# Accurate Kinetic Studies of OH + HO 2 Radical–Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy

I-Yun Chen, Che-Wei Chang, Christa Fittschen, Pei-Ling Luo

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I-Yun Chen, Che-Wei Chang, Christa Fittschen, Pei-Ling Luo. Accurate Kinetic Studies of OH + HO 2 Radical–Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy. Journal of Physical Chemistry Letters, 2024, 15 (14), pp.3733-3739. 10.1021/acs.jpclett.4c00494. hal-04781915

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# 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

I-Yun Chen, Che-Wei Chang, Christa Fittschen, and Pei-Ling Luo*

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Cite This: J. Phys. Chem. Lett. 2024, 15, 3733–3739

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**ABSTRACT:** The radical–radical reaction between OH and HO₂ has been considered for a long time as an important reaction in tropospheric photochemistry and combustion chemistry. However, a significant discrepancy of an order of magnitude for rate coefficients of this reaction is found between two recent experiments. Herein, we investigate the reaction OH + HO₂ via direct spectral quantification of both the precursor (H₂O₂) and free radicals (OH and HO₂) upon the 248 nm photolysis of H₂O₂ using infrared two-color time-resolved dual-comb spectroscopy. With quantitative and kinetic analysis of concentration profiles of both OH and HO₂ at varied conditions, the rate coefficient k<sub>OH+HO₂</sub> is determined to be (1.10 ± 0.12) × 10<sup>-10</sup> cm³ molecule<sup>-1</sup> s<sup>-1</sup> at 296 K. Moreover, we explore the kinetics of this reaction under conditions in the presence of water, but no enhancement in the k<sub>OH+HO₂</sub> can be observed. This work as an independent experiment plays a crucial role in revisiting this prototypical radical–radical reaction.

As atmospheric detergents, hydroxyl (OH) and hydroperoxyl (HO₂) radicals play critical roles in the catalytic cycle of ozone formation and controlling secondary pollution in the troposphere.<sup>1,2</sup> The reaction of OH with HO₂ is a key pathway for removal of both OH and HO₂ radicals in the upper troposphere, and it is also of special importance in low-NOₓ atmosphere and low-temperature combustion systems.<sup>3–5</sup> In addition, being a prototype of the radical–radical reaction, the reaction between OH and HO₂ is also of great interest in both theoretical and physical chemistry. A brief description of the previous studies of the reaction OH + HO₂ is shown in Note S1. Although numerous investigations on the kinetics of the reaction OH + HO₂ have been conducted during the past couple of decades,<sup>6–17</sup> the reported rate coefficients k<sub>OH+HO₂</sub> from both experimental and theoretical studies are scattered in a wide range of (0.1–1.7) × 10<sup>-10</sup> cm³ molecule<sup>-1</sup> s<sup>-1</sup> at around room temperature, as shown in Figure S1. In particular, the most recent measurements of the rate coefficient carried out by Assaf et al.<sup>6</sup> and Speak et al.<sup>7</sup> show a significant discrepancy between each other by a factor of 10. Assaf et al. employed laser-induced fluorescence (LIF) and near-infrared cavity ring down spectroscopy (CRDS) to respectively monitor the OH and HO₂ radicals generated upon laser flash photolysis (LFP) of the mixture of H₂O₂/(COCl)₂/CH₃OH/O₂ to investigate the kinetics of the reaction OH + HO₂.<sup>6</sup> By analyzing the time traces of OH and HO₂ obtained at different experimental conditions and kinetic simulations with adjustable parameters of initial concentrations of H₂O₂ and OH,

Assaf et al. obtained the rate coefficient k<sub>OH+HO₂</sub> with a value of (1.02 ± 0.06) × 10<sup>-10</sup> cm³ molecule<sup>-1</sup> s<sup>-1</sup> at 298 K. Most recently, Speak et al. conducted kinetic measurements of the reaction OH + HO₂ in the LFP and LIF systems.<sup>7</sup> By adopting the multiple reactions such as OH + H₂O₂, OH + CH₃OH, and HO₂ + NO, Speak et al. calibrated the LIF signals of OH as well as the relative yields of HO₂ in the reactors. The rate coefficients k<sub>OH+HO₂</sub> at 298 K were obtained with the values of (1.34 ± 0.14) and (1.00 ± 0.32) × 10<sup>-11</sup> cm³ molecule<sup>-1</sup> s<sup>-1</sup> from the high-pressure and low-pressure studies, respectively. Furthermore, a significant enhancement in the rate coefficient k<sub>OH+HO₂</sub> in the presence of water was also observed in the high-pressure water photolysis experiments with excessive oxygen by Speak et al. Both experimental studies have been carefully conducted; however, in these experiments, the initial concentrations of the precursors such as hydrogen peroxide (H₂O₂) and the radical concentrations were estimated mainly based on earlier kinetic results of the reactions such as OH + H₂O₂ from IUPAC recommendation,<sup>18</sup> and the indirect determinations of concentration for the reaction species

