Real time observation of the excimer formation dynamics of a gas phase benzene dimer by picosecond pump–probe spectroscopy

Abstract

We observed the real-time excimer (EXC) formation dynamics of a gas phase benzene dimer (Bz₂) cluster after photo-excitation to the S₁ state by applying an ionization detected picosecond transient absorption method for probing the visible EXC absorption for the first time. The time evolution of the EXC absorption from the S₁ 0⁰ level shows a rise that is well fitted by a single exponential function with a time constant of 18 ± 2 ps. The structure of the Bz dimer has a T-shaped structure in the ground electronic state, and that in the EXC state is a parallel sandwich (SW) structure. Thus, the observed rise time corresponds to the structural change from the T to the SW structures, which directly shows the EXC formation. On the other hand, the EXC formation after excitation of the S₁ 6¹ vibrational level of the stem site showed a faster rise of the time constant of 10 ± 2 ps. Supposing equilibrium between the EXC and the local excited states, it followed that the intramolecular vibrational energy redistribution rate of the 6¹ level is largely enhanced and becomes faster than the EXC formation reaction.