  With the high-order harmonic generation (HHG) from plasma sur- faces it is
possible to turn a laser pulse into a train of attosecond or even zeptosecond
pulses in the backward radiation. These attosecond pulses may have amplitude
several orders of magnitude higher than that of the laser pulse under
appropriate conditions. We study this process in detail, especially the
nanobunching of the plasma electron density. We derive an analytical expression
that describes the electron density pro- file and obtain a good agreement with
particle-in-cell simulation results. We investigate the most efficient case of
HHG at moderate laser intensity (I = 2*10^20 W/cm^2 ) on the over-dense plasma
slab with an exponential profile pre-plasma. Subsequently we calculate the
spectra of a single at- tosecond pulse from the backward radiation using our
expression for the density shape in combination with the equation for the
spectrum of the nanobunch radiation.
