Dynamics of excimer formation and relaxation in the T-shaped benzene dimer

The excited-state dynamics of the T-shaped van der Waals (vdW) dimer of ben zene has been studied by probing fluorescence after S-1 excitation. Excimer fluorescence is observed when the dimer is excited into the S-1 origin and the v(6) vibronic level. The results demonstrate that the excimer formatio n proceeds by tunneling through an energy barrier between vdW and excimer p otential wells. Use of a pump-probe fluorescence depletion technique showed that the excimer absorbs strongly at 500 nm, consistent with the result in solution. Evidence is observed for an equilibrium between the vdW dimer an d excimer states in the vs excitation. These results are explained by invok ing an excimer of a parallel stacked geometry, having a charge-transfer cha racter. The excimer formation dynamics of the benzene dimer is discussed ba sed on comparison with those of other aromatic dimers and cluster systems.