Ionization of atoms and molecules using 200-keV protons and 5.5-MeV/u bare C ions: Energy-dependent collision dynamics

Abstract

Using electron spectroscopy technique, we measure the absolute double-differential cross sections (DDCSs) of electrons emitted in collisions of 200-keV protons on He, ${\mathrm{CH}}_{4}$, and ${\mathrm{O}}_{2}$ and 5.5-MeV/u bare C ions colliding on ${\mathrm{O}}_{2}$. The emitted electrons are measured in the energy range from 1 to 400 eV for He and ${\mathrm{CH}}_{4}$ targets and up to 600 eV for ${\mathrm{O}}_{2}$ to include the $K\text{\ensuremath{-}}LL$ Auger line of oxygen. The electrons are detected over different emission angles varying from ${20}^{\ensuremath{\circ}}$ in the extreme forward direction to ${160}^{\ensuremath{\circ}}$ in the backward direction. The single-differential cross section (SDCS) and total cross section are deduced from the measured DDCSs spectra for all the targets. The DDCS and SDCS are compared with the state-of-the-art continuum distorted wave--eikonal initial state (CDW-EIS) theoretical model. The CDW-EIS model provides excellent agreement with the oxygen data at MeV energy, whereas the deviation in the case of keV energy is substantial, in spite of having nearly the same perturbation strength. The forward-backward angular asymmetry shows a saturation effect in the case of keV energy protons but no such signature is observed for the high-energy collision. A systematic analysis reveals that the asymmetry at low electron energy is sensitive to the associated atomic or molecular structure and is in close agreement with the theory.