Direct ab initio molecular dynamics (MD) study of the ionization on a benzene dimer

The ionization dynamics of a benzene dimer have been investigated by means of a direct ab initio molecular dynamics (MD) method in order to elucidate the reaction mechanism. Following the ionization, the T-shaped neutral benzene dimer was gradually changed to a p-stacked benzene dimer cation. The structural change and time evolution of the electronic absorption spectrum were completely visualized for the first time. The time scale of the dimer formation was estimated to be 1.0-1.5 ps. First, the benzene molecule at the stem position (Bz(')) was ionized, and the structure of Bz'(+) was rapidly deformed due to the Jahn-Teller effects. Next, the rotation of (Bz')(+) gradually occurred relative to Bz. Finally, a pi-stacked benzene dimer cation was formed. TD-DFT calculations indicated that the absorption spectrum of (Bz)(2)(+) is blue-shifted as a function of time. The formation mechanism of the benzene dimer cation was discussed on the basis of the theoretical results.