Direct absorption spectra of higher excited states of jet-cooled monosubstituted benzenes: Phenylacetylene, styrene, benzaldehyde, and acetophenone

The second π→π* absorption bands of phenylacetylene, styrene, benzaldehyde, and acetophenone cooled in supersonic molecular jets are investigated by direct ultraviolet absorption spectroscopy. New vibronic structure is observed and vibrational assignments are discussed. The most prominent vibronic bands arise from motions centered in the substituent and from substituent-sensitive benzene modes. This contrasts with the spectra of the first π→π* transitions of these molecules, which exhibit structure involving ring-localized motions primarily. The spectra presented here demonstrate the utility of the direct absorption technique in the investigation of higher electronic transitions of jet-cooled molecules.