Emergence of a higher energy structure in strong field ionization with   inhomogeneous electric fields
  Studies of strong field ionization have historically relied on the strong field approximation, which neglects all spatial dependence in the forces experienced by the electron after ionization. More recently, the small spatial inhomogeneity introduced by the long-range Coulomb potential has been linked to a number of important features in the photoelectron spectrum, such as Coulomb asymmetry, Coulomb focusing, and the low energy structure (LES). Here, we demonstrate by combined quantum and classical simulations that a small time-varying spatial dependence in the laser electric field creates a prominent higher energy peak at energies above the "classical cut-off" for direct electrons. This higher energy structure (HES) originates from direct electrons ionized near the peak of a single half-cycle of the laser pulse. The HES is separated from all other ionization events (providing sub-cycle resolution) and is highly sensitive to the carrier envelope phase (CEP). The large accumulation of electrons with tuneable energy suggests a promising method for creating a localized source of electron pulses of attosecond duration using tabletop laser technology. 
L. Ortmann, J. A. P\'erez-Hern\'andez, M. F. Ciappina, A. Chac\'on, G.   Zeraouli, M. F. Kling, L. Roso, M. Lewenstein, and ...
physics.atom-ph physics.optics