Photoelectron spectra of Al2O2− and Al3O3−via slow electron velocity-map imaging

Abstract

High-resolution photoelectron spectra of cryogenically-cooled Al₂O₂⁻ and Al₃O₃⁻ cluster anions are obtained using slow electron velocity-map imaging. These spectra show vibrationally-resolved detachment from the (²B_3u) ground state of Al₂O₂⁻ to the ¹A_g and ã³B_3u neutral electronic states, giving an electron affinity of 1.87904(4) eV for neutral Al₂O₂ and a term energy of 0.4938(4) eV for the triplet excited state. Additionally, there is evidence for autodetachment from photoexcited anions as well as influences from vibronic coupling between excited states of the neutral Al₂O₂ cluster. Detachment from both the “kite” and “book” isomers of Al₃O₃⁻ is observed, yielding electron affinities of 2.0626(4) and 2.792(3) eV for the corresponding neutral isomers. Experiments carried out at different anion temperatures suggest that the two anionic isomers are nearly isoenergetic but clearly identify the kite isomer as the global minimum structure, in contrast to prior studies. This finding is supported by density functional theory calculations, which show that the relative ordering of the anion isomers is sensitive to basis set size; calculations for the anion isomers at the B3LYP/aug-cc-pVQZ level find the kite isomer to lie 0.011 eV below the book isomer.