Continuum-orientation phenomena in ionization by positron impact

Abstract

Ionization collisions by positron impact at low and intermediate energies are investigated by means of a classical trajectory Monte Carlo (CTMC) simulation. Calculations of fully differential cross sections unveil a remarkable dynamical orientation of the electron-positron continuum dipole. Actually, by means of the CTMC simulation we show that, for electron velocities close to the final positron velocity, the dipole is narrowly oriented along the direction of motion of its centre-of-mass, with the negative charge pointing towards the residual target, i.e. emitted at lower energies than the positron. The characteristic features of this phenomenon are studied. Also, several future experiments and calculations are proposed. The possible outcomes of this further research might help to unveil the mechanisms responsible for the appearance of this orientation effect.

Classical trajectory Monte Carlo calculation of the recoil-ion momentum distribution for positron-impact ionization collisions

“…This means that the inner sphere is more sharply defined when the electron-positron centre of mass moves in the forward direction. This result is consistent with CTMC calculations of the electron momentum distribution [30,39,40]. This means that the orientation effect of low-energy electrons described in the previous section is much stronger when θ is smaller.…”

“…Up to our knowledge, no similar result has ever been reported or investigated in ionization processes. We assume that this surprising effect might be related to a strong orientation of low-energy electrons into the direction of motion of the electron-positron centre of mass, already observed in our most recent CTMC calculations of the electron and positron momentum distributions [39,40].…”

Vortices in ionization collisions by positron impact

“…Even before any positron reaction microscope becomes operational, experimental and theoretical studies of one and two-body spectra have already leaded to a clear understanding of some of the main features of the FDCS for positron-impact ionisation collisions. This is the case, for instance, of the electron capture to the continuum (ECC) effect (Kövér and Laricchia 1998), whose characteristics in the momentum spectra of both the electron (Fiol and Barrachina 2011) and the recoil-ion (Barrachina and Fiol 2012) are now well understood.…”

Collisional Classical Dynamics at the Quantum Scale

“…This work was followed soon after by differential analyses in positron collisions with He, Kr and H 2 [81,82]. Positron studies at the differential level were also carried out within the CTMC method by groups at Bariloche [83][84][85], Debrecen [86] and Bahía Blanca [16]. These studies, provided further insight on the physical mechanisms mediating the ECC peak structure, as well as the dynamical orientation of the positron-emitted electron pair in the continuum.…”