Ionization dynamics of the benzene-HF complex: A direct ab initio dynamics study

Direct ab initio dynamics calculations have been carried out for the ionization processes of the benzene-HF (Bz-HF) complex in order to elucidate its mechanism and dominant factor on the reaction channels. The full dimensional potential energy surface (PES) including all degrees of freedom and its gradient were calculated at each time step in the dynamics calculation. The dynamics calculations of the neutral complex were carried out at constant temperatures 10 and 50 K in order to obtain the relevant structures at finite temperatures. It was found that the angle between a dipole moment of HF and the benzene C-6 axis (theta) fluctuates largely due to the thermal energy. The Bz-HF complex has a nonrigid structure and a wide Franck-Condon region for the ionization. Using the geometrical configurations selected randomly from the neutral structures of Bz-HF, the trajectories for Bz(+)-HF system following the vertical ionization were calculated. Two reaction channels were obtained as products: one is the complex formation channel in which the complex composed of Bz(+)-FH is formed, while the other one is dissociation channel in which the trajectory directly leads to the dissociation product Bz(+) + HF. In the latter,channel, the rotational quantum number of the dissociating HF molecule is calculated to be J = 1-3. The ionization from C-6v structure of neutral Bz-HF complex, calculated for comparison directly leads to the dissociation products Bz+ + HF without the rotational excitation of the HF molecule. The calculations indicated that the ionization from the geometrical configuration with a large angle theta leads to the complex formation Bz(+)-FH, whereas the trajectory with a small angle theta leads to the dissociation products (Bz(+) + HF). The present calculations indicated that the angle theta dominants strongly the preference of the reaction channels in the photoionization of Bz-HF complex. The mechanism was discussed on the basis of the theoretical results.