  Interaction of a high intensity short laser pulse with near-critical plasmas
allows to achieve extremely high coupling efficiency and transfer laser energy
to energetic ions. One dimensional Particle-In-Cell (PIC) simulations are
considered to detail the processes involved in the energy transfer. A
confrontation of the numerical results with the theory highlights a key role
played by the process of stimulated Raman scattering in the relativistic
regime. The interaction of a 1 ps laser pulse (I $\sim$ 6.10$^{18}$ W.cm$^2$)
with an under-critical (0.5 $n_c$) homogeneous plasma leads to a very high
plasma absorption reaching 68 % of the laser pulse energy. This permits a
homogeneous electron heating all along the plasma and an efficient ion
acceleration at the plasma edges and in cavities.
