Energies and decay channels of negative ion resonances in acetaldehyde

Abstract

Negative ion mass spectroscopy and electron impact spectroscopy were employed for a detailed study of the electronic structure and decay channels of the acetaldehyde negative ion. The energy-dependence of the vibrational excitation revealed a very broad σ ★ shape resonance peaking at 6.8 eV, in addition to the already known π ★ shape resonance at 1.2 eV. Energy dependence of the excitation of the 3(π, π ★) state revealed a 2(π, π ★2) core excited resonance at 8.1 eV. This resonance, besides decaying into its parent valence excited state, also dissociates to form O −. Two Feshbach resonances, 2( n3 s 2 ) at 6.34 eV and 2( n,3s3 p) at 6.64 eV, are observed in the dissociative attachment yields of CH − 3 and H −, as well as in energy-loss spectra near threshold. Time resolved energy-loss spectra suggest that a fraction of the CH 3 − ions are formed in low vibrationally excited states, which subsequently undergo slow autodetachment on the 0.1–1 μs time scale. Study of partially deuterated acetaldehyde showed no 'hydrogen scrambling' prior to dissociation of the Feshbach resonances, and further permitted the assignment of the vibrations excited in the two Feshbach resonances as v 6 (C-H bend) and v 7, (CH 3 deformation).