Excited state spectroscopy of para di-substituted benzenes in a supersonic beam using resonant two photon ionization

Excited state vibronic spectra of p-aminophenol, p-cresol, p-fluoroaniline, p-fluorophenol, hydroquinone and p-toluidine have been obtained using resonant two photon ionization supersonic beam mass spectrometry. Despite marked similarities in the spectra, notable differences exist and different para polyatomic substituents in the same molecule show vibronic evidence of their real molecular symmetry of C_2ν. Expansion of the ring is also noted upon excitation in all cases. Further, it is now evident that the assignment of some vibronic bands historically interpreted as sequence structure must be reconsidered since molecules like hydroquinone are mixtures of cis and trans and others have a vibronic structure arising from the polyatomic nature of the substituents (cƒ. CH₃).