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

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 or the absorption cross-section at 220 turn. The diode laser wavelength was calibrated by atmospheric cavity ringdown spectra of CH4, CO2, and H2O. 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 2,3dimethyl-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. Index Headings: Near-infrared absorption; NIR absorption cross-sections; Isoprene; Volatile organic compounds; VOCs; Continuous wave cavity ringdown spectroscopy; cw-CRDS.