Ruling out a saddle-point mechanism of ionization in intermediate-energy ion-atom collisions

Is there a saddle-point mechanism for ionization in intermediate-energy ion-atom collisions? Since Olson [Phys. Rev. A 33, 4397 (1986)] proposed the idea that the electrons stranded in the potential saddle between the two Coulomb centers dominate the ejected-electron spectra, multiple experimental and theoretical attempts have been made to answer this question. However, the topic has remained controversial. Here we provide a theoretical analysis of this question which can contribute significantly to a definitive answer, at least for intermediate and large projectile energies. To this end we calculate the energy and angular distribution of electrons emitted in proton-helium collisions. We use the two-center four-body wave-packet convergent close-coupling method based on the correlated two-electron structure for the helium target. The doubly differential cross sections obtained at 52 and 103 keV show no sign of a hump near one-half the relative velocity of the collision, which is expected according to the saddle-point ionization theory. At the same time, the results are in excellent agreement with measurements by Meckbach et al. [J. Phys. B 24, 3763 (1991)]. Two mechanisms for the production of electrons are clearly identified: direct ionization (direct knockout) and electron capture to the continuum (ECC) of the projectile. The electron speed (equivalently, energy) where direct ionization peaks is found to be practically independent of the ejection angle. However, the ECC peak is shown to shift towards one-half the relative velocity with increasing electron angle. It is concluded that the signatures of the suggested saddle-point mechanism may actually be due to a shift of the well-known ECC peak when electrons are emitted into angles away from the forward direction. Thus, the answer to the question is in the negative.