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 Ni²⁴⁺ impact was explored in a kinematically complete experiment by combining a high-resolution recoil-ion momentum spectrometer with a novel 4π low-energy electron analyzer. More than 90% of the "soft electrons" (Eₑ50 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 He¹⁺ ion. Energy losses of the 0.2 GeV projectiles as small as ΔEPEP=3.4×10^-7 are observable.