Coupling of electronic and nuclear motion in a negative ion resonance: Experimental and theoretical study of benzene

We present calculated and measured elastic and vibrational excitation cross sections in benzene with the objective to assess the reliability of the theoretical method and to shed more light on how the electronic motion of the incoming electron is coupled with the nuclear motion of the vibrations. The calculation employed the discrete momentum representation method which involves solving the two-channel Lippmann-Schwinger equation in the momentum space. The electron-molecule interaction was described by the exact static-exchange potential extended by a density-functional theory correlation-polarization interaction that models the molecular response in the field of the incoming electron. Cross sections were calculated for all 20 vibrational modes from near threshold until 20 eV. They were convoluted with a simulated instrumental profile for comparison with electron energy-loss spectra or appropriately summed for overlapping vibrations for comparison with measured cross sections plotted as a function of electron energy. An electron spectrometer with hemispherical analyzers was employed for the measurements. Good agreement of theory with experiment was obtained for the spectral profiles at 8 eV, and a nearly quantitative agreement was obtained at 3 and 4.8 eV. The theoretical results provided new insight into the excitation process, and it showed that more modes are excited than predicted by simple symmetry rules. Spectra showing the details of boomerang structure in the 1.15 eV π* resonance were recorded and are presented, although this aspect of experiment cannot be compared with the current theory.We present calculated and measured elastic and vibrational excitation cross sections in benzene with the objective to assess the reliability of the theoretical method and to shed more light on how the electronic motion of the incoming electron is coupled with the nuclear motion of the vibrations. The calculation employed the discrete momentum representation method which involves solving the two-channel Lippmann-Schwinger equation in the momentum space. The electron-molecule interaction was described by the exact static-exchange potential extended by a density-functional theory correlation-polarization interaction that models the molecular response in the field of the incoming electron. Cross sections were calculated for all 20 vibrational modes from near threshold until 20 eV. They were convoluted with a simulated instrumental profile for comparison with electron energy-loss spectra or appropriately summed for overlapping vibrations for comparison with measured cross sections plotted as a function of elec...