ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The technique of velocity map imaging has been used to determine the dissociation energies of the van der Waals complexes p-difluorobenzene–Ar and p-difluorobenzene–Kr. The values determined for the S0, S1, and D0 states, respectively, are 337±4, 367±4, and 572±6 cm−1 for p-difluorobenzene–Ar and 398±7, 445±7, and 720±6 cm−1 for p-difluorobenzene–Kr. An ionization potential of 73 549±4 cm−1 for p-difluorobenzene–Kr has been determined by velocity map imaging of photoelectrons. The dissociation energies determined here are inconsistent with dispersed fluorescence spectra 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-difluorobenzene emits, suggesting that dissociation is occurring from these levels. 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-difluorobenzene 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 performed for the related benzene–Ar system [B. Fernandez, H. Koch, and J. Makarewicz, J. Chem. Phys. 111, 5922 (1999)], the emitting levels are most likely above the barrier separating different p-difluorobenzene–partner configurations. The fluorescence observations are consistent with those of other techniques if the p-difluorobenzene–partner interaction is the same in the ground and excited electronic states for such highly excited levels. Emission then occurs at the p-difluorobenzene monomer position since the energy shift is the same for the initial and final states. Deducing van der Waals binding energies from the observation of spectral transitions at the free chromophore position following excitation of the complex can be confounded by such an effect. The dispersed fluorescence spectra reveal that the rate of intramolecular vibrational redistribution is reduced for the Kr complex compared with the Ar complex. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1405440
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