Impact ionization of molecular oxygen by 3.5-MeV/u bare carbon ions

We have measured the absolute double-differential cross sections (DDCSs) for electron emission in ionization of O-2 molecules under the impact of 3.5-MeV/u C6+ ions. The data were collected between 10 and 600 eV, in an angular range of 30 degrees. to 150 degrees. The single-differential cross sections (SDCSs) in emission angle and electron energy are deduced from the electron DDCS spectra. Also, the total cross section has been obtained from the SDCS spectra. The DDCS spectra as well as the SDCS spectra are compared with continuum distorted-wave eikonal initial-state calculations which employ molecular wave functions built as linear combinations of atomic orbitals. The DDCS ratio i.e. sigma(O2)/2 sigma(O), derived by dividing the experimental DDCS for molecular oxygen with the theoretical DDCS for atomic oxygen, does not show any primary or secondary oscillations arising from Young-type interference, which is apparently in contrast to what has been observed earlier for H-2 and in agreement with the model calculation. Similarly, the forward-backward angular asymmetry increases monotonically with the velocity of the emitted electrons. However, the results on the DDCSs, SDCSs, the asymmetry parameter, and the nonexistence of oscillations are in qualitative agreement with the predictions of the model used.