Electronic spectra of butadiene and its methyl derivatives: A multiphoton ionization study

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Abstract
The multiphoton ionization (MPI) spectra of trans‐1,3‐butadiene and three of its methyl substituted derivatives have been recorded using a tunable visible dye laser. The effect of methyl substitution on the electronic structure of butadiene is investigated, resulting in a reassignment of the spectra of these molecules. Quantum defects are found to be independent of principal quantum number, indicating little mixing between Rydberg and core orbitals. The validity of retaining conventional s, p, d, and f descriptions of Rydberg orbitals in these quasilinear molecules is demonstrated. Two electronic states of butadiene, assigned to the 3d and 4s Rydberg states, are observed for the first time in optical spectroscopy. No evidence for an excited 1Ag valence state is found in the MPI spectra.