The 1B2u←←1A l g two-photon spectra of several isotopes of benzene by supersonic beam–multiphoton ionization spectroscopy

The two-phonon spectra of the 1B2u←←1Alg transitions of benzene, perdeuterobenzene, sym-C6H3D3, p-C6H4D2, and C6H5D have been recorded in a supersonic beam–multiphoton ionization experiment. A thorough assignment of these spectra includes a number of new bands as well as some reassignments of features seen in previous room temperature studies, notably in transitions concerning the ν12 vibration. Efficient rotational cooling has allowed the direct measurement of rotationless band origins with better than 0.5 cm−1 accuracy, uncovering structure due to naturally occurring 13C substituted molecules and to dimers created in the expansion.