  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.
