David C. Easter

Structure and dynamics of intermediate benzene–argon clusters: (C6H6)Arn,n=13–40

Size-resolved benzene–argon clusters, (C6H6)Arn, n=13–40, were generated by supersonic expansion and studied by ultraviolet resonant two-photon ionization spectroscopy through benzene’s B2u←A1g601 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=13–40 range, each cluster spectrum is well described by the sum of between two (n=13) and five (n=17–24,26–29) Gaussian features. Only two spectral features (D,E) span the 28-cluster sequence, while each of the other four features appears over a limited consecutive cluster size range (A:n=14–39; B:n=17–29; C:n=14–24; F:n=26–40). 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 s...

EVAPORATION AND ISOMERIZATION DYNAMICS LEADING TO THE FREE-JET FORMATION OF ISOTOPICALLY LABELED (BENZENE)13 : A SPECTROSCOPIC OBSERVATION

Isotopically labeled (benzene)13 clusters, (C6H6)(C6D6)12, were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization (R2PI) spectroscopy through the C6H6B2u←A1g601 transition. Because of the spectrum’s simplicity, it serves as a sensitive monitor of the environment and dynamics of the C6H6 chromophore. We report experimental evidence for both evaporation and isomerization dynamics. Initially, the observed (C6H6)(C6D6)12 cluster population undergoes a transition from fluxional to rigid, resulting from the evaporation of a single C6D6 molecule from (C6H6)(C6D6)13. “Solidification” is followed by isomerization, in which the C6H6 moiety migrates from the surface of ordered, rigid clusters to their interior. The “freezing” temperature of (C6H6)(C6D6)12 is inferred to be near 137 K, in good agreement with theoretical simulations [Bartell and Dulles, J. Phys. Chem. 99, 17107 (1995)].

Toward the Experimental Structure Determination of Larger Molecular Clusters: Application and Limitations of the Weak Interaction Model to the (C6H6)13B2u ← A1g000 Spectrum

The experimental B2u ← A1g000 spectrum of (C6H6)13 was analyzed within the weak-interaction model using minimum energy structures calculated from six different potential energy surfaces. The coexistence of two isomers—both of C3 symmetry and with nearly equal populations—is supported. Structures predicted by two of the six potential energy surfaces are strongly favored. The transition dipole of benzene moieties within the cluster has a magnitude of ∼0.23 Debye. Weak transition dipole–dipole interactions fall between −1.95 and +2.24 cm−1 and site shifts of ligand molecules range from −160.3 to −153.8 cm−1. Although the weak-interaction transition dipole–dipole model falls short of unambiguously determining isomeric structures of benzene-13, it establishes a solid foundation on which modeling can be based for determining structures of larger, high-symmetry, molecular clusters.

Isomerization of isotopically-labeled benzene13 during free jet expansion

Abstract Isotopically-labeled benzene13 clusters, (C6H6)(C6D6)12, were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization (R2PI) spectroscopy through the C6H6 B2u←A1g(601) transition. Because of the spectrums simplicity, it serves as a sensitive monitor of the environment and dynamics of the C6H6 chromophore. We report for the first time evidence for isomerization within this cluster, where the C6H6 moiety migrates from the surface of a rigid cluster to the clusters interior.