Energy measurements of electrons submitted to an ultrastrong laser field

We report a new experimental situation where the main contribution to the energy of electrons submitted to an ultrastrong laser field is given by the ponderomotive force. Electrons released from a low-density gas medium with energies up to 5 keV have been detected, which is far above the energy acquired during the ionization process. Numerical simulations of the electron trajectories are in good agreement with the experimental results.