POLARIZATION AND FORWARD SCATTERING EFFECTS IN LOW-ENERGY POSITRON INTERACTIONS WITH H₂

Аннотация

Positron physical-chemistry has been a significant area of scientific investigation in recent decades. However, low-energy positron scattering by atoms and molecules still presents several open questions, particularly regarding the effects of low-angle scattering on measured cross sections and the role of target polarization in the comparison between theoretical and experimental results. In this study, we examine low-energy positron collisions with H₂ molecules, focusing on the convergence of the polarization contribution in the scattering potential. The interaction between the positron and the molecule is represented by a model potential, which combines a free-electron gas correlation term with an asymptotic polarization potential derived from perturbation theory. Specifically, we analyze how polarization effects beyond second-order perturbation influence scattering observables. Our findings indicate that a model incorporating up to the quadrupole polarization contribution shows improved agreement with recent experimental data when corrected for forward scattering effects, as these measurements were obtained using a transmission beam technique. We also examine angular distributions by comparing our results with available folded differential cross-section measurements. A simple correction scheme is proposed for experimental folded differential cross sections at energies below 1 eV, which we argue aligns well with the quadrupole polarization model. Finally, a comparison between our phase shifts and scattering lengths with recent many-body ab initio calculations—including virtual positronium effects—suggests that these effects are inherently accounted for within the adopted model correlation potential. Keywords: molecular hydrogen, positron, elastic scattering, polarization.