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 So 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.