Photofragmentation of mass-selected (C6H6) + n clusters: Measurement of monomer-cluster binding energy for n=7-15

Benzene cluster cations up to 15 molecular units in size, produced in the throat of a supersonic nozzle by laser-induced plasma formation, are studied by mass-selected photofragmentation spectroscopy. All of the clusters exhibit a strong, broad absorption which extends at least from 690 to 1064 nm, centered near 900 nm. This absorption is assigned to the dimer based intervalence transition observed in solid state spectra of the dimer ions. Excitation into this band produces prompt ejection of neutral benzene molecules from the cluster ion. At low fragmentation laser intensities, a single daughter ion species is observed. We observe a simple dependence of the number of neutral molecules ejected on the fragmentation laser wavelength, strongly suggesting a sequential evaporation process of neutral benzene molecules. For clusters larger than six, an average bond energy ranges from 0.37-0.34 eV/molecule, quite close to the heat of vaporization for liquid benzene. The 14 molecule cluster is found to have an anomalously high binding energy, providing strong evidence that it forms a stable icosahedral structure. The general trend of the monomer bond energy with cluster size is well accounted for by a simple model based on macroscopic constructs and liquid benzene dielectric constant, surface tension, and heat of vaporization values.