Higher order polarizabilities and the positron forward scattering problem: Convergence between calculated and measured cross sections in the very low energy regime

Semiempirical Calculations on Low-Energy Electron Scattering by Zn and Cd Atoms

“…Opposed to ab initio and model potential formulations, semiempirical approaches are based on adjustable parameters, usually tuned to reproduce some external previously known quantity. In practice, we are going to work with a single-body potential to represent the electron-atom interaction whose polarization component has asymptotically correct form up to the first order [16,17], and its short range component is adjusted to reproduce the position of the p-wave resonances at the energies observed by Burrow et al [4].…”

Low Energy Positron Scattering by F and F2

“…This makes other terms necessary in the polarization component of the scattering potential, such as the quadrupole polarization term, first mixed hyperpolarizability, and second hyperpolarizability. When this polarization potential is combined with a model correlation potential based on density functional theory for the positron correlation energy with an electron gas, the cross sections obtained for positron scattering by N 2 and O 2 satisfactorily describe the respective experimental data …”

“…If higher order polarization effects had to be somewhat considered in a SMC calculation, this would certainly be one of the most adequate basis to this end available in the literature. If the hypothesis that higher order polarization effects are critical to successfully describe the low energy positron molecule cross sections, ,, it will be hard to avoid the inference that the majority of the positron scattering results obtained with the SMC methodology up to this date are underpolarized, a point of fundamental relevance for nonpolar molecules. The idea that it is enough to choose a basis of Gaussians that describes the fundamental energy of the target and its intrinsic and induced multipole moments to perform calculations with the SMC is not verified in practice, , that is, validation criteria for the elected basis are necessary.…”

“…One may argue that this procedure is physically legitimate, and surely it is if the experimental cross sections were absolutely correct. Unfortunately this is not the case. , Recently, we showed that the missing angle in transmission based spectrometers may lead to large errors in the measured cross sections for low energies (≤1 eV), that is, the positron cross sections cannot be extrapolated to very low energies without being corrected for the forward scattering missing angle effect . After fixing it, the collision parameters as the scattering length, for example, can be obtained in a reliable manner.…”

“…In this sense, the purpose of this work is to study the elastic scattering of positrons by F 2 molecules using the model potential approach cited above. Due to the encouraging results found in the study of the forward problem with such theoretical model, we find it interesting to study a system that does not have any experimental data yet. To assist in this task, the positron scattering by F atoms will also be investigated since the positron-F 2 cross sections can be obtained from the F scattering amplitude when employing the IAM.…”