A PRECISE MEASURE OF OFF-ENERGY-SHELL EFFECTS IN ION-ATOM COLLISIONS

The forward spectrum of ionized electrons in ion-atom collisions is do minated by the capture to continuum cusp and the quasi-elastic electro n scattering peak, usually referred to as the binary encounter peak. T he cusp is replaced by a pronounced dip when antiprotons are considere d as projectiles, but the binary encounter peak prevails. The peak pos ition is, however, shifted in energy relative to the corresponding pos ition in the proton impact case. This relative shift is solely due to off-shell effects in the quasi-free scattering of the target electron in the field of the incident projectile. The relative shift is a slowl y decreasing function of projectile energy. A typical value in the cas e of helium targets is 25 eV for protons and antiprotons at 1 MeV whic h is well within the mach of the present experimental resolution power of forward electron spectroscopy.