电子诱导分子非顺序解离与顺序解离动力学研究

The fragmentation of highly charged molecular ion induced by particle collision has been always classified to Coulomb explosion in which the bonds of the molecule broken simultaneously and the molecule dissociates due to Coulomb repulsion. However, for some molecular ions, recent studies indicate that the non-sequential and sequential fragmentation both contribute to their dissociation. The Coulomb explosion model is no longer valid in the case of sequential fragmentation. It is not yet fully understanding the facts (collision energy, collision time scale, etc.) which control the non-sequential and sequential fragmentation due to the complexity of the dissociation of polyatomic molecules. At the same time, the studies of non-sequential and sequential fragmentation dynamics induced by electron have not been carried out due to the low cross section of multiple ionization. In this project, we use electron and triatomic molecules collision to produce highly charged molecular ions. The final state ions are detected in coincidence and the momentum of the three ions are determined by ion imaging method. By analyzing the momentum correlation among the ions, the non-sequential and sequential fragmentation channels are distinguished. And also the angular distributions and kinetic energy releases (KERs) are determined. Then, we will get the information of geometries and potential energy surfaces of highly charged molecular ions. By changing the electron energy, the dependences between the branching ratio of non-sequential and sequential fragmentation channels and the collision energy will be studied in detail.

对于粒子碰撞得到的高电荷态分子离子，一般认为其解离过程为库仑爆炸过程，即分子的多个共价键同时断裂，分子沿着库仑势解离。然而近年的研究结果表明，对于某些分子离子，其解离过程有非顺序、顺序两种机制。对于顺序解离的情况，库仑爆炸模型不再成立。由于多原子分子电离解离过程的复杂性，人们对于控制非顺序与顺序解离的主导因素（碰撞能量、相互作用时间等）还没有认识清楚。同时，由于电子碰撞多重电离的截面较低，电子碰撞的相关工作还未见报道。本项目拟利用电子与三原子分子碰撞，产生高电荷态的分子离子，利用三重符合技术对末态三个离子进行符合测量，利用离子成像技术对三个离子的动量进行测量，通过它们之间的动量关联区分非顺序与顺序解离通道，同时得到离子的相对角分布和能量信息，进而获得高电荷态分子离子的几何构型和势能曲面等基本信息。通过改变电子能量，深入探索非顺序解离与顺序解离通道分支比对碰撞能量的依赖关系。

对于高电荷态分子离子，通常认为其解离过程为库仑爆炸，即分子的多个共价键同时断裂，分子沿着库仑势解离。然而由于分子离子可以处于不同的电子态，其解离的势能面十分复杂，导致其解离机制也极其复杂。本项目针对分子解离过程中的顺序解离及非顺序解离机制展开研究，目的在于探讨控制顺序解离、非顺序解离的控制条件，对分子化学键的断裂有更深入的了解。本项目利用电子与多原子分子碰撞，产生高电荷态的分子离子，利用符合测量技术对末态产生的所有离子进行符合测量。同时，利用离子成像技术对离子的三维动量进行成像测量。数据分析中，利用末态碎片之间的动量关联区分非顺序与顺序解离通道。实验上，我们详细研究了CO2，OCS，CS2等分子的三体解离过程，通过动量关联观察到了顺序解离及非顺序解离过程，通过对比不同解离机制对应的离子能量分布，我们发现，所有的顺序解离都会导致较低的离子能量。这说明顺序解离主要发生在分子离子的电子基态及较低的激发态。本项目在理论计算上主要取得了两方面的进展，首先采用高精度的量化计算，对CO2三重电离分子离子的势能面进行了计算，确定了顺序解离的电子态信息；其次采用分子动力学模拟方法，对CO2顺序解离进行了细致的研究。本项目还针对NF3，C4H4O，CF4，CH3CH2OH等复杂分子进行了实验研究。