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Received: February 16, 2024

Revised: March 22, 2024

Accepted: March 26, 2024

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<table><caption>Table 1. Reactions and Rate Coefficients Employed in the Kinetic Model Simulation</caption><thead><tr><th></th><th>reaction</th><th>rate coefficient<sup>a</sup></th><th>ref.</th></tr></thead><tbody><tr><td>R1</td><td>H<sub>2</sub>O<sub>2</sub> + hν<sub>248</sub> → 2OH</td><td></td><td>21–23</td></tr><tr><td>R2</td><td>OH + H<sub>2</sub>O<sub>2</sub> → H<sub>2</sub>O + HO<sub>2</sub></td><td>1.92 × 10<sup>-12</sup></td><td>19</td></tr><tr><td>R3</td><td>OH + HO<sub>2</sub> → H<sub>2</sub>O + O<sub>2</sub></td><td>k<sub>OH+HO<sub>2</sub></sub> determined in this work</td><td></td></tr><tr><td>R4a</td><td>OH + OH → H<sub>2</sub>O + O</td><td>1.48 × 10<sup>-12</sup></td><td>18</td></tr><tr><td>R4b</td><td>OH + OH + M → H<sub>2</sub>O<sub>2</sub> + M</td><td>6.9 × 10<sup>-31</sup> × [N<sub>2</sub>]</td><td>18</td></tr><tr><td>R5</td><td>HO<sub>2</sub> + HO<sub>2</sub> + M → H<sub>2</sub>O<sub>2</sub> + O<sub>2</sub> + M</td><td>1.6 × 10<sup>-12</sup> + 5.2 × 10<sup>-32</sup> × [N<sub>2</sub>]</td><td>18</td></tr></tbody></table>

<sup>a</sup>Rate coefficient in cm<sup>3</sup> molecule<sup>-1</sup> s<sup>-1</sup>; [N<sub>2</sub>] in molecules cm<sup>-3</sup>.

<figure><img src="image_10.png" /><figcaption>**Figure 1.** Time-resolved dual-comb spectra in the regions (a) 3407.56–3408.02 cm⁻¹ and (b) 1122.78–1123.22 cm⁻¹. The spectra were recorded simultaneously upon photolysis of a flowing mixture of H₂O₂/N₂ ([H₂O₂]₀ = 1.91 × 10¹⁵ molecules cm⁻³, P<sub>T</sub> = 30.3 Torr, 296 K) over 10000 excimer laser shots at 248 nm with the photolysis energy of 38 mJ cm⁻². Here, the spectral sampling spacing is 291 MHz (9.7 × 10⁻³ cm⁻¹). The temporal resolutions of panels a and b are set to be 40 and 80 µs, respectively.</figcaption></figure>

might cause additional and unknown systematic errors. 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 H₂O₂ and OH as well as the rate coefficient of the reaction OH + H₂O₂ 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 H₂O₂ and OH were simultaneously measured before and after 248 nm photolysis of the flowing mixtures of H₂O₂/N₂ at varied conditions to investigate the kinetics of the reaction between OH and H₂O₂. By analyzing the temporal profiles of OH with the kinetic model including self-reactions of the OH and HO₂ and taking into account the reaction of OH + HO₂ with the rate coefficients of 1 × 10⁻¹¹ and 1 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, the rate coefficients for the reaction OH + H₂O₂ were respectively determined to be (1.97 ± 0.10) × 10⁻¹² and (1.92 ± 0.10) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ at 296 K, which are in excellent agreement with the recent experimental value of (2.00 ± 0.15) × 10⁻¹² cm³ 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 cm³ molecule⁻¹ s⁻¹ from the IUPAC recommendation.¹⁸

