Abstract
The excited-state dynamics of small naphthalene clusters that occur upon selective excitation into S1 vibronic levels have been investigated by mass-selective pump-probe photoionization spectroscopy. The results provide direct evidence for structural isomerization of the initially excited van der Waals cluster into an excimer geometry, consistent with the fluorescence results. The excimer has been identified as a cluster having a strongly bound dimer core surrounded by ground-state molecules, thus ruling out the possibility of excimer formation via monomer evaporation. We have also found that ionization is enhanced when the initially excited cluster undergoes excimer formation, which provides a new ionization mechanism for aromatic clusters. The ionization enhancement is explained by the Franck–Condon factors associated with the ionizing transition from the excimer state.