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

Direct ab initio dynamics calculations have been carried out for the ioniza tion processes of the benzene-HF (Bz-HF) complex in order to elucidate its mechanism and dominant factor on the reaction channels. The full dimensiona l potential energy surface (PES) including all degrees of freedom and its g radient were calculated at each time step in the dynamics calculation. The dynamics calculations of the neutral complex were carried out at constant t emperatures 10 and 50 K in order to obtain the relevant structures at finit e temperatures. It was found that the angle between a dipole moment of HF a nd the benzene C-6 axis (theta) fluctuates largely due to the thermal energ y. The Bz-HF complex has a nonrigid structure and a wide Franck-Condon regi on for the ionization. Using the geometrical configurations selected random ly from the neutral structures of Bz-HF, the trajectories for Bz(+)-HF syst em following the vertical ionization were calculated. Two reaction channels were obtained as products: one is the complex formation channel in which t he complex composed of Bz(+)-FH is formed, while the other one is dissociat ion channel in which the trajectory directly leads to the dissociation prod uct 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 excitat ion of the HF molecule. The calculations indicated that the ionization from the geometrical configuration with a large angle theta leads to the comple x formation Bz(+)-FH, whereas the trajectory with a small angle theta leads to the dissociation products (Bz(+) + HF). The present calculations indica ted that the angle theta dominants strongly the preference of the reaction channels in the photoionization of Bz-HF complex. The mechanism was discuss ed on the basis of the theoretical results.