Low-Energy Electrons and Their Dynamical Correlation with Recoil Ions for Single Ionization of Helium by Fast, Heavy-Ion Impact

Helium single ionization by 3.6 me V/u ${\mathrm{Ni}}^{24+}$ impact was explored in a kinematically complete experiment by combining a high-resolution recoil-ion momentum spectrometer with a novel $4\pi$ low-energy electron analyzer. More than 90% of the "soft electrons" ($E_e\lesssim 50$ eV) are ejected in the forward direction in agreement with classical-trajectory Monte Carlo predictions. The electron longitudinal momentum is not balanced by the longitudinal momentum change of the projectile but mainly by the backwards recoiling ${\mathrm{He}}^{1+}$ ion. Energy losses of the 0.2 GeV projectiles as small as $\frac{\Delta E_P}{E_P}=3.4\mathrm{}\times {10}^{-7\mathrm{}}$ are observable.