-ray laser source. The transient absorption experiment described in this manuscript combines the latest generation of femtosecond lasers with a vacuum spectroscopy system enabling time-resolved measurements of samples at the low femtosecond timescale with a transmission geometry. The technique relies on the generation of high order harmonics (HHG). Using the atomic selectivity of high energies and the temporal coherence of laser technology, we demonstrate the observation of the first stages of chemical transformation of matter in the gas and liquid phases.