Abstract
Well-resolved spectra of benzene–acetonitrile binary clusters BAn, with n=1–4 have been obtained by the (one-color) resonant two-photon ionization technique using the benzene’s B2u←A1g 000 and 601 resonances. The spectra reveal a rapid increase in complexity with the number of acetonitrile molecules in the cluster, associated with van der Waal modes and isomeric forms. While only single cluster origins are found for the benzene–acetonitrile (BA) and the BA2 clusters, two and four distinct isomers are identified for the BA3 and BA4 clusters, respectively. The origins of the BA and BA2 clusters are blueshifted with respect to the free benzene molecule by 38 cm−1 and 26 cm−1, respectively. Monte Carlo (MC) simulations reveal two types of isomeric structures of the BAn clusters. The clusters containing an even number of the acetonitrile molecules (BA2, BA4, and BA6) are dominated by acetonitrile anti-parallel paired dimers. The BA3 cluster consists of a cyclic acetonitrile trimer parallel to the benzene ring. In the BA5 clusters, the acetonitrile molecules are assembled in a cyclic trimer + a paired dimer configuration or in two paired dimers + a single monomer structure. The R2PI spectra, in conjunction with the MC structural models and simple energetic arguments, provide a reasonably compelling picture of the spectroscopic and dynamical phenomena associated with dipole pairing molecular cluster systems.