Speaker
Description
Within quantum mechanics, understanding ionisation in ion-atom collisions has been an ongoing project for decades. The problem has resisted solution due to the complexity of modelling an electron's motion in the presence of long-ranged attractive potentials. This is especially true in the intermediate-energy region, as the probability of capture by the projectile is significant making the ionisation process more difficult to study both theoretically and experimentally. Differential cross sections are necessary in this endeavour as they are the most detailed quantities that may be used to examine ionisation. However, even for the simplest four-body system consisting of proton collisions with atomic helium, debate within the field regarding the underlying mechanisms governing single ionisation is not settled.
Thus far, the two widely recognised mechanisms are direct ionisation (DI) of the target and electron capture to the continuum (ECC) of the projectile. The former is characterised by a maximum when an electron is ejected with near-zero energy and the latter by a maximum in the forward direction when the electron is ejected with a velocity close to that of the incident projectile, known as the matching velocity. The saddle-point mechanism was first proposed by Olson et al. [1]. According to this idea, as the scattered projectile and the residual target move apart, the ejected electron is stranded at the point at which the Coulombic attraction from the target and projectile is equal, leading to an increase in the ionisation probability in this region.
Subsequent theoretical and experimental investigations reported conflicting findings, thus, there was insufficient evidence to support or refute the existence of saddle-point ionisation. The two-centre four-body wave-packet convergent close-coupling (WP-CCC) approach has been applied to the calculation of the energy and angular distribution of electrons in intermediate-energy proton-helium collisions to address this question [2]. Results are in excellent agreement with the measurements of Meckbach et al. [3], identifying both the DI and ECC peaks but no sign of an increase in the ionisation cross section at the saddle point is seen. In the calculated doubly differential cross sections, it is noted that the location of the ECC peak moved from the energy corresponding to the matching-speed at 0 deg to smaller ejection energies as the electron ejection angle increased. This phenomenon may have previously been mistaken as evidence of the saddle-point mechanism. Therefore, the WP-CCC method does not support the existence of a saddle-point mechanism in this energy regime.
References
[1] R. E. Olson, T. J. Gay, H. G. Berry, E. B. Hale, and V. D. Irby, Phys. Rev. Lett. 59, 36 (1987).
[2] K. H. Spicer, C. T. Plowman, N. W. Antonio, M. S. Schöffler, M. Schulz, and A. S. Kadyrov, Phys. Rev. A 109, 062805 (2024).
[3] W. Meckbach, S. Suarez, P. Focke, and G. Bernardi, J. Phys. B 24, 3763 (1991).
