  Using 1D and 2D PIC simulations, we describe and model the backward ejection
of electron bunches when a laser pulse reflects off an overdense plasma with a
sharp density gradient on its front side. The dependence on the laser intensity
and gradient scale length is studied. It is found that during each laser period
the incident laser pulse generates a large charge-separation field, or plasma
capacitor, which accelerates an attosecond bunch of electrons towards vacuum.
This process is maximized for short gradient scale lengths and collapses when
the gradient scale length is comparable to the laser wavelength. We develop a
model that reproduces the electron dynamics and the dependence on laser
intensity and gradient scale length. This process is shown to be strongly
linked with high-harmonics generation via the Relativistic Oscillating Mirror
mechanism.
