Browsing by Author "Chang, Chun-Hung"

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Detailed mechanism of the CH2I + O-2 reaction: Yield and self-reaction of the simplest Criegee intermediate CH2OO
(2014-09-14)
Ting, Wei-Lun
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Chang, Chun-Hung
;
Lee, Yu-Fang
;
Matsui, Hiroyuki
;
Lee, Yuan-Pern
The application of a new reaction scheme using CH2I + O-2 to generate the simplest Criegee intermediate, CH2OO, has stimulated lively research; the Criegee intermediates are extremely important in atmospheric chemistry. The detailed mechanism of CH2I + O-2 is hence important in understanding kinetics involving CH2OO. We employed ultraviolet absorption to probe simultaneously CH2I2, CH2OO, CH2I, and IO in the reaction system of CH2I + O-2 upon photolysis at 248 nm of a flowing mixture of CH2I2, O-2, and N-2 (or SF6) in the pressure range 7.6-779 Torr to investigate the reaction kinetics. With a detailed mechanism to model the observed temporal profiles of CH2I, CH2OO, and IO, we found that various channels of the reaction CH2I + O-2 and CH2OO + I play important roles; an additional decomposition channel of CH2I + O-2 to form products other than CH2OO or ICH2OO becomes important at pressure less than 60 Torr. The pressure dependence of the derived rate coefficients of various channels of reactions of CH2I + O-2 and CH2OO + I has been determined. We derived a rate coefficient also for the self-reaction of CH2OO as k = (8 +/- 4) x 10(-11) cm(3) molecule(-1) s(-1) at 295 K. The yield of CH2OO from CH2I + O-2 was found to have a pressure dependence on N-2 and O-2 smaller than in previous reports; for air under 1 atm, the yield of similar to 30% is about twice of previous estimates. (C) 2014 AIP Publishing LLC.
Direct kinetic measurement of the reaction of the simplest Criegee intermediate with water vapor
(2015-02-13)
Chao, Wen
;
Hsieh, Jun-Ting
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Chang, Chun-Hung
;
Lin, Jim Jr-Min
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應用化學系
Carbonyl oxides, or Criegee intermediates, are important transient species formed in the reactions of unsaturated hydrocarbons with ozone. Although direct detection of Criegee intermediates has recently been realized, the main atmospheric sink of Criegee intermediates remains unclear. We report ultraviolet absorption spectroscopic measurements of the lifetime of the simplest Criegee intermediate, CH2OO, at various relative humidity levels up to 85% at 298 kelvin. An extremely fast decay rate of CH2OO was observed at high humidity. The observed quadratic dependence of the decay rate on water concentration implied a predominant reaction with water dimer. On the basis of the water dimer equilibrium constant, the effective rate coefficient of the CH2OO + (H2O)(2) reaction was determined to be 6.5 (+/-0.8) x 10(-12) cubic centimeters per second. This work would help modelers to better constrain the atmospheric concentrations of CH2OO.
Kinetics and absolute UV absorption spectrum of a methyl substituted Criegee intermediate CH3CHOO
(2014-08-10)
Smith, Mica
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Ting, Wei-Lun
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Chang, Chun-Hung
;
Boering, Kristie
;
Lin, Jim Jr-Min
UV absorption spectrum of the C2 Criegee intermediate CH3CHOO
(2014-08-21)
Smith, Mica C.
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Ting, Wei-Lun
;
Chang, Chun-Hung
;
Takahashi, Kaito
;
Boering, Kristie A.
The UV spectrum of CH3CHOO was measured by transient absorption in a flow cell at 295 K. The absolute absorption cross sections of CH3CHOO were measured by laser depletion in a molecular beam to be (1.06 +/- 0.09) x 10(-17) cm(2) molecule-1 at 308 nm and (9.7 +/- 0.6) x 10(-18) cm(2) molecule-1 at 352 nm. After scaling the UV spectrum of CH3CHOO to the absolute cross section at 308 nm, the peak UV cross section is (1.27 +/- 0.11) x 10(-)1(7) cm(2) molecule-1 at 328 nm. Compared to the simplest Criegee intermediate CH2OO, the UV absorption band of CH3CHOO is similar in intensity but blue shifted by 14 nm, resulting in a 20% slower photolysis rate estimated for CH3CHOO in the atmosphere. (C) 2014 AIP Publishing LLC.