In this work, to further explore the kinetics of the reaction between OH and HO₂, hydrogen peroxide (H₂O₂) was chosen as the precursor for generation of OH radicals upon 248 nm laser photolysis, because the photodissociation of H₂O₂ has

been well evaluated in previous experiments.²¹⁻²³ The quantum yield for OH production was determined to be two in the 248 nm photolysis system,²¹,²² and no evidence for vibrationally excited OH was observed on laser irradiation of H₂O₂ at 222 nm.²³ By employing photodissociation of H₂O₂ at 248 nm (R1), the OH radicals can be generated immediately and they will subsequently react with H₂O₂ to form H₂O and HO₂ radicals (R2). In the meantime, radical–radical reactions such as OH + HO₂ (R3), OH + OH (R4), and HO₂ + HO₂ (R5) will occur as well. The rate coefficients of these involved reactions are shown in Table 1 and will be used in the kinetic model simulation. Because the rate coefficients for the self-reactions of OH and HO₂ (R4 and R5) are much slower compared to that of the reaction OH + HO₂ (R3) by 1–2 orders of magnitude, the reaction OH + HO₂ (R3) can be considered as the dominant secondary reaction in the OH + H₂O₂ reaction system and plays a major role influencing the yield of HO₂ radicals. Based on the clear kinetic model, we were able to investigate the kinetics of the reaction OH + HO₂ through direct determination of precursor (H₂O₂) and radical (OH and HO₂) concentrations in a flash photolysis reactor and simultaneous analysis of measured temporal profiles of the OH and HO₂ radicals.

The schematic of the experimental setup is shown in Figure S2. In the experiment, we determined well the water-free rate coefficient for the OH + HO₂ reaction and further explored the reaction kinetics under the conditions in the presence of water. The water-free gaseous H₂O₂ was prepared by utilizing thermal decomposition of the H₂O₂–urea complex which has been
<figure><img src="image_11.png" /><figcaption>**Figure 2.** Comparison of the dual-comb spectra obtained before and after photolysis of a flowing mixture of H₂O₂/N₂ ([H₂O₂]₀ = 1.91 × 10¹⁵ molecules cm⁻³, P_T = 30.3 Torr, 296 K) in the regions (a–c) 3407.52–3408.08 cm⁻¹ and (d–f) 1122.78–1123.22 cm⁻¹. The spectra in panels a and d were recorded before the 248 nm irradiation. The difference absorbance spectra were extracted at 0–40 µs (b), 160–240 µs (e), and 1–2 ms (c and f) from the measured time-resolved dual-comb spectra (Figure 1). The insert in panel c depicts the enlarged spectra in region 3407.76–3408.04 cm⁻¹.</figcaption></figure>

adopted in various experiments for generation of stable and gaseous H₂O₂.²⁴,²⁵ By warming up a glass container containing a mixture of H₂O₂—urea powder and SiO₂ in the water bath to ~45 °C, water-free gaseous H₂O₂ can be produced stably, and then it was carried by a stream of nitrogen (N₂) and probed through a UV absorption cell before being introduced to the main reactor. The mixing ratio of the H₂O₂/N₂ mixtures in the UV absorption cell was determined based on the measured absorption spectra and the absorption cross section of H₂O₂ in 215–220 nm region.²⁶ Other streams of N₂ were used for purging the windows and mirrors of the multipass cell and to dilute the concentration of H₂O₂ in the reactor. The initial concentration of H₂O₂, [H₂O₂]₀, in the reactor was estimated by using the flow rate of each stream, mixing ratios of the H₂O₂/N₂ premixtures, and the total pressure of the reactor. Furthermore, the [H₂O₂]₀ was also directly determined by using the high-resolution absorption spectra and the line strengths of several H₂O₂ v₆ transitions¹⁹ for double confirming that no loss of H₂O₂ was caused in the flowing system. By monitoring the precursor mixtures with both UV and IR spectroscopy, we confirmed that the pure H₂O₂ can be produced without other impurities such as H₂O, HNO₃, and HNCO²⁷ from the H₂O₂—urea/SiO₂ mixture after 30 min of warming up and during stable heating at below 50 °C.

To directly determine the concentrations of H₂O₂ and H₂O (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 HO₂ 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.¹⁹,²⁸

Employing dual-comb spectrometers near 2.9 and 8.9 µm, high-resolution time-resolved spectra of OH and HO₂ radicals were simultaneously measured upon 248 nm irradiation of the flowing mixture of H₂O₂/N₂ ([H₂O₂]₀ = 1.91 × 10¹⁵ molecules cm⁻³, P_T = 30.3 Torr, 296 K), as shown in Figure 1. Both OH and HO₂ can be probed with the detection limit down to ~1 × 10¹¹ molecules cm⁻³ at the time resolution of tens of µs. In high-resolution time-resolved dual-comb spectroscopy, the spectral sampling spacing is equal to the repetition frequency of the dual-comb source, and the spectral sampling points can be increased by interleaving several dual-comb spectra measured with different spectral sampling spacings or varied central wavelengths. The temporal resolution of time-resolved dual-comb spectra is adjustable from the µs to ms level, depending on the length of measured interferogram used for generation of each time-dependent Fourier transform dual-comb spectrum. A detailed description of the experimental approach with time-resolved dual-comb spectroscopy has been presented in our previous works.²⁸⁻³⁰

