Ionization Energies and Rydberg States of Tetraaminoethylenes

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Absorption spectra and photo-ionization curves of tetrakis (dimethylamino) ethylene, (TMAE), and 1,1′,3,3′-tetramethyl-<I>Δ</I><SUP>2,2′</SUP>-bi(imidazolidine), (TMBI), have been studied. It has been found that these compounds have very low ionization potentials (about 5.4 eV), which are even comparable with that of the lithium atom. The first ionization potentials of these compounds are nearly the same, although both their absorption spectra and their photo-ionization curves are considerably different from each other. These results can be explained semi-quantitatively by use of the results of simple, semi-empirical molecular orbital calculation with inclusion of direct interaction between four lone-pair orbitals on the N atoms, which was found to be important at the "perpendicular" structure. The first absorption bands of these molecules are assigned to the Rydberg bands, lying quite unusually near the visible region. The quantum defect δ is found to be 0.7 for the lowest Rydberg state of TMAE. The size of the Rydberg orbital for this state is estimated to be 2.9 Å from the maximum radial density, which is comparable with the core size. Higher Rydberg series are also observed for TMBI.
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