Br2 molecular elimination in photolysis of (COBr)2 at 248 nm by using cavity ring-down absorption spectroscopy: a photodissociation channel being ignored.

A primary dissociation channel of Br(2) elimination is detected following a single-photon absorption of (COBr)(2) at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br(2) fragment in the B(3)Π(ou)(+)-X(1)Σ(g)(+) transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br(2) contribution from a secondary reaction. By means of spectral simulation, the ratio of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 ± 0.09):(0.34 ± 0.07), corresponding to a Boltzmann vibrational temperature of 893 ± 31 K. The quantum yield of the ground state Br(2) elimination reaction is determined to be 0.11 ± 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr)(2) via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br(2) + 2CO. The resulting Br(2) is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism.