Abstract : This thesis is about the study of dynamics of atoms and molecules on the femtosecond time scale by the pump-probe technique. In a first part the oscillation of a wave packet created by a superposition of fine structure states in atomic potassium has been observed. This oscillation is interpreted as the precession of the spin momentum around the total angular momentum. This is followed by a theoretical study showing that this oscillation can be used to produce spin polarised electrons. Another experiment performed has permitted the observation of interferences of free electron wave packets created by ionising potassium atoms with a sequence of two time delayed ultrashort pulses. The pump-probe technique has been applied to the study of the population transfer in a two level system interacting with a chirp pulse. This experiment has led to the observation of coherent transients due to interferences between the amplitude of probability transferred at and after resonance. In a second part, I have studied the predissociation of highly excited states of acetylene, which leads to very short lifetimes of these states. A first step consists in a nanosecond REMPI (3+1) experiment. The photoelectron spectra of the different states were collected. They show a strong Rydberg-Valence mixing. The dynamics of these states has been then studied in a pump-probe experiment. The pump step consists in the excitation by a VUV (132 nm) photon while the probe step is done by ionising the system with UV (396 nm) pulse. We show that the determination of the lifetimes must be done carefully by taking into account the pulses characteristics.