Absorption Cross-Sections of the C—H Overtone of Volatile Organic Compounds: 2 Methyl-1,3-Butadiene (Isoprene), 1,3-Butadiene, and 2,3-Dimethyl-1,3-Butadiene

Abstract

Many molecules or transient radicals have well-documented absorption cross-sections in the ultraviolet (UV) region, but their absorption cross-sections in the near-infrared (NIR) region are much less often known and are difficult to measure. We propose a method to determine the unknown NIR absorption cross-sections using the known absorption cross-sections in the UV region, in which single-path UV absorption spectroscopy and NIR continuous wave cavity ringdown spectroscopy (cw-CRDS) are employed in a cross-arm reaction chamber for simultaneous measurements. Without knowing the actual sample partial pressures (or concentrations), the NIR absorption cross-sections can be accurately determined through the two sets of measurements. The method is demonstrated by measuring the NIR absorption cross-section of the first overtone of the asymmetric C-H stretch of 2-methyl-1,3-butadiene (isoprene) (3.24 (+/-0.16) x 10(-22) cm(2) molecule(-1)) at 1651.52 nm using the known value of the absorption cross-section at 220 nm. The diode laser wavelength was calibrated by atmospheric cavity ringdown spectra of CH(4), CO(2), and H(2)O. By comparison with sample pressure measurements, this method can also be used as a pressure calibration means for the reaction chamber, and this has been demonstrated with two additional measurements of the absorption cross-sections of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene (2.50 (+/- 0.08) x 10(-22) and 2.82 (+/-0.16) x 10(-22) cm(2) molecule(-1), respectively) at 1651.52 nm. The applicability of the method to determining absorption cross-sections using the simultaneous measurements of cw-CRDS and single-path absorption spectroscopy is discussed.

Near-infrared broad-band cavity enhanced absorption spectroscopy using a superluminescent light emitting diode

“…2) are in reasonable agreement with other reported values determined with a diode laser (2.5 Â 10 À22 cm 2 at 1651.52 nm). 21 Some data are used to extract the wavelength dependent mirror reflectivity when analysing the cavity enhanced datasets, and the resulting reflectivity is then used to correct the cavity enhanced absorption of different datasets (see Fig. 3).…”

Quantitative infrared absorption cross sections of isoprene for atmospheric measurements

“…While the consensus in the literature supports this conclusion, there has been some dispute regarding the scis conformer (Fig. 2b), its fractional abundance at room temperature, and whether the conformer is planar or gauche (Traetteberg et al, 1984;Bock et al, 1987;Cias et al, 2007;Panchenko and De Mare, 2008;Squillacote and Liang, 2005;Allodi et al, 2008). Compton Squillacote and Liang (2005) using the G3B3 method.…”

“…Despite the obvious need for quantitative infrared data and the potential for infrared observation of isoprene, as well as the relative abundance of UV cross sections (e.g., Campuzano-Jost et al, 2004, 2000, the only quantitative infrared intensity studies of isoprene found in the literature report absorption cross sections in the near-infrared (Denzer et al, 2011;Cias et al, 2007). While it is not unusual to record quantitative measurements of isoprene in conjunction with other experiments or for calibration purposes, these results are seldom reported in the literature.…”

“…Very few quantitative mid-infrared band strengths of isoprene are available in the literature, but a few pa- pers report near-infrared absorption cross sections (σ ). For example, Cias et al (2007) determined σ of the C-H overtone at 1651.52 nm (6055 cm −1 ) to be 3.24 (±0.16) ×10 −22 cm 2 molecule −1 . Unfortunately, that mode is not observed in our spectra; thus no comparison can be made.…”

Cavity Ringdown Laser Absorption Spectroscopy

“…This method has been successfully used in several ringdown research studies and applications. (17,46,57,58,62,64,113,114) …”