| Research Abstract |
Due to the difference in charge, electrons (e^-) and positrons (e^+) are expected to provide quite different interaction scheme and dynamical mechanisms upon collisions with atoms and molecules. We have investigated vibrational excitation in CO_2 and OCS molecules by (e^-) and e^+ impacts, because both molecules are linear molecules, but OCS molecule has a weak dipole moment while CO_2 has no dipole. We have observed a strong vibrational mode-dependence for vibrational excitation in e^+ and e^- scattering from CO_2 for the first time ; for the symmetric stretching (100) mode, vibrational excitation cross section by e^- scattering is larger by two-three orders of magnitude than that by e^+ scattering, while the cross sections for asymmetric stretching (001) and bending (010) modes are rather similar. Our experimental results are qualitatively consistent to theoretical findings. The weak quadrupole interaction and long-range strong dipole interaction in a combination with the coupling matrix element are responsible to the cause. In addition, the presence of the resonance-like structures was abundantly observed.
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