A structural study on the excimer state of an isolated benzene dimer using infrared spectroscopy in the skeletal vibration region.

We applied infrared (IR) spectroscopy on electronic excited states of a benzene dimer (Bz2) isolated in a supersonic expansion to investigate the vibrational structure and geometry of the excimer (EXC) state where the electronic excitation is equally shared between the two Bz units. The IR spectrum of an EXC produced via the electronic origin of Bz2 gives a simpler spectral appearance than that in the electronic ground state, in which it has a T-shaped structure. Each band position locates nearly at the average of the corresponding vibrations in the electronic ground and excited states of the Bz monomer. This frequency averaging is explained by an excitation exchange model that takes into account vibrational excitations. From the observed frequency averaging, a highly symmetric parallel stacking structure in the EXC state is concluded with the help of a DFT calculation. This model clarifies that Franck-Condon factors between the S1-S0 transition of the monomer govern not only the magnitude of the EXC interaction, but also the configuration of vibrational states. The IR spectrum of the vibrationally excited EXC state produced by excitation to the 61 level of the stem site, on the other hand, shares the IR features both of the EXC state and the local excited (LE) state in which the excitation localizes on one of the benzene rings, making a T-shape contact. The structural interconversion equilibrium between parallel stacking (EXC) and T-shaped (LE) structures due to the vibrational excess energy has been established.