Electronic spectra of 1,4-cyclohexadiene and 1,3-cyclohexadiene: A combined experimental and theoretical investigation

The electronically excited states of the two isomers 1,3- and 1,4-cyclohexa diene are examined by means of multiconfigurational second-order perturbati on (CASPT2) theory with extended ANO-type basis sets. The calculations comp rise the lower valence excited states plus all singlet 3s, 3p, and 3d membe rs 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 resonantl y enhanced multiphoton ionization spectroscopic measurements were made on t he 1,3 isomer. The computed vertical excitation energies were found to be i n agreement with the available experimental energies for both systems. New assignments are proposed on the basis of combined experimental and theoreti cal efforts.