Measurements of absolute line strength of the ν1 fundamental transitions of OH radical and rate coefficient of the reaction OH + H2O2 with mid-infrared two-color time-resolved dual-comb spectroscopy

Abstract

Absolute line strengths of several transitions in the ν1 fundamental band of the hydroxyl radical (OH) have been measured by simultaneous determination of hydrogen peroxide (H2O2) and OH upon laser photolysis of H2O2. Based on the well-known quantum yield for the generation of OH radicals in the 248-nm photolysis of H2O2, the line strength of the OH radicals can be accurately derived by adopting the line strength of the well-characterized transitions of H2O2 and analyzing the difference absorbance time traces of H2O2 and OH obtained upon laser photolysis. Employing a synchronized two-color dual-comb spectrometer, we measured high-resolution time-resolved absorption spectra of H2O2 near 7.9 µm and the OH radical near 2.9 µm, simultaneously, under varied conditions. In addition to the studies of the line strengths of the selected H2O2 and OH transitions, the kinetics of the reaction between OH and H2O2 were investigated. A pressure-independent rate coefficient kOH+H2O2 was determined to be [1.97 (+0.10/−0.15)] × 10−12 cm3 molecule−1 s−1 at 296 K and compared with other experimental results. By carefully analyzing both high-resolution spectra and temporal absorbance profiles of H2O2 and OH, the uncertainty of the obtained OH line strengths can be achieved down to <10% in this work. Moreover, the proposed two-color time-resolved dual-comb spectroscopy provides a new approach for directly determining the line strengths of transient free radicals and holds promise for investigations on their self-reaction kinetics as well as radical–radical reactions.

Accurate Kinetic Studies of OH + HO2 Radical–Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy

“…After photolysis of the precursor mixture, transient infrared absorption spectra of two OH lines were identified at 3407.612 and 3407.989 cm –1 in the early reaction time (Figure b) and several HO 2 absorption lines were observed in both spectral regions at the later reaction time, as shown in Figure c,e,f. To avoid the interference from the absorption of HO 2 near 3408 cm –1 , we mainly analyzed the temporal profiles of OH by monitoring the absorption line of OH at 3407.612 cm –1 with the measured line strength of (3.38 ± 0.25) × 10 –20 cm molecule –1 . The temporal profiles of HO 2 can be extracted from the time-resolved spectra of the five absorption peaks of HO 2 in the region of 1122.78–1123.22 cm –1 .…”

“…Direct quantification of both precursors and free radicals in the reaction cell would thus be crucial to perform accurate reaction kinetic studies, particularly for the radical–radical reactions. Recently, we have carried out the measurements on absolute line strength of the fundamental transitions of H 2 O 2 and OH as well as the rate coefficient of the reaction OH + H 2 O 2 using synchronized two-color time-resolved dual-comb spectroscopy in the mid-infrared spectral ranges . Utilizing two sets of dual-comb spectrometers near 7.9 and 2.9 μm, high-resolution time-resolved spectra of H 2 O 2 and OH were simultaneously measured before and after 248 nm photolysis of the flowing mixtures of H 2 O 2 /N 2 at varied conditions to investigate the kinetics of the reaction between OH and H 2 O 2 .…”

“…The initial concentration of H 2 O 2 , [H 2 O 2 ] 0 , in the reactor was estimated by using the flow rate of each stream, mixing ratios of the H 2 O 2 /N 2 premixtures, and the total pressure of the reactor. Furthermore, the [H 2 O 2 ] 0 was also directly determined by using the high-resolution absorption spectra and the line strengths of several H 2 O 2 ν 6 transitions for double confirming that no loss of H 2 O 2 was caused in the flowing system. By monitoring the precursor mixtures with both UV and IR spectroscopy, we confirmed that the pure H 2 O 2 can be produced without other impurities such as H 2 O, HNO 3 , and HNCO from the H 2 O 2 –urea/SiO 2 mixture after 30 min of warming up and during stable heating at below 50 °C.…”

“…To directly determine the concentrations of H 2 O 2 and H 2 O (for the experiments in the presence of water) inside the reactor before starting the time-resolved measurements and to simultaneously record the time-dependent absorption spectra of OH and HO 2 radicals during reaction processes, two sets of dual-comb sources in the short-wave (near 2.9 μm) and long-wave (near 7.9 or 8.9 μm) mid-infrared spectral regions were operated and coupled into the multipass cell. The flash photolysis reactor was designed with a multipass path length of ∼41.8 m for the dual-comb beams and an overlap path of ∼13.4 m between the photolysis and dual-comb beams. , …”

“…Recently, we have carried out the measurements on absolute line strength of the fundamental transitions of H 2 O 2 and OH as well as the rate coefficient of the reaction OH + H 2 O 2 using synchronized two-color time-resolved dual-comb spectroscopy in the mid-infrared spectral ranges. 19 Utilizing two sets of dual-comb spectrometers near 7.9 and 2.9 μm, high-resolution time-resolved spectra of H 2 O 2 and OH were simultaneously measured before and after 248 nm photolysis of the flowing mixtures of H 2 O 2 /N 2 at varied conditions to investigate the kinetics of the reaction between OH and H 2 O 2 . By analyzing the temporal profiles of OH with the kinetic model including self-reactions of the OH and HO 2 and taking into account the reaction of OH + HO 2 with the rate coefficients of 1 × 10 –11 and 1 × 10 –10 cm 3 molecule –1 s –1 , the rate coefficients for the reaction OH + H 2 O 2 were respectively determined to be (1.97 ± 0.10) × 10 –12 and (1.92 ± 0.10) × 10 –12 cm 3 molecule –1 s –1 at 296 K, which are in excellent agreement with the recent experimental value of (2.00 ± 0.15) × 10 –12 cm 3 molecule –1 s –1 at 298 K, reported by Jiménez et al, 20 but up to 16% higher than the value of 1.7 × 10 –12 cm 3 molecule –1 s –1 from the IUPAC recommendation.…”