Direct observation of IVR under white light excitation: Fluorescence spectra of p-difluorobenzene by controlled electron impact

Fluorescence spectra of p-difluorobenzene cooled in supersonic expansions have been measured following electron impact. The spectra consist of structured emission from optically active (from the S₀ zero point level) states and unstructured emission from optically inactive states (bath modes) populated by IVR, similarly to those in earlier laser experiments. The fraction of the unstructured emission to the total emission has been determined for three optically active states. The results demonstrate that above 1600 cm^-1 of excess energy from the S₁ origin, the IVR rate is at least one order of magnitude faster than that from the ns laser experiments.