Abstract

The C1s and O1s core shake-up and HeII excited valence photoelectron spectra of formaldehyde, acetaldehyde and acetone have been studied experimentally and by INDO-CI calculations. It is found that the presence of the ? type orbitals on the CH₃ groups of acetaldehyde and acetone is responsible for the large differences found in the core shake-up spectra between these molecules and formaldehyde. Due to the redistribution of charge in the oxygen acetaldehyde and acetone core ionised systems, tire energy ordering between the C=O ? orbital and the CH₃ ¹b₁ ? type orbital is reversed compared to the neutral molecules. The ionization from the 6a₁ ?(C-O) orbital in acetone gives rise to a clearly observable vibrational progression whereas the corresponding orbital on formaldehyde is structureless. This indicates a substantial stabilization of the bonds due to the methyl substitution.

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