Interference effects in fully differential cusp electron production cross sections for p + He collisions

We have measured fully momentum analyzed He⁺ recoil ions and scattered projectiles in coincidence for 75 keV p + He collisions.  From the data, we obtained fully differential ionization cross sections (FDCS) for electrons with an energy of 43.9 eV ejected into the scattering plane.  This energy corresponds to an electron speed close to the projectile speed (velocity matching).  The measurements were performed for a small collimator slit - target distance corresponding to a relatively small transverse projectile coherence length of about 1.0 a.u.

Previously, we reported FDCS for several ejected electron energies in the velocity matching regime, measured for a relatively large transverse projectile coherence length.  At large projectile scattering angles, apart from the well-established “binary peak”, another well-separated peak structure in the forward direction was observed.  This was explained by interference between the first- and higher-order amplitudes.  At the matching velocity, one higher-order process which is known to be particulalrly important is post-collision interaction (PCI), leading to the so-called cusp peak in the electred electron energy spectra.  If this explanation is correct, the double-peak structure should disappear for a coherence length approaching 0.  Indeed, the new data, taken for small coherence length, show a less pronounced double peak structure.