Perturbation selection rules for multiphoton electronic spectroscopy of neutral alternant hydrocarbonsa)

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Abstract
A one‐electron perturbation which is to significantly enhance a ‘‘forbidden’’ single‐photon or multiphoton optical transition between any two states of an alternant hydrocarbon must satisfy pseudoparity selection rules in addition to the usual group theoretical selection rules. Pseudoparity predicts whether vibronic or inductive perturbations, e.g., will be successful, while group theory specifies the proper perturbation symmetries. A principal result is that for a given electronic transition, the roles of vibronic, and inductive perturbations are complementary. Their roles reverse depending on whether the transition requires an even or odd number of photons.