Direct and resonant vibrational excitation of C₂H₂by electron impact from 0 to 3.6 eV

Energy and angular dependences of the elastic scattering and of the excitation of the nu 2 , nu 3 and nu 5 vibrational modes have been measured in the angular range from 5 degrees to 108 degrees and for impact energies of the electrons from threshold to 3.6 eV. Absolute differential and integrated cross sections are presented. The study of the vibrations nu 1 and nu 4 is more limited because these Raman active modes are not excited strongly in this energy region. A previous theoretical treatment allows the determination of the amplitudes for direct excitation and for excitation via the 2 Pi g shape resonance and also the relative phases between these two amplitudes as a function of the impact energy. Branching ratios are presented for the decay of the resonance into the different vibrational channels of the molecule.