Electronic excitation of gaseous acetic acid studied by K-shell electron energy loss spectroscopy and ab initio calculations

Abstract

Ab initio Configuration Interaction calculations have been carried out in order to assign the bands observed in the carbon and oxygen K-shell spectra of gaseous acetic acid, measured using the inner-shell electron energy loss spectroscopy (ISEELS) method with better energy resolution than in previous studies. The good agreement between the theoretical and the measured spectra allows us to assign precisely most of the peaks, especially for the Rydberg states. Some of them have been shown to have strong valence character. The lowest energy band at the carbon edge is assigned to the transitions 1sC1→3pπ/σ*(C1–H) and 1sC1→π*(C2–O2). Simple Franck–Condon calculations, based on the linear coupling approximation, were performed in order to reproduce the vibrational structure observed for the first time in the oxygen and carbon core excited species as well as in the previously measured X-ray photoelectron spectroscopy spectra of the core ionised molecule. Finally, the calculated structural parameters of the core states of acetic acid match well those of the corresponding valence states of their Z+1 molecules, as predicted by the equivalent core approximation. However, significant differences between the geometry of the 1sC1→π*(C2–O2) state and the CH3NOOH ground state are obtained.