中文摘要：

采用密度泛函和耦合簇理论从过渡态的观点对CH3I。十离子的解离过程进行了计算和分析．优化得到了CH31，CH3I^＋和CH3I^2＋的稳定结构及解离过渡态的几何构型并给出了相应能量，计算的第一、二电离能与实验结果符合．基于母体离子、过渡态和解离碎片的几何结构和能量，对CH3I^2＋的两体解离过程和三体解离过程进行了详细分析和讨论．给出了二体、三体解离的可能解离通道，并分析了实验上观测到的解离碎片离子的产生机制．计算结果表明，三重和单重绝热势能面上的一些解离过程表现出明显的差异．

英文摘要：

As one of the simplest alkyl halides, methyl iodide is extensively investigated in the research fields of the photodissociation and photoionization. In the experimental investigations of ionization and dissociation, many molecular fragments, such as Iq＋（q ≤ 3）, CH＋（n ≤ 3）, H＋, etc., are observed in the mass spectrum of CH3I. While the mechanisms for dissociation and ionization are not completely understood. As the doubly-ionized product, CH3I2＋ exhibits different isomer structures and isomerization reactions. The dissociation channels of different isomers in combination with the corresponding transition states of CH3I2＋ are helpful for better understanding the dissociation and ionization dynamics of CH3I in an intense laser field. In our present work, the dissociation channels of CH3I2＋ are investigated by the density functional and couple clus- ter theory. The geometries and energies corresponding to the local isomers and the transition states of CH3I, CHaI＋ and CH3I2＋ are computed. The first and second ionization energies we measured are in good agreement with experimental values. Our computational results show that the ground state of the CH3I2＋ is a triplet one with 3A2 symmetry. Totally 11 two-body and 15 three-body dissociation channels of the CH^I2＋ on both the lowest singlet and the lowest triplet potential energy surfaces are computed and analyzed in detail. Our computations indicate that seven two-body dissocia- tions channels, i.e., six singlet and one triplet ones, are exergonic, in which CH3I2＋ （1A＇） →CH＋＋HI＋ （4A1） is the easiest process to achieve; four exergonic three-body dissociation channels with three on singlet potential energy surface and one on triplet potential energy surface are found. The possible mechanisms for producing the dissociative ionized fragments observed in experiments, CH＋, H＋, and I＋, are presented; furthermore, the dissociation channels generating other ions not observed in experiments, such as H3＋ et al, are also