Abstract
A model for the description of proton collisions from molecules composed of atoms such as hydrogen, carbon, nitrogen, oxygen and phosphorus (H, C, N, O, P) was recently extended to treat collisions with multiply charged ions with a focus on net ionization. Here we complement the work by focusing on net capture. The ion–atom collisions are computed using the two-center basis generator method. The atomic net capture cross sections are then used to assemble two models for ion–molecule collisions: An independent atom model (IAM) based on the Bragg additivity rule (labeled IAM-AR), and also the so-called pixel-counting method (IAM-PCM) which introduces dependence on the orientation of the molecule during impact. The IAM-PCM leads to significantly reduced capture cross sections relative to IAM-AR at low energies, since it takes into account the overlap of effective atomic cross sectional areas. We compare our results with available experimental and other theoretical data focusing on water vapor (H2O), methane (CH4) and uracil (C4H4N2O2). For the water molecule target we also provide results from a classical-trajectory Monte Carlo approach that includes dynamical screening effects on projectile and target. For small molecules dominated by a many-electron atom, such as carbon in methane or oxygen in water, we find a saturation phenomenon for higher projectile charges (q=3) and low energies, where the net capture cross section for the molecule is dominated by the net cross section for the many-electron atom, and the net capture cross section is not proportional to the total number of valence electrons.
Highlights
Collisions of multiply charged ions with biologically relevant molecules are recognized as being important for future developments in radiation medicine and related fields
Another approach is to use collision information obtained for the atomic constituents and to combine them into molecular cross sections, most notably independent atom models (IAM), which either follow the simple Bragg additivity rule, or more sophisticated versions that take the molecular structure of the target into account, and allow for the fact that the effective cross section should be reduced due to overlap effects
For a detailed description of the other method for which we show results for water molecule vapor targets, namely the classical-trajectory Monte Carlo (CTMC) model, we refer to previous literature
Summary
Collisions of multiply charged ions with biologically relevant molecules are recognized as being important for future developments in radiation medicine and related fields. E.g., ionization at very high energies for multiply charged projectiles single-electron processes may dominate and the difference between net and total (non-weighted summed) cross sections for ionization or capture becomes small or even negligible. The CTMC approach takes the (frozen) molecular orientation into account during the collision, and total ionization cross sections, as well as some charge-state correlated cross sections have been compared to experimental data [6] Another approach is to use collision information obtained for the atomic constituents and to combine them into molecular cross sections, most notably independent atom models (IAM), which either follow the simple Bragg additivity rule (labeled IAM-AR), or more sophisticated versions that take the molecular structure of the target into account, and allow for the fact that the effective cross section should be reduced due to overlap effects.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
