Evaporation and isomerization dynamics leading to the free-jet formation of isotopically labeled (benzene)13: A spectroscopic observation

Abstract

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.

Stacked Clusters of Polycyclic Aromatic Hydrocarbon Molecules

“…In larger assemblies, isotropic interactions favor an icosahedral growth scheme over stacking. In the structure displayed in Figure b, the molecular centers of mass indeed form a distorted icosahedron, similar to what has been found in (benzene) 13 by several authors. ,,, Such structures are significantly easier to locate than stacked isomers, because the orientational degrees of freedom can be neglected in a first approximation. Here, the third trick suggested in the previous subsection for improving the basin-hopping algorithm should be efficient.…”

“…Generally speaking, very little is known about the structure of PAH clusters. In comparison, assemblies of benzene molecules have received much more attention. − At empirical levels of theory, van de Waal 29,30 and the groups of Stace, Bartell, and Whetten and Easter 37-39 have investigated assemblies containing up to 13 benzene molecules. These studies showed a marked preference for the Wefelmeier growth scheme also observed in argon clusters .…”

Structural Features of Small Benzene Clusters (C6H6)n (n ≤ 30) As Investigated with the All-Atom OPLS Potential

“…The benzene trimer and tetramer were examined by mass-selective resonantly enhanced two-photon ionization (R2PI) and ultraviolet–ultraviolet hole burning spectroscopy. , The experimental data are consistent with a cyclic trimer structure with C 3 symmetry and a distorted tetrahedral structure with S 4 symmetry . For benzene clusters with n = 7–19, Easter and co-workers − studied these structures formed in free jet expansions by R2PI spectroscopy. The numbers of configurations of the clusters with 13–19 molecules were estimated to be one to three referring to the transitions observed in the spectra of isotopically substituted benzene clusters .…”

Low‐energy structures of benzene clusters with a novel accurate potential surface

“…It became apparent from these data that the structures of the benzene trimer and tetramer are compatible with a C 3 and S 4 symmetry, respectively. In addition, Easter et al have employed R2PI spectroscopy to study benzene clusters with up to 19 molecules. For n = 13, the spectrum shows no spectral evidence of the absorption due to the central molecule, which may indicate the presence of a high symmetric structure, since such transition is forbidden for C 3 or, even, higher symmetry.…”