Selectivity in the excitation of fermi-coupled vibrations in CO2 by impact of slow electrons.

Improvement of resolution of a gas phase hemispherical electron spectrometer (to 7 meV FWHM) permitted separate measurements of the excitation of each member of the well known Fermi dyad in CO2 by impact of slow electrons. Absolute differential cross sections at a scattering angle of 135 degrees were measured as a function of electron energy. The excitation was found to be highly selective both via the 2Pi(u) shape resonance around 3.6 eV and via the virtual CO2- state at low energy. The selectivity is surprising because both vibrational states ensuing from the Fermi resonance involve the same type of nuclear motion (bending and stretch) and excitation of both is symmetry allowed.