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The electronically excited states of the two isomers 1,3-and 1,4-cyclohexadiene are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprise the lower valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging to the first ionization limit for the 1,3 isomer and the two first limits for 1,4. Companion optical and resonantly enhanced multiphoton ionization spectroscopic measurements were made on the 1,3 isomer. The computed vertical excitation energies were found to be in agreement with the available experimental energies for both systems. New assignments are proposed on the basis of combined experimental and theoretical efforts.

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