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Title:Measuring Time-resolved Concentrations Of Free Radicals In Chemical Reactions With Cavity Ringdown Spectroscopy
Author(s):Jones, Ian
Contributor(s):Stanton, John F.; Adamovich, Igor V.; Miller, Terry A.; Jans, Elijah R.
Subject(s):Dynamics and kinetics
Abstract:Numerous spectroscopic techniques have been used to follow the concentration of reactive intermediates, such as free radicals, during chemical reactions. However, it is often necessary to measure absolute concentrations to test proposed kinetic mechanisms. Such measurements have always posed more of a challenge because of the difficulty of experimentally measuring the required value of the dipole transition moment to whose square the absorption cross section is proportional. In the-just presented talk involving HO$_2$, the issue was partially resolved since an empirical value of the absorption cross section was available$^1$ for the 2$\nu_2$ overtone band. However, the cross-section is temperature and pressure dependent and the kinetic experiment was performed at different conditions than the cross section measurements. To resolve this problem, we used the pGopher software$^2$ with the HO$_2$ molecular constants$^3$, $\boldsymbol{\mu}$, to simulate the HO$_2$ spectrum from which the cross sections were derived, and thereby get the transition moment, $\boldsymbol{\mu}$. A second simulation under the experimental conditions then determined the absolute HO$_2$ concentration. In addition, we carried out a quantum chemical calculation of $\boldsymbol{\mu}$ using the CFOUR package. Despite the overtone nature of the transition, the calculation produced the same values of $\boldsymbol{\mu}$ as our simulation, within experimental error. We are now investigating C$_2$H$_5$O$_2$ to compare experimental and calculated values of $\boldsymbol{\mu}$ for its $\tilde{A}-\tilde{X}$ electronic transition. We hope to establish when it is reasonable to use calculated $\boldsymbol{\mu}$ values instead of the difficult-to-measure, experimental ones, to determine concentrations of small, free radicals important in combustion and atmospheric reactions. $^1$Thiebaud, et. al, J. Phys. Chem. A 2007, 111, 6959-6966; $^2$ C. M. Western, J. Quant. Spectro. Rad. Tran., 186 221-242 (2017); $^3$DeSain, et. al, J. Mol. Spectro. 219 (2003) 163–16.
Issue Date:2021-06-23
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper / Presentation
Date Available in IDEALS:2021-09-24

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