Ionization of small molecules induced by H$^+$, He$^+$ and N$^+$ projectiles: comparison of experiment with quantum and classical calculations

TL;DR

This study compares experimental measurements of electron emission from molecules impacted by various ions with quantum and classical calculations, revealing insights into ionization processes and the effectiveness of theoretical models.

Contribution

It provides a comprehensive comparison of experimental data with CTMC and CDW-EIS calculations for ion impact on molecules, highlighting the strengths and limitations of each approach.

Findings

Excellent agreement for H+ projectile with both theories.

Good agreement of CTMC with dressed ion impacts, including screening effects.

Overestimation of electron loss in some CDW-EIS calculations.

Abstract

We report the energy and angular distribution of ejected electrons from CH$_4$ and H$_{2}$O molecules impacted by 1 MeV H$^+$, He$^+$ and 650 keV N$^+$ ions. Spectra were measured at different observation angles, from 2 eV to 2000 eV. The obtained absolute double-differential-electron-emission cross sections (DDCS) were compared with the results of CTMC and CDW-EIS calculations. For the bare H+ projectile both theories show remarkable agreement with the experiment at all observed angles and energies. The CTMC results are in similarly good agreement with the DDCS spectra obtained for impact by dressed He$^+$ and N$^+$ ions, where screening effects and electron loss from the projectile gain importance. The CDW-EIS calculations slightly overestimate the electron loss for 1 MeV He$^+$ impact, and overestimate both the target and projectile ionization at low emitted electron energies for 650 keV N$^+$ impact. The contribution of multiple electron scattering by the projectile and target centers (Fermi-shuttle) dominates the N$^+$-impact spectra at higher electron energies, and it is well reproduced by the non-perturbative CTMC calculations. The contributions of different processes in medium velocity collisions of dressed ions with molecules are determined.