248 nm photolysis of CH{sub 2}Br{sub 2} by using cavity ring-down absorption spectroscopy: Br{sub 2} molecular elimination at room temperature

Journal Article · Sat Oct 07 04:00:00 EDT 2006 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.2218514· OSTI ID:20864308

Wei, P -Y; Chang, Y -P; Lee, W -B; Zhengfa, Hu; Huang, H -Y; Lin, K -C; Chen, K T; Chang, A H. H. ^[1]

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

Following photodissociation of CH{sub 2}Br{sub 2} at 248 nm, Br{sub 2} molecular elimination is detected by using a tunable laser beam, as crossed perpendicular to the photolyzing laser beam in a ring-down cell, probing the Br{sub 2} fragment in the B {sup 3}{pi}{sub ou}{sup +}-X {sup 1}{sigma}{sub g}{sup +} transition. The nascent vibrational population is obtained, yielding a population ratio of Br{sub 2}(v=1)/Br{sub 2}(v=0) to be 0.7{+-}0.2. The quantum yield for the Br{sub 2} elimination reaction is determined to be 0.2{+-}0.1. Nevertheless, when CH{sub 2}Br{sub 2} is prepared in a supersonic molecular beam under cold temperature, photofragmentation gives no Br{sub 2} detectable in a time-of-flight mass spectrometer. With the aid of ab initio potential energy calculations, a plausible pathway is proposed. Upon excitation to the {sup 1}B{sub 1} or {sup 3}B{sub 1} state, C-Br bond elongation may change the molecular symmetry of C{sub s} and enhance the resultant 1 {sup 1,3}A{sup '}-X-tilde{sup 1}A{sup '} (or 1 {sup 1,3}B{sub 1}-X-tilde{sup 1}A{sub 1} as C{sub 2v} is used) coupling to facilitate the process of internal conversion, followed by asynchronous concerted photodissociation. Temperature dependence measurements lend support to the proposed pathway.

• OSTI ID: 20864308
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 13 Vol. 125; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ABSORPTION SPECTROSCOPY
BROMINATED ALIPHATIC HYDROCARBONS
BROMINE
DISSOCIATION
INTERNAL CONVERSION
MASS SPECTRA
MOLECULAR BEAMS
PHOTOLYSIS
POTENTIAL ENERGY
REACTION KINETICS
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE 0273-0400 K
TIME-OF-FLIGHT MASS SPECTROMETERS
TIME-OF-FLIGHT METHOD
VIBRATIONAL STATES