This thesis is devoted to the study of dynamics induced by intense femtoseconds lasers pulses. The first studied dynamics deals with molecular alignment of CO2-X mixtures (X=CO2, Ar, N2), in dense gases (up to 20 bar). Up to now, this regime has never been studied experimentally. In the field-free regime (after laser/matter interaction), molecular alignment exhibits two components : a permanent alignment and a transient one. The influence of collisions appears through population transfers between rotational states, which leads to a decrease of these two contributions. Permanent alignment relaxation time is only tied to inelastics collisions whereas transient alignment relaxation time is tied to both inelastics and elastics ones. We show that the determination of the elastic collisions contribution (for which the experimental determination is uneasy), is then possible thanks to the analysis of molecular alignment measurements. This analysis is based on the modelling of inelastics rotational state-to-state transfer rates by ECS-(E)P semi-empirical laws. The elastic contribution of collisions is experimentally determined and happened to be in a good agreement with classically calculated ones. The second studied dynamics is the intensity dependence of the electronic Kerr effect. We pursue the works led by Loriot and al. in 2009 which showed that electronic Kerr index saturated, before nullifying and then presenting a negative contribution when the intensity increases (inversion of the sign for some tens of terawatts by square centimeter). We complete the previous study by performing similar measurements in air at 400 nm (800 nm in the original study).
