Ionization dynamics of aminopyridine dimer: a direct ab initio molecular dynamics (MD) study.

The ionization dynamics of an aminopyridine dimer (AP)(2) has been investigated by means of the direct ab initio molecular dynamics (MD) method. It was found that the reaction process was composed of three steps after the vertical ionization of (AP)(2): dimer approach, proton transfer and energy relaxation. The timescales of these processes were 50-100, 10-20, and 200 fs, respectively. The timescale of the dimer approach was dependent on the initial separation between AP(+) and AP. After the ionization, AP approached gradually the ionized AP(+). The proton of AP(+) was transferred to AP at the nearest intermolecular distance, while the potential energy was quickly dropped according to the proton transfer. The energy relaxation of the dimer cation was significantly faster than that of the monomer cation. The mechanism of ionization dynamics of (AP)(2) was discussed on the basis of the theoretical results.