Experimental study of vibrational excitation of allene by slow electron impact: Vibronic coupling in the shorthyphen;lived negative ion states

摘要

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 Cndash;Cndash;C stretch, CH2twist and scissoring, CH stretch, and Cndash;Cndash;C bending are all excited. The antisymmetric Cndash;Cndash;C stretch and CH2twist are excited by Jahnndash;Teller activity of the degenerate2Eresonance, the bending by vibronic coupling with higher lying resonances. The essential features of the excitation are qualitatively rationalized by a Hartreendash;Fock (HF)/6hyphen;31Gast; 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 2ndash;16 eV range, mainly in the excitation functions of the Cndash;H stretch and the CH2scissoring vibrations. A moderately broad resonance peak at 11.5 eV, observed in the excitation of the symmetric and antisymmetric Cndash;Cndash;C stretch vibrations, is assigned to two overlapping sgr;ast; shape resonances. Absolute elastic cross sections are given for reference.