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CRDS & Kinetics

A requirement for a simple quantitative measurements of concentration-time profiles by means of CRDS is a (quasi) steady concentration of the detected species during the whole ringdown (typically several tens of μs). In this case, the concentration of the species can be easily extracted from the single-exponential ringdown
time constants by pump-probe schemes with variable time delays between photolysis (pump) and ringdown (probe) laser pulses. For "fast reactions" that occur on the same time scale as the ringdown, the ringdown becomes nonexponential due to the convolution with the concentration change caused by the reactions. In this case, a more sophisticated ringdown model has to be applied.


CRDS for Fast Reactions: The eSKaR Model

Typically, kinetic applications of cavity-ringdown spectroscopy (CRDS) are limited to the investigation of relatively slow reactions with lifetimes of the measured species longer than the ringdown time. In this case, the concentration of the reactive species remains almost constant during the ringdown period and complete concentration-time profiles can be obtained from the single-exponential ringdown time constants by pump-probe schemes with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses.

crds 1

 

For fast reactions that occur on the same time scale as the ringdown, the ringdown becomes nonexponential due to the convolution with the concentration change caused by the reactions. In this case, rate constants can be extracted from the nonexponential ringdown signals by the simultaneous kinetics and ringdown (SKaR) model developed by Brown et al. (J. Phys. Chem. A 2000, 104, 7044 and 8600). However, nonexponential signals are also obtained in the absence of a reaction when the probe laser line width is comparable or exceeds the absorpion line width of the detected species. In this case, different frequency components of the probe laser light in the ringdown cavity experience different absorptions and thus decay with different time constants. The resulting ringdown exhibits multiexponential character even without convoluted kinetics. This is called the bandwidth effect.

We have developed an extended version (eSKaR) of the SKaR model to extract rate constants of fast reactions from nonexponential ringdown profiles by deconvolution of the ensuing reaction and the bandwidth effect. Detailed measurements of the rate constants of the reactios NH2 + NO, SiH2 + O2, and SiH2 + alkenes have shown that the eSKaR model provides a reliable method for extracting rate constants from the ringdown profiles. Small inaccuracies in the assumed absorption and probe laser line shapes were shown to induce only minor errors in the finally determined rate constant. The eSKaR method thus extends the applicability of CRDS measurements of fast reactions of small radicals using conventional pulsed dye lasers as the probe laser source.

 

[1] Y. Q. Guo, M. Fikri, G. Friedrichs, F. Temps, "An Extended Simultaneous Kinetics and Ringdown Model: Determination of the Rate Constant for the Reaction SiH2 + O2", Phys. Chem. Chem. Phys. 5 (2003) 4622-4630.

[2] G. Friedrichs, M. Colberg, M. Fikri, Z. Huang, J. Neumann, and F. Temps, "Validation of the extended Simultaneous Kinetics and Ringdown Model by Measurements of the Reaction NH2 + NO", J. Phys. Chem. A 109 (2005) 4785-4795.

[3] G. Friedrichs, "Review paper: Sensitive Absorption Methods for Quantitative Gas Phase Kinetic Measurements. Part2: Cavity Ringdown Spectroscopy", Z. Phys. Chem. 222 (2008) 31-61.

[4] G. Friedrichs, M. Fikri, Y. Q. Guo, F. Temps, "Time-resolved cavity ringdown measurements and kinetic modeling of the pressure dependences of the recombination reactions of SiH2 with the alkenes C2H4, C3H6, and t-C4H8", J. Phys. Chem. A 112 (2008) 5636-5646.



Contributing researchers: G. Friedrichs, F. Temps and (formerly) Y. Guo, M. Fikri, J. Neumann, Z. Huang


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Last Updated on Monday, 09 January 2017 09:56
 
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