Photoionization of Excimers Produced in Naphthalene Clusters

The excited-state dynamics of small naphthalene clusters that occur upon selective excitation into S 1 vibronic levels have been investigated by mass-selective pump-probe photoionization spectroscopy. Theresults provide direct evidence for structural isomerization of the initially excited van der Waals clusterinto an excimer geometry, consistent with the fluorescence results. The excimer has been identified asa cluster having a strongly bound dimer core surrounded by ground-state molecules, thus ruling out thepossibility of excimer formation via monomer evaporation. We have also found that ionization is enhancedwhen the initially excited cluster undergoes excimer formation, which provides a new ionizationmechanism for aromatic clusters. The ionization enhancement is explained by the Franck–Condon factorsassociated with the ionizing transition from the excimer state.