The binding energies of p-difluorobenzene-Ar,-Kr measured by velocity map imaging: Limitations of dispersed fluorescence in determining binding energies

The technique of velocity map imaging has been used to determine the dissoc iation energies of the van der Waals complexes p-difluorobenzene-Ar and p-d ifluorobenzene-Kr. The values determined for the S-0, S-1, and D-0 states, respectively, are 337 +/-4, 367 +/-4, and 572 +/-6 cm(-1) for p-difluoroben zene-Ar and 398 +/-7, 445 +/-7, and 720 +/-6 cm(-1) for p-difluorobenzene-K r. An ionization potential of 73 549 +/-4 cm(-1) for p-difluorobenzene-Kr h as been determined by velocity map imaging of photoelectrons. The dissociat ion energies determined here are inconsistent with dispersed fluorescence s pectra of the complexes when these are assigned in the usual way. The issue is that spectra for levels below dissociation show bands where free p-difl uorobenzene emits, suggesting that dissociation is occurring from these lev els. For the dispersed fluorescence and velocity map imaging results to be consistent, these fluorescence bands must arise from transitions of the van der Waals complexes shifted such that they appear at the free p-difluorobe nzene wavelengths. It is proposed that these bands are due to emission from highly excited van der Waals modes populated by intramolecular vibrational redistribution from the initially excited level. From calculations perform ed for the related benzene-Ar system [B. Fernandez, H. Koch, and J. Makarew icz, J. Chem. Phys. 111, 5922 (1999)], the emitting levels are most likely above the barrier separating different p-difluorobenzene-partner configurat ions. The fluorescence observations are consistent with those of other tech niques if the p-difluorobenzene-partner interaction is the same in the grou nd and excited electronic states for such highly excited levels. Emission t hen occurs at the p-difluorobenzene monomer position since the energy shift is the same for the initial and final states. Deducing van der Waals bindi ng energies from the observation of spectral transitions at the free chromo phore position following excitation of the complex can be confounded by suc h an effect. The dispersed fluorescence spectra reveal that the rate of int ramolecular vibrational redistribution is reduced for the Kr complex compar ed with the Ar complex. (C) 2001 American Institute of Physics.