DUAL FLUORESCENCE FROM THE PHOTOEXCITED VANDERWAALS DIMER OF DIBENZOFURAN IN A SUPERSONIC JET - INTERMEDIATE-CASE LEVEL STRUCTURE OF THE COUPLING BETWEEN THE DIMER EXCITED-STATE AND THE LOWER-LYING EXCIMER STATE

We show here that electronic excitation of the ground-state van der Waals dimer of dibenzofuran to its lowest excited singlet state leads to the appearance of dual fluorescence, which is a superposition of an emission from the excited state and an emission from an excimer state. Comparison of the excitation spectra of the two emissions indicates that the coupling between the excited state of the dimer and the lower-lying excimer state belongs to the strong coupling, intermediate-case level structure for electronic relaxation. We also report a direct optical transition from the ground state of a van der Waals dimer to an excimer state, which has not been previously observed for homodimers.