Autodetachment dynamics of acrylonitrile anion revealed by two-dimensional electron impact spectra.

Two-dimensional electron energy-loss spectra of acrylonitrile are presented in the incident energy range 0.095-0.9 eV, where the incoming electron is temporarily captured in the lowest π* orbital. The cross section is plotted as a function of the incident electron energy that determines which vibrational resonant anion state is populated, and of the electron energy loss, that shows into which final vibrational states each resonance decays. The striking result is that besides the expected horizontal (for given resonances) and vertical (for given final states) patterns of peaks, diagonal arrangements of peaks are observed. They reveal restrictive selectivity with respect to which vibrations are deexcited in the detachment process, with ν(4) (C ≡ N stretch) and ν(7) (C-H rocking) dominating. Surprisingly, all spectral features could be assigned to vibrational levels of the valence π* resonance, and none to dipole-bound vibrational Feshbach resonance, although the latter must also exist in this energy range.