Br{sub 2} elimination in 248-nm photolysis of CF{sub 2}Br{sub 2} probed by using cavity ring-down absorption spectroscopy

Journal Article · Sat Oct 01 00:00:00 EDT 2005 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.2047570· OSTI ID:20723153

Hsu, C -Y; Huang, H -Y; Lin, K -C ^[1]

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1. Department of Chemistry, National Taiwan University, Taipei 106, Taiwan and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan (China)

By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br{sub 2} molecular elimination following photodissociation of CF{sub 2}Br{sub 2} at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br{sub 2} fragment in the B {sup 3}{pi}{sub ou}{sup +}-X {sup 1}{sigma}{sub g}{sup +} transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) is determined to be 0.4{+-}0.2, slightly larger than the value of 0.22 evaluated by Boltzmann distribution at room temperature. The quantum yield of the Br{sub 2} elimination reaction is also measured to be 0.04{+-}0.01. This work provides direct evidence to support molecular elimination occurring in the CF{sub 2}Br{sub 2} photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF{sub 2}Br{sub 2} is excited probably to the {sup 1}B{sub 1} and {sup 3}B{sub 2} states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the {sup 1}A{sup '}({sup 1}B{sub 1}) state to the high vibrational levels of the ground state X-tilde {sup 1}A{sup '}({sup 1}A{sub 1}) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF{sub 2}Br{sub 2} feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF{sub 2}Br+Br(CF{sub 2}Br{center_dot}{center_dot}{center_dot}Br) and the products CF{sub 2}+Br{sub 2}. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br{sub 2} elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br{sub 2} fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) agrees with the proposed mechanism.

• OSTI ID: 20723153
• Journal Information: Journal of Chemical Physics, Vol. 123, Issue 13; Other Information: DOI: 10.1063/1.2047570; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ABSORPTION SPECTROSCOPY
ATOM-MOLECULE COLLISIONS
BOND LENGTHS
BROMINATED ALIPHATIC HYDROCARBONS
DISSOCIATION
EXCITATION
FLUORINATED ALIPHATIC HYDROCARBONS
GROUND STATES
INTERMEDIATE STATE
INTERNAL CONVERSION
PHOTOLYSIS
PHOTON-MOLECULE COLLISIONS
REACTION KINETICS
TEMPERATURE RANGE 0273-0400 K
VIBRATIONAL STATES