Forward-backward asymmetry in electron impact ionization of CO.

We experimentally investigate the molecular-orientation dependence of high-energy electron-impact ionization of CO. The direction of the molecular-axis with respect to the momentum transfer vector K is deduced from the angular correlation between the fragment ion and the scattered electron. The experimental results on the 3 Π ionization reveal that at small momentum transfer, the ionization probability near the threshold is higher when K points toward the C atom along the molecular axis than when it is in the opposite direction. Such a forward-backward asymmetry does not appear in single-photon ionization and requires non-dipole contributions. It is also shown that the {4 Σ + 5 Σ + 6 Σ} ionization preferentially takes place in the vicinity of the molecular orientation parallel to K at small momentum transfer, while non-dipole contributions cause the decrease in the relative intensity of the parallel direction.