C-Br Bond Dissociation Mechanisms of 2-Bromothiophene and 3-Bromothiophene at 267 nm

C-Br bond dissociation mechanisms of 2-bromothiophene and 3-bromothiophene at 267 nm were investigated using ion velocity imaging technique. Translational energy distributions and angular distributions of the photoproducts, Br(2P3/2) and Br*(2P½), were obtained and the possible dissociation channels were analyzed. For these two bromothiophenes, the Br fragments were produced via three channels: (i) the fast predissociation following the intersystem crossing from the excited singlet state to repulsive triplet state; (ii) the hot dissociation on highly vibrational ground state following the internal conversion of the excited singlet state; and (iii) the dissociation following the multiphoton ionization of the parent molecules. Similar channels are involved for photoproduct Br* of the 2-bromothiophene dissociation at 267 nm; whereas for the photoproduct Br* of 3-bromothiophene, the dissociation channel via internal conversion from the excited singlet state to highly vibrational ground state became dominating and the fast predissociation channel via the excited triplet state almost disappeared. Informations about the relative contribution, energy disposal, and the anisotropy of each channel were quantitatively given. It was found that with the position of Br atom in thienyl being far from S atom, the relative ratios of products from channels (i) and (ii) decreased obviously and the anisotropies corresponding to each channel became weaker.