Figure 2 shows the comparison of the dual-comb spectra obtained before and after 248 nm irradiation of the flowing mixture of H₂O₂/N₂ ([H₂O₂]₀ = 1.91 × 10¹⁵ molecules cm⁻³, P_T = 30.3 Torr, 296 K). Figures 2a and 2d exhibit the absorbance spectra of H₂O₂ obtained before laser photolysis in the regions of 3407.52–3408.08 cm⁻¹ (near 2.9 µm) and 1122.78–1123.22 cm⁻¹ (near 8.9 µm), respectively. Although some absorption lines of H₂O₂ were observed near 2.9 µm, their line strengths are much smaller compared to that of OH by 2–3 orders of magnitude, and thus, the difference absorbance spectra would not be interfered by H₂O₂. 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 2b) and several HO₂ absorption lines were observed in both spectral regions at the later reaction time, as shown in
<figure><img src="image_12.png" /><figcaption>**Figure 3.** Comparison of the measured and the simulated temporal profiles of OH and HO₂ at varied conditions. The concentration temporal profiles of (a) OH and (b) HO₂ were recorded after the 248 nm irradiation of the flowing mixture of H₂O₂/N₂ ([H₂O₂]₀ ≈ 1.9 × 10¹⁵ molecules cm⁻³, P_T = 30.3 Torr, 296 K) with different photolysis energies. The concentration temporal profiles of (c) OH and (d) HO₂ were recorded after the 248 nm irradiation of the flowing mixture of H₂O₂/N₂ ([H₂O₂]₀ ≈ 9.7 × 10¹⁴ molecules cm⁻³, P_T = 30.3 Torr, 296 K) with different photolysis energies. Here, the OH time traces were obtained by analyzing the time-resolved spectra of the OH X²Π₃/₂ (1 ← 0) P(4.5)f transition at 3407.612 cm⁻¹ and the HO₂ time traces were obtained by analyzing the time-resolved spectra of five absorption peaks near 1123 cm⁻¹. The open symbols represent the measured temporal profiles with a resolution of 40 µs. The solid lines represent the simulated profiles using the kinetic model, as shown in Table 1, with k<sub>OH+HO₂</sub> = 1.10 × 10<sup>-10</sup>.</figcaption></figure>

**Figure 2c,e,f.** To avoid the interference from the absorption of HO₂ 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⁻¹.¹⁹ The temporal profiles of HO₂ can be extracted from the time-resolved spectra of the five absorption peaks of HO₂ in the region of 1122.78–1123.22 cm⁻¹. The total line strength of these five peaks were currently determined to be (4.18 ± 0.17) × 10⁻²⁰ cm molecule⁻¹ by simultaneous analysis of high-resolution time-resolved spectra of HCl and HO₂ in the Cl + CH₃OH reaction system. For precise determination of the concentration temporal profiles of OH and HO₂ radicals, multiple measurements of time-resolved dual-comb spectra with different spectral sampling spacings were conducted to increase the spectral sampling points of the rotationally resolved infrared absorption spectra of OH and HO₂. **Figures S3 and S4** show the interleaved dual-comb spectra of OH and HO₂, respectively. These spectra were curve-fitted with the multipeak Voigt function to obtain the integrated absorption area of each absorption transition, which can be further used to derive the time-dependent concentrations of OH and HO₂. Based on the well-determined line strengths of OH and HO₂ and their effective absorption path, the time-dependent concentration of each radical can be accurately determined with an uncertainty less than 10%. Moreover, the overall systematic errors could be reduced

through simultaneous measurements of OH and HO₂ and using the same effective absorption path.

