High energy neutral atom emission from rear side of foils

Intense laser pulses focused onto a solid micron foil target are best known for generation of large electrostatic sheath fields at the target rear and acceleration of protons to MeV energies. The fact that such acceleration schemes also generate fast neutral atoms has received comparatively less attention. A quantitative assessment of the neutralization fraction reveals novel and often unexpected aspects of electron-ion interaction in regions far from dense plasmas. At the rear side of laser irradiated foil, a few hundred keV accelerated neutral Hydrogen atoms are observed. Neutralisation fraction is much larger than that anticipated from the collisional charge transfer and is about 300% larger for ions of ⩽ 100 keV energy. Corroborated by particle-in-cell simulations, the larger neutralisation fraction is attributed to electron-recombination as the protons co-propagate with the low energy electrons.