Positronium formation in collisions of fast positrons impacting on vapour water molecules

Abstract

Positronium formation through electron capture by fast positrons impinging on vapour water molecules is studied theoretically at intermediate and high impact energies. This multi-electron system is treated within the framework of the independent electron model. The charge transfer process is described by employing the continuum distorted-wave final-state approximation. In this model, the final state of the collision is distorted by two Coulomb wavefunctions associated with the interactions of both the positron and the active electron (the captured one) with the residual ionic target. The water molecule is described by an expansion of monocentric Slater-like functions centred on the oxygen atom. Total cross sections are computed by using a partial-wave technique and compared with results obtained using the simpler Coulomb–Born approximation. Theoretical results for the production of H2O+ ions in the final channel of the reaction through charge transfer processes (positronium formation and ionization) are also presented.

Comment on ‘Total and positronium formation cross sections for positron scattering from H2O and HCOOH’

“…It is clear that the Ps formation threshold is different from and lower in energy than the threshold for ionization of about 12.6 eV. As a matter of fact, this can be clearly seen in figure 2 of [2] where we show our theoretical CDW-FS results for Ps formation cross sections from different water molecular orbitals. As may be inferred from these data, the threshold for our predictions of Ps formation in its ground state from the 1B 1 is approximately 6 eV.…”

“…The theoretical Ps formation cross sections we presented in [2] have been misinterpreted in [1], in particular above the ionization threshold. Indeed, the above paragraph extracted from [1] was based on the results introduced in figure 5 of [2].…”

“…Continuum distorted wave-final state (CDW-FS) calculations for Ps formation and distorted-wave Born results for single target ionization by positron impact are shown. In figure 5 of our work [2], the cross section for Ps formation into 1 s and 2 s states was deliberately depicted beyond its threshold energy of about 6 eV. It is clear that the Ps formation threshold is different from and lower in energy than the threshold for ionization of about 12.6 eV.…”

“…We would like to comment on the following sentence in Casten Makochekanwa's paper 'Total and positronium formation cross sections for positron scattering from H 2 O and HCOOH' [1] with regard to our theoretical cross sections [2] for positronium (Ps) formation (QPs as stated in [1]) and the corresponding threshold energy (EPs as stated in [1]):…”

“…Indeed, the above paragraph extracted from [1] was based on the results introduced in figure 5 of [2]. In this figure, the cross sections for the production of the molecular ion H 2 O + during the collision of water molecules with a positron beam are displayed.…”