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Direct frequency comb measurement of OD + CO → DOCO kinetics

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Science  28 Oct 2016:
Vol. 354, Issue 6311, pp. 444-448
DOI: 10.1126/science.aag1862
  • Fig. 1 Energetics of the OH + CO → H + CO2 reaction.

    (A) Potential energy surface, with energies taken from Nguyen et al. (2). OH + CO → H + CO2 proceeds through vibrationally excited HOCO*, which is either deactivated by bath gas M or reacts to form H + CO2. The inset shows the simplified OH + CO reaction mechanism. TS, transition state. (B) Schematic showing the most important reactions in our system. Time-dependent concentrations of trans-DOCO, OD(v = 0), OD(v = 1), and D2O (red) are measured by cavity-enhanced absorption spectroscopy; the concentrations of the precursors (purple) are set by flow controllers or meters. O3 is measured by UV absorption.

  • Fig. 2 Spectral acquisition and fitting.

    (A) Experimental spectra (black) were recorded with an integration time of 50 μs and offsets of –50 (“before photolysis”), 100, and 4000 μs from the photolysis pulse. These spectra were then fitted to the known line positions of OD (blue), D2O (green), and trans-DOCO (orange) to determine their temporal concentration profiles. The P, Q, and R branches of trans-DOCO are indicated above the 100-μs experimental trace. (B) An analytical functional form for [OD](t) was obtained by fitting the data (black circles) to a sum of boxcar-averaged exponential functions (red line). At each time, the data point represents ~300 averaged spectra, and the error bars are from statistical uncertainties in the spectral fit. (C) The bimolecular trans-DOCO rise rate was obtained by fitting the data (black circles) to Eq. 3 (red line). The data in (B) and (C) were obtained at a 10-μs camera integration time and precursor concentrations of [CO] = 5.9 × 1017, [N2] = 8.9 × 1017, [D2] = 7.4 × 1016, and [O3] = 1 × 1015 molecules cm−3.

  • Fig. 3 Determination of the termolecular trans-DOCO formation rate.

    The bimolecular trans-DOCO formation rate coefficient, k1a, is plotted as a function of [CO] and [N2] to determine the termolecular rate coefficients Embedded Image and Embedded Image. Each point represents one of 26 experimental conditions tabulated in table S1. In both panels, the error bars represent uncertainties from fits to Eqs. 2 to 4 and the measured densities of the gases. (A) k1a is plotted as a function of [CO] while [N2] = 8.9 × 1017 molecules cm−3 is held constant. (B) k1a is plotted as a function of [N2] while [CO] = 5.6 × 1017 molecules cm−3 is held constant. In both plots, D2 and O3 concentrations are fixed at 7.4 × 1016 and 1 × 1015 molecules cm–3, respectively. Blue and red data points indicate 50- and 10-μs camera integration times, respectively. The data in (A) and (B) are simultaneously fitted to Eq. 4. The black lines in (A) and (B) are obtained from weighted linear fits (Embedded Image). The y offsets in the data arise from the nonzero concentrations of N2 and CO in (A) and (B), respectively.

  • Fig. 4 Rate equation model fitting.

    The OD (blue circles) and trans-DOCO (red circles) traces are weighted fits to the model (solid and dashed lines for OD and trans-DOCO, respectively) described in the supplementary materials. The integration time was 50 μs. The error bars are from uncertainties in the spectral fit, in the same manner as for Fig. 2B. The input k1a values for both CO and N2 were from the early-time trans-DOCO rise analysis and were fixed in the fit. The floated parameters included a single scaling factor for the OD and trans-DOCO intensities and an extra DOCO loss channel. (A) [CO] = 5.9 × 1017 molecules cm−3. (B) [CO] = 1.2 × 1018 molecules cm–3. For both data sets, [N2] = 8.9 × 1017, [D2] = 7.4 × 1016, and [O3] = 1 × 1015 molecules cm–3 were fixed.

Supplementary Materials

  • Direct frequency comb measurement of OD + CO → DOCO kinetics

    B. J. Bjork, T. Q. Bui, O. H. Heckl, P. B. Changala, B. Spaun, P. Heu, D. Follman, C. Deutsch, G. D. Cole, M. Aspelmeyer, M. Okumura, J. Ye

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

    Download Supplement
    • Materials and Methods
    • Figs. S1 to S13
    • Tables S1 to S4
    • References

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