Measurements of the Absolute Absorption Cross Sections of theÃ←X̃Transition in Organic Peroxy Radicals by Dual-Wavelength Cavity Ring-Down Spectroscopy

Abstract

We demonstrate an experimental method for the accurate measurement of the absorption cross section of transient species, such as organic peroxy radicals in which the concentration of the radicals is determined via the absorption of a stable coproduct that is produced stochiometrically. The requirements for the experimental apparatus, a dual-wavelength cavity ring-down spectrometer, and the chemical protocol for transient species generation are discussed. The capability of this approach is demonstrated by measuring the peak absorption cross section of the ethyl peroxy radical, C₂2H₅O₂, whose value for the Ã←X electronic transition at 7596 cm⁻¹ (λ = 1316.5 nm) is found to be σ(p)(EP) = 5.29(20) × 10⁻²¹ cm². These present results are compared to those obtained from other methods of measurement of σ(p)(EP). Possible random and systematic errors are discussed.

Absolute Absorption Cross-Section of the Ã←X˜ Electronic Transition of the Ethyl Peroxy Radical and Rate Constant of Its Cross Reaction with HO2

“…The upper graph shows that our spectrum (green symbols and green axis apply) agrees well with the results of Melnik et al [17] if our data are shifted by 4 cm −1 . Possibly, there is a mistake in the Melnik figure (T. Miller, private communication), because the peak absorption is given in the text at 7596 cm −1 , just as in our case, however in the figure the peak is located at 7600 cm −1 , indicated by a blue vertical line.…”

“…Also, the overall shape is, after consideration of a baseline shift, in excellent agreement between both works (see Figure 7). A possible explanation might be that Melnik et al and Rupper et al both consider the complete conversion of Cl-atoms into C2H5O2 radicals: a simple model is presented by Melnik et al [17] showing the complete conversion of Cl-atoms into C2H5O2. However, the very fast reactions of Cl-atoms with C2H5O2 (k10 = 1.5 × 10 −10 cm 3 molecule −1 s −1 ) [41] and C2H5 (k = 3 × 10 −10 cm 3 molecule −1 s −1 ) [42] are omitted in this model, even though these reactions are non-negligible under their conditions of very high initial Cl-atom concentrations, well above 10 15 cm −3 , combined with relatively low C2H6 concentrations (1 × 10 16 cm −3 ).…”

“…In this case, a broadband absorber generated simultaneously to the C2H5O2 radical would induce a baseline shift. To take into account this shift (horizontal dashed [17] (upper graph, Reprinted with permission from [17], Copyright 2010 American Chemical Society) and Atkinson and Spillman [11] (lower graph, Reprinted with permission from [11], Copyright 2002 American Chemical Society). In the upper graph the data have been shifted by 4 cm −1 , and in both graphs our data have been scaled on the y-axis, i.e., apparently there is a baseline shift in both comparisons.…”

“…The next work on the ethyl peroxy spectrum from the Miller group [17] used a different method to obtain the absorption cross-section: in a dual-path CRDS set-up, the concentration of HCl (generated from the reaction of Cl-atoms with C 2 H 6 ) was measured on one path while the absorption of C 2 H 5 O 2 was measured simultaneously on the other path. Again supposing that each HCl-molecule had generated one C 2 H 5 O 2 radical, they obtained σ = (5.29 ± 0.20) × 10 −21 cm 2 for C 2 H 5 O 2 at 7596 cm −1 .…”

“…In this work we present a new determination of the absorption cross-section, based on two different approaches. The first one is comparable to one of the Miller methods [17] and will be called back-to-back method: in our dual-path CRDS set-up we generate Cl-atoms and transform them to HO 2 through reaction with CH 3 OH, with HO 2 being quantified on one path at 6638.2 cm −1 . Directly after, the Cl-atoms were transformed to C 2 H 5 O 2 by adding C 2 H 6 instead of CH 3 OH to the reaction mixture and the C 2 H 5 O 2 absorption was measured on the second path.…”