Electronic spectra of jet-cooled anthracene dimer: Evidence of two isomers in the electronic ground state

The broad and sharp absorption systems of anthracene in the 362-376 nm region have been confirmed to be due to dimeric species by measuring the resonantly enhanced two-photon ionization mass spectrum. The hole-burning spectrum indicated that two stable isomers exist in the S₀ state. The decay time of the excimer emission (300 ns) from the broad system is much longer than that of the emission from the sharp system (15 ± 5 ns), indicating that the electronic nature of the excited state is very different between the two systems. The difference in the excited-state potentials of the two systems has been discussed on the basis of the vibronic structures and the emission lifetimes.