Direct imaging of the Fermi resonance interaction in para-difluorobenzene: A special insight into energy redistributions in the S1 low-energy regime

We have experimentally observed the dynamics of the Fermi resonance interaction in the S-1 low-energy regime of para-difluorobenzene at +819 cm(-1) by femtosecond time-resolved photoelectron imaging technique. The Fermi resonance interaction is clearly demonstrated by the time dependence of the photoelectron spectra; furthermore, the corresponding photoelectron angular distributions (PADs) clearly reflect each Fermi component character of the resonance 5(1) . . . 6(2). The oscillation period for the Fermi resonance is estimated to be less than 20 ps from the time behavior of the photoelectron spectra, which is very close to our calculated oscillation period value of 13.3 ps. Our results demonstrate the vibrational energy transfer between the two Fermi resonance components of 5(1) and 6(2), suggestive of a special insight into the vibrational energy redistribution in the S-1 low-energy regime of para-difluorobenzene.