利用速度成像技术研究碘乙烷多光子电离解离动力学

卤代烷烃会破坏臭氧层，而碘乙烷（C₂H₅I）是卤代烷烃中重要代表物质之一.采用离子速度成像技术、飞秒激光技术和飞行时间质谱技术，探究了C₂H₅I的多光子电离解离动力学.通过分析C₂H₅I在强场作用下多光子电离解离得到的解离通道、碎片的动能、角度分布和各向异性参数等信息来研究碘乙烷离子（C₂H₅I⁺）C–I键裂解机理.根据飞行时间质谱实验，C₂H₅I在飞秒激光脉冲作用下发生多光子电离解离得到的碎片有C₂H₅⁺，I⁺，CH₂I⁺，C₂H₂⁺，C₂H₃⁺，C₂H₄⁺等.与C–I键相关的碎片为C₂H₅⁺和I⁺，解离机制分别对应于C₂H₅I⁺→C₂H₅⁺+I和C₂H₅I⁺→C₂H₅+I⁺.同时，采用离子速度成像技术研究C₂H₅I⁺的C–I键裂解产生的C₂H₅⁺和I⁺的速度影像，得出两者的速度分布和动能分布，分析结果表明C–I键裂解产生C₂H₅⁺和I⁺的过程都存在高能通道和低能通道.进一步分析解离碎片离子的角度分布发现C₂H₅⁺解离时各向异性参数接近于0，可能对应于慢速的振动预解离过程.I⁺在解离时各向异性参数较高，可能源于排斥势能面上的快速解离过程.最后采用密度泛函理论计算了C₂H₅I分子电离前后构型变化、离子态的能级强度及谐振强度，对C₂H₅I⁺的解离机制做了更进一步的分析和讨论.

Multiphoton ionization dissociation dynamics of iodoethane studied with velocity map imaging technique

Halogenated alkanes destroy the ozone layer, and iodoethane is one of the important representative halogenated alkanes. Time-of-flight mass spectrometry and velocity map imaging technique are used for investigating the photoionization dissociation dynamics of iodoethane, induced by 800 nm femtosecond laser. The dissociation mechanisms of iodoethane are obtained and discussed by analyzing the velocity distributions and angular distributions of the fragment ions generated in the dissociation. The measurements by time-of-flight mass spectrometry show that iodoethane cations generates C₂H₅⁺, I⁺, CH₂I⁺, C₂H₂⁺, C₂H₃⁺ and C₂H₄⁺. The fragments related to C–I bond fragmentation are C₂H₅⁺ ions and I⁺ ions, and the dissociation mechanisms are C₂H₅I⁺→ C₂H₅⁺+I and C₂H₅I⁺→ C₂H₅+I⁺ respectively. Comparison between the configurations before and after ionization shows that the C–I bond length is 0.2220 nm before ionization and turns longer and becomes 0.2329 nm after ionization. This indicates that the C–I bond becomes more unstable after ionization and is more prone to dissociation. Moreover, the velocity map images of C₂H₅⁺ and I⁺ ions are acquired, from which the speed and angular distribution of C₂H₅⁺ and I⁺ are obtained. The analysis of speed distribution of the fragment ions shows that there are two channels, i.e. high energy channel and low energy channel in the dissociation process for producing C₂H₅⁺ and I⁺ ion. The difference between the ratios of the high energy channel and the low energy channel is small, indicating that the high energy channel and the low energy channel of the two dissociation processes are similar. According to the further analysis of the angular distribution of the fragment ions, it is found that the anisotropy parameter of C₂H₅⁺ is close to 0 (isotropic), the production channel of which may correspond to the slow vibration predissociation process. The anisotropy parameters of I⁺ ions are higher, which may be due to the rapid dissociation process on the repulsive potential energy surface. In addition, the density functional theory is used to calculate the configuration change of the iodoethane molecule before and after ionization, the energy level and oscillator strength for the ionic state in order to obtain more insights into the photodissociation dynamics.