Band intensities in the absorption spectrum of benzene vapour

The transition intensities of a large number of different progressions in the absorption spectra of benzene-h6 and -d6 were carefully measured, photo-electrically. Overall, the results are in agreement with the Franck-Condon factors predicted on the basis of Herzberg-Teller theory for a C-C bond length change of between 0·032 to 0·034 Å. Significant deviations do exist, not all of which can be readily ascribed to assigned Fermi resonances.