FDCS in atomic units for SI of helium using a 3-body model without PCI (dashed), 4-body model without PCI (solid), 4-body model with PCI (dotted), and 3-body model with PCI (dash-dotted) L HarrisA MorrisonK 2013 <p><strong>Figure 4.</strong> FDCS in atomic units for SI of helium using a 3-body model without PCI (dashed), 4-body model without PCI (solid), 4-body model with PCI (dotted), and 3-body model with PCI (dash-dotted). The theoretical calculations are compared to the experiments of (b) Schlemmer [<a href="http://iopscience.iop.org/0953-4075/46/14/145202/article#jpb469773bib52" target="_blank">52</a>], (d), (f) Jung [<a href="http://iopscience.iop.org/0953-4075/46/14/145202/article#jpb469773bib54" target="_blank">54</a>], and (g)–(i) Röder [<a href="http://iopscience.iop.org/0953-4075/46/14/145202/article#jpb469773bib55" target="_blank">55</a>]. Incident projectile energies <em>E</em><sub>0</sub>, ejected electron energies <em>E</em><sub>2</sub>, scattering angles θ<em><sub>s</sub></em>, and momentum transfers <em>q</em> are shown in the figure.</p> <p><strong>Abstract</strong></p> <p>The frozen core approximation has been successfully used for many years to model 4-body collisions as 3-body collisions. We present a comprehensive comparison of 3-body and 4-body models for the process of single ionization of helium by electron impact using our 4-body distorted wave model. Differences between the two models are observed in both magnitude and peak locations. We identify four possible sources for the discrepancies between the models, and isolate the specific physical causes of the discrepancies.</p>