Mechanism of the collision energy and reagent vibration’s effects on the collision time for the reaction Ca + HCl

The collision time which describes the speed of the collision process in a reaction is an important concept to an elementary chemical reaction. In this study, the quasiclassical trajectory method is applied to investigate the collision time of the reaction Ca + HCl (v = 0–2, j = 0) → CaCl + H. In order to provide a clear image of the reaction, the integral cross section we calculated is compared with corresponding quantum result and shows fairly good agreement. The results indicate that the collision energy and the initial vibrational level affect the average collision time remarkably. As the collision energy or the initial vibrational level increases, the average collision time decreases. The difference of average collision time for different initial vibrational level decreases with the increasing of collision energy. The product distributions as functions of scattering angle, attack angle and impact parameter are computed. Observing the functions, it can be found that the features could be caused by a competition among different parts of the product molecules with different collision time. For all the investigated initial vibrational levels, most of the reactive trajectories have the shorter collision times and are focused in several concentrated regions. Two possible mechanisms could be responsible for the HCl (v = 0) reaction in the concentrated regions. One is the sideway scattering and the system would fall into the deep potential well once in the collision process. The other is the weak forward scattering and strong backward scattering. The system would go around the deep potential well in the collision process. It is shown that the character of the weak forward scattering and strong backward scattering for the HCl (v = 1 and 2) reactions in the concentrated regions. However, the reactions outside the concentrated regions have the longer collision times and no particular mechanism. In the collision process, the system could fall into the deep potential well many times. We also explored the dynamics of the reaction at the same total energies but for different initial vibrational levels and found that the role of the insertion well becomes less and less important with the increasing of total energy.