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

Abstract

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 σ* shape resonances. Absolute elastic cross sections are given for reference.