title: Br2 molecular elimination in photolysis of (COBr)2 at 248 nm by using cavity ring-down absorption spectroscopy: a photodissociation channel being ignored.
abstract: 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.
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei, Taiwan.
authors.forename: Chia-Ching
authors.initials: CC
authors.lastname: Wu
authors.forename: Hsiang-Chin
authors.initials: HC
authors.lastname: Lin
authors.forename: Yuan-Bin
authors.initials: YB
authors.lastname: Chang
authors.forename: Po-Yu
authors.initials: PY
authors.lastname: Tsai
authors.forename: Yu-Ying
authors.initials: YY
authors.lastname: Yeh
authors.forename: He
authors.initials: H
authors.lastname: Fan
authors.forename: King-Chuen
authors.initials: KC
authors.lastname: Lin
authors.forename: J S
authors.initials: JS
authors.lastname: Francisco