Emission spectra from single rovibronic quantum states in S1 benzene after Doppler-free two-photon excitation

Dispersed emission from single rovibronic quantum states in S1
benzene is measured after Doppler-free two-photon excitation under
low pressure conditions (0.3 Torr). This was made possible by a
long-term stabilization of the single-mode dye laser yielding a
stability of better than 1 MHz/h. The emission spectra of
unperturbed rotational levels in the 141 and the 14111 vibronic
states reveal a great number of detailed results on Duschinsky
rotation and long-range Fermi resonances in the electronic ground
state. By contrast, it is seen that the emission spectra from
perturbed rovibronic states are contaminated by additional bands.
The analysis of these bands leads in most cases to an
identification of the coupled dark background state and the
responsible rotation–vibration coupling process (H42 resonances).
The emission spectra clearly demonstrate that even for a density of
states of 60 1/cm−1, coupling in S1 benzene is still selective and
far from the statistical limit. It is further demonstrated that the
dark and the light states are more efficiently mixed by short-range
couplings with coupling matrix elements of some GHz than by
long-range Fermi resonances. The Journal of Chemical Physics is
copyrighted by The American Institute of Physics.