A screened independent atom model for the description of ion collisions from atomic and molecular clusters

TL;DR

This paper introduces a geometrical independent-atom model for calculating ion-impact electron transfer and ionization cross sections in atomic and molecular clusters, validated against experimental data and revealing size and energy dependencies.

Contribution

The study develops a novel geometrical model based on overlapping atomic disks and uses DFT-based calculations to accurately predict cross sections for ion collisions with clusters.

Findings

Cross sections scale with cluster size as n^α, with α between 2/3 and 1.

At 100 keV, capture cross sections have α close to 1, differing from previous models.

Results are consistent across water, neon, and carbon clusters, with simple parametrizations fitting the data.

Abstract

We apply a recently introduced model for an independent-atom-like calculation of ion-impact electron transfer and ionization cross sections to proton collisions from water, neon, and carbon clusters. The model is based on a geometrical interpretation of the cluster cross section as an effective area composed of overlapping circular disks that are representative of the atomic contributions. The latter are calculated using a time-dependent density-functional-theory-based single-particle description with accurate exchange-only ground-state potentials. We find that the net capture and ionization cross sections in p-X$_n$ collisions are proportional to $n^\alpha$ with $2/3 \le \alpha \le 1$. For capture from water clusters at 100 keV impact energy $\alpha$ is close to one, which is substantially different from the value $\alpha=2/3$ predicted by a previous theoretical work based on the simplest-level electron nuclear dynamics method. For ionization at 100 keV and for capture at lower energies we find smaller $\alpha$ values than for capture at 100 keV. This can be understood by considering the magnitude of the atomic cross sections and the resulting overlaps of the circular disks that make up the cluster cross section in our model. Results for neon and carbon clusters confirm these trends. Simple parametrizations are found which fit the cross sections remarkably well and suggest that they depend on the relevant bond lengths.