关键词: 共振增强多光子电离, 飞行时间质谱, 光解动力学, 分支比

The photodissociation dynamics of CH₂Br₂, C₂H₄Br₂, C₃H₆Br₂ and C₄H₈Br₂ 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_3/2⁰) and spin-orbit state Br(²P_1/2⁰).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 ³Q₀ and ¹Q₁ states is responsible for the reduction in the symmetry of the molecule in the photodissociation process.To C₄H₈Br₂, 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.

Key words: resonance-enhanced multiphoton ionization, time-of-flight mass spectrometry, photodissociation dynamics, branching ratio