Existence of two internal energy distributions in jet-formed van der Waals heteroclusters: example of the anthracene–argonn system

Abstract

In this paper we report on the structure of van der Waals anthracene–argonn (n=1–6) clusters, which were interrogated by a complete set of experimental diagnoses based upon mass-selective, fragmentation-free, two-color, resonant two-photon ionization and laser induced fluorescence as well as a theoretical modeling of the ionization energy. We were able to provide a global assignment of the spectral features which rests on several independent sources of information: the spectral shifts δν and linewidths of the S1←S0 transition, the ionization energy shifts δI.E. and threshold widths and their comparison with the theoretical simulations, the hole-burning spectra, the structure-specific predissociation dynamics. The existence of two cluster ensembles, which present qualitatively different spectral features and significant relative intensity, has been evidenced for each cluster size. The first ensemble exhibits spectral features, typical of cold, rigid-like clusters. Only one prominent stable structure is found for each cluster size of this ensemble: (1|0) for n=1, (2|0) for n=2, (2|1) for n=3, (2|2) for n=4, (3|2) for n=5, (3|3) for n=6, where the notation (n-p|p) refers to the distribution of the argon atoms between the two molecular sides. The second ensemble has been assigned to more or less `hot' clusters having probably the same (n-p|p) arrangement as the first ones, but excited in high van der Waals vibrational levels through an intracluster vibrational relaxation of anthracene molecules. Results on similar heteroclusters reported in the literature are discussed on the light of our new assignment of the spectral features of small Ant–Arn clusters to structures differing by their internal energy content and not by their configuration.