Selectivity in resonant vibrational excitation by e − impact (0.3–4.5 eV) on formaldehyde, acetaldehyde and acetone

Abstract

Vibrational excitation by e − impact via low-energy resonances has been investigated in acetaldehyde and acetone and compared with similar results in formaldehyde. Despite the large number of vibrational modes involved, the three systems exhibit a selective excitation of only several modes. Besides an important excitation of the CO stretch modes (dominant in H 2CO), excitation of CH stretch, CH 3 stretch and CH 3 deformation are also observed in CH 3CHO and (CH 3) 2CO. Interpretation of the energy loss spectra is given in terms of recently developed symmetry considerations together with the character of the LUMO occupied by the extra electron to form the transitory negative ion (resonance). Differential cross sections versus electron energy are presented for elastic and several inelastic processes. Weak oscillations (of the “boomerang” type) are observed on the inelastic cross sections for acetaldehyde, whereas no structure appears for acetone. This is in contrast with the pronounced oscillations observed for H 2CO, and reveals a shorter lifetime for the CH 3CHO − and (CH 3) 2CO − resonant states, compared to H 2CO −.