Intensity distribution in rotational line spectra

Completely resolved Doppler-free rotational line spectra of six
vibronic two-photon bands in benzene C6 H6 and C6 D6 are presented.
The excited final states possess different excess energies in S1
(1567 to 2727 cm−1 ) and are embedded in dense manifolds of
background states with differing densities of states (1