  We present a detailed quantitative description of the recently proposed
"slingshot effect" [Fiore, Fedele, De angelis 2014]. Namely, we determine a
broad range of conditions under which the impact of a very short and intense
laser pulse normally onto a low-density plasma (or matter to be locally
completely ionized into a plasma by the pulse) causes the expulsion of a bunch
of surface electrons in the direction opposite to the one of propagation of the
pulse, and the detailed, ready-for-experiments features of the expelled
electrons (energy spectrum, collimation, etc). The effect is due to the
combined actions of the ponderomotive force and the huge longitudinal field
arising from charge separation. Our predictions are based on estimating 3D
corrections to a simple, yet powerful plane magnetohydrodynamic (MHD) model
where the equations to be solved are reduced to a system of Hamilton equations
in one dimension (or a collection of) which become autonomous after the pulse
has overcome the electrons. Experimental tests seem to be at hand. If confirmed
by the latter, the effect would provide a new extraction and acceleration
mechanism for electrons, alternative to traditional radio-frequency-based or
Laser-Wake-Field ones.
