BrCl(+) elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields

By using a dc-slice imaging technique, photodissociation of 1,2-C(2)H(4)BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl(+) in an 80 fs laser field. The occurrence of fragment ion BrCl(+) in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl(+) in this experiment. The relative quantum yield of the BrCl(+) channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl(+) and its partner ion C(2)H(4)(+), we concluded that BrCl(+) came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C(2)H(4)BrCl(2+). With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl(+) detachment from 1,2-C(2)H(4)BrCl(2+) has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C(2)H(4)BrCl(2+) and the products BrCl(+) + C(2)H(4)(+). In this entire dissociation process, the C–Br and C–Cl bond lengths were observed to elongate asymmetrically, that is, the C–Br chemical bond broke firstly, and subsequently a new Br–Cl chemical bond started to emerge while the C–Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl(+) detachment.