The excited states of 1, 3-butadiyne determined by electron energy loss spectroscopy

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Abstract
Dipole forbidden electronic transitions in butadiyne were studied in the range 2–12 eV using a trochoidal electron spectrometer. The lowest observed feature is weak, structureless, with an onset sround 2.7 eV, and is probably due to the lowest excited state3Σ+ u . The second triplet state 3Δ u is observed at 3.216 ±0.01 eV(00 0), respectively 4.2±0.2 eV (vertical). It exhibits long progressions in the ν′ 2 C≡C stretch and ν6 C–H bend vibrations. The latter indicates a bent equilibrium configuration. The UV inactive 00 0 transition of the 1Δ u state is observed at 5.06±0.01 eV with low electron energies. This value is in excellent agreement with the prediction of Jungen e t a l. based on the interpretation of the UV absorption. At higher electron energies only the UV active vibronic transitions of this state are observed. With low electron energies a number of narrow bands corresponding to dipole forbidden transitions are observed above 7 eV. They are tentatively grouped into three new Rydberg series with δ=0.46, 0.5, and 0.87. Finally, three narrow anion states (Feshbach resonances) are observed at 6.71, 6.82, and 7.00±0.04 eV. They decay preferentially by emission of a slow (≤0.3 eV) electron.