二溴代烷烃的紫外光解动力学研究

利用飞行时间质谱装置研究了234和267nm激光作用下二溴甲烷、二溴乙烷、二溴丙烷和二溴丁烷分子的光解离过程．研究表明二溴代烷烃分子在紫外激光的作用下主要是断裂C—Br键解离出一个Br原子，并且存在两种可能的布居：基态Br(^2P3/2^0)和激发态Br^*(^2P1／2^0)．通过共振增强多光子电离技术探测两种光解产物布居的分支比．对比得到了分子构型对称性不同的二溴代烷烃的分支比，提出了两种假设的光解离模型．

Ultraviolet Photodissociation Dynamics of Dibromoalkane

The photodissociation dynamics of CH 2Br 2, C 2H 4Br 2, C 3H 6Br 2 and C 4H 8Br 2 near 234 and 267 nm have been studied using time-of-flight mass spectrometry. It was found that photoexcitation of dibromoalkane in UV absorption band results in prompt fission of the C-Br bond, producing two distinguishable photodissociation products: ground state Br( 2P 0 3/2) and spin-orbit state Br( 2P 0 1/2). To achieve state-selective detection of Br and Br *, a (2+1) resonance-enhanced multiphoton ionization (REMPI) scheme was employed. The branching ratio of the photodissociation products was extracted from the relative signal intensities of Br * and Br in the REMPI TOF spectra. The findings of this work reveal that a nonadiabatic curve-crossing between the 3Q 0 and 1Q 1 states is responsible for the reduction in the symmetry of the molecule in the photodissociation process. To C 4H 8Br 2, the further reduction in the molecular symmetry induces level-splitting to more electronic states, then the coupling between these states causes an avoided crossing, which can change the photodissociation paths accordingly. The wavelength dependence of the branching ratio for both Br and Br * channels provides insight into the nature of the electronic excited states which contribute to the absorption spectrum and the role of nonadiabatic curve crossing in the photodissociation process.