Unexpectedly large cross sections of high-energy electrons ejected from water vapor by 6.0-10.0 MeV/u He2+ ions

We present absolute doubly differential cross sections (DDCS) of electron emission (⩽∼12 keV, 20°-160°) in collisions of 6.0 and 10.0 MeV/u He2+ ions with water vapor, in which unexpectedly large DDCS values were observed at the backward angles, particularly in the high-energy region, as well as singly differential cross sections larger than the Rudd model by a factor of 2-3 at ∼10 keV. The experimental results imply that these high-energy electrons are accelerated through the Fermi-shuttle acceleration. We evaluated the partial cross sections, which contribute to this acceleration, to be 7.1 and 4.5×10−19cm2, respectively, leading to ∼0.7% of the total ionization cross sections.