EXPERIMENTAL-STUDY OF VIBRATIONAL-EXCITATION OF ALLENE BY SLOW-ELECTRON IMPACT - VIBRONIC COUPLING IN THE SHORT-LIVED NEGATIVE-ION STATES

Vibrational electron energy loss spectra, vibrational excitation functions, and angular dependence of vibrational energy losses were measured in relative units for propadiene (allene, H2C=C=CH2) in an incident energy range up to 16 eV. Resonant excitation via the 2 eV resonance is not very selective; symmetric and antisymmetric C-C-C stretch, CH2 twist and scissoring, CH stretch, and C-C-C bending are all excited. The antisymmetric C-C-C stretch and CH2 twist are excited by Jahn-Teller activity of the degenerate 2E resonance, the bending by vibronic coupling with higher lying resonances. The essential features of the excitation are qualitatively rationalized by a Hartree-Fock (HF)/6-31G* anion potential energy surface. Unspecific excitation of high vibrational levels, accompanied by detachment of slow electrons, is also observed to result from attachment of 2 eV electrons, and is rationalized as a consequence of temporary trapping of part of the nuclear wave packet on the bound (not autodetaching) part of the anion potential surface. Very broad resonance features are observed in the 2-16 eV range, mainly in the excitation functions of the C-H stretch and the CH2 scissoring vibrations. A moderately broad resonance peak at 11.5 eV, observed in the excitation of the symmetric and antisymmetric C-C-C stretch vibrations, is assigned to two overlapping sigma* shape resonances. Absolute elastic cross sections are given for reference.