Fragmentation energetics and dynamics of fluorobenzeneAr n (n=1-3) clusters studied by mass analyzed threshold ionization spectroscopy New assignments in the UV spectroscopy of the small benzene-argon n clusters: The effects of a structureselective vibrational predissociation Size-resolved benzene-argon clusters, ͑C 6 H 6 ͒Ar n , nϭ13-40, were generated by supersonic expansion and studied by ultraviolet resonant two-photon ionization spectroscopy through benzene's B 2u ←A 1g 6 0 1 transition. The size-specific sequence of cluster spectra reveals six features that can be isolated, allowing for an independent analysis of each feature's evolution. In the n ϭ13-40 range, each cluster spectrum is well described by the sum of between two (nϭ13) and five (nϭ17-24,26-29) Gaussian features. Only two spectral features ͑D,E͒ span the 28-cluster sequence, while each of the other four features appears over a limited consecutive cluster size range ͑A:nϭ14-39; B:nϭ17-29; C:nϭ14-24; F:nϭ26-40͒. The evolution of the spectral shift, width, and relative amplitude is traced for each feature. The observations are rationalized through a model that has been previously developed for the discussion of benzene-argon cluster data. We report for the first time a ''high shift'' spectral feature, observed at a spectral shift of Ϫ80 cm Ϫ1 , which approaches the bulk shift more closely than anything previously reported.

Isotopically labeled (benzene)13 clusters, (C6H6)(C6D6)12, generated in supersonic expansion, were studied by resonance-enhanced two-photon ionization (R2PI) spectroscopy as a function of nozzle-to-laser distance using two distinct sets of expansion conditions. In this report we present spectroscopic evidence for both evaporation and isomerization within the supersonic jet. The observed (C6H6)(C6D6)12 population initially undergoes a fluxional-to-rigid transition; this transition is followed by isomerization within a subpopulation of the ordered, rigid clusters, in which the unique C6H6 moiety migrates from the surface to the interior of an otherwise homogeneous C6D6 cluster. These experiments are unique in that analogous isomerization dynamics have never before been spectroscopically observed. The observed kinetics are generally insensitive to differences between the benzene:helium ratio in the expansion mixture.

Isotopically labeled (benzene) 13 clusters, (C 6 H 6 ͒͑C 6 D 6 ) 12 , were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization ͑R2PI͒ spectroscopy through the C 6 H 6 B 2u ←A 1g 6 0 1 transition. Because of the spectrum's simplicity, it serves as a sensitive monitor of the environment and dynamics of the C 6 H 6 chromophore. We report experimental evidence for both evaporation and isomerization dynamics. Initially, the observed (C 6 H 6 ͒͑C 6 D 6 ) 12 cluster population undergoes a transition from fluxional to rigid, resulting from the evaporation of a single C 6 D 6 molecule from (C 6 H 6 ͒͑C 6 D 6 ) 13 . ''Solidification'' is followed by isomerization, in which the C 6 H 6 moiety migrates from the surface of ordered, rigid clusters to their interior. The ''freezing'' temperature of (C 6 H 6 ͒͑C 6 D 6 ) 12 is inferred to be near 137 K, in good agreement with theoretical simulations ͓Bartell and Dulles, J. Phys. Chem. 99, 17107 ͑1995͔͒.