Strong Field Acceleration of Attosecond Electron Pulses emitted by a Sharp Metallic Nanoprobe

We report on the observation of strong near-field acceleration of attosecond electron pulses emitted from a sharp nanometer-sized gold tip. Kinetic energy spectra extending over tens of eV and varying qualitatively with laser wavelength and intensity are explained in terms of the spatiotemporal electron dynamics in the strong field gradient at the tip apex.