To further evaluate the kinetics of the radical–radical reaction between OH and HO₂, over 24 measurements in six sets were conducted under the water-free conditions with the initial concentrations of hydrogen peroxide [H₂O₂]₀ ≈ 9.7 × 10¹⁴ and 1.9 × 10¹⁵ molecules cm⁻³; photolysis energies E₂₄₈ nm = 19.0, 28.5, and 38.0 mJ cm⁻²; and the total pressure P_T ≈ 30.3 Torr at 296 K. A summary of experimental conditions of each experimental set is listed in **Table S1**. **Figure 3** shows the comparison of the measured and simulated time traces of OH and HO₂ under various conditions. The concentration temporal profiles of OH were obtained by analyzing the time-resolved spectra of the OH X²Π₃/₂ (1 ← 0) P(4.5)f transition at 3407.612 cm⁻¹, and the HO₂ time traces were obtained by analyzing the time-resolved spectra of five absorption peaks near 1123 cm⁻¹. All concentration profiles were simultaneously analyzed using the kinetic model, taking into account the reactions of OH + H₂O₂, OH + HO₂, and the self-reactions of OH and HO₂, as listed in **Table 1**. For these experiments carried out at 30.3 Torr and 296 K, the diffusion rates of OH and HO₂ were estimated below 10 s⁻¹ and could be neglected in the kinetic analysis. In order to fit all concentration time traces of OH and HO₂ obtained at varied conditions with the kinetic simulations, the rate coefficient of the reaction OH + HO₂ was evaluated to be 1.10 × 10⁻¹⁰ cm³
molecule⁻¹ s⁻¹ at 296 K. Moreover, an additional two sets of experiments were carried out by determining the OH concentrations using another transition near 3484 cm⁻¹ and measurement of HO₂ concentrations with the spectra near 1123 cm⁻¹ to double confirm the proposed results, as shown in Figure S5. According to the kinetic simulations, the 5% error of the measured k<sub>OH+HO₂</sub> could cause the uncertainty of 5% on the determination of k<sub>OH+HO₂</sub>. The influences on k<sub>OH+HO₂</sub> caused by the possible errors of the self-reactions of OH and HO₂ (30%) were also considered and estimated to be less than 1%. In addition, by simultaneous analysis of the OH and HO₂ time traces obtained under different experimental conditions, the uncertainty of k<sub>OH+HO₂</sub> caused by the errors of the line strengths of OH (7%) and HO₂ (4%), spectral analysis (4%) as well as the effective absorption path (7%) was evaluated to be ~9%. Considering all possible errors, the overall uncertainty of k<sub>OH+HO₂</sub> was estimated to be ~11% and the rate coefficient k<sub>OH+HO₂</sub> was hence obtained to be (1.10 ± 0.12) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 296 K, which is in excellent agreement with the result reported by Assaf et al.,⁶ but it is ~10 times larger than the most recent experimental value from Speak et al.⁵

Moreover, experiments in the presence of water were also conducted in this work to explore the possible enhancement effect on the rate coefficient of the reaction OH + HO₂. By employing rotationally resolved infrared absorption spectra, the concentrations of both H₂O₂ and H₂O in the reactor were calibrated and measured directly. Figure 4 shows the

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Temporal concentration profiles of OH (red open symbol) and HO₂ (blue open symbol) obtained at the condition in the presence of H₂O. The concentration profiles were measured after 248 nm irradiation of the flowing mixture of H₂O₂/H₂O/N₂ ([H₂O₂]₀ = 1.41 × 10¹⁵ molecules cm⁻³, [H₂O]₀ = 6.32 × 10¹⁶ molecules cm⁻³, P_T = 31.6 Torr, 296 K) with the photolysis energy of 33.3 mJ cm⁻². The solid curves represent the simulated profiles derived using the kinetic model, as shown in Table 1, with k<sub>OH+HO₂</sub> = 1.1 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The long-dashed and short-dashed lines represent the simulated profiles derived using the kinetic model, as shown in Table 1, with k<sub>OH+HO₂</sub> = 0.2 × 10⁻¹⁰ and 2.0 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively.</figcaption></figure>

concentration profiles of OH and HO₂ which were measured after 248 nm irradiation of the flowing mixture of H₂O₂/H₂O/N₂ ([H₂O₂]₀ = 1.41 × 10¹⁵ molecules cm⁻³, [H₂O]₀ = 6.32 × 10¹⁶ molecules cm⁻³, P_T = 31.6 Torr, 296 K) with the photolysis energy of 33.3 mJ cm⁻². The comparison of measured time traces and simulated profiles derived using the kinetic model with different k<sub>OH+HO₂</sub> values is also displayed in

**Figure 4.** The long-dashed, solid, and short-dashed curves represent the simulated profiles derived using the kinetic model, as shown in Table 1, with k<sub>OH+HO₂</sub> = 0.2 × 10⁻¹⁰, 1.1 × 10⁻¹⁰, and 2.0 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively. Because the yields of HO₂ are strongly sensitive to the value of k<sub>OH+HO₂</sub> in the kinetic simulations, k<sub>OH+HO₂</sub> can be clearly determined. The measured OH and HO₂ time traces were both fitted well with the kinetic simulations using the rate coefficient k<sub>OH+HO₂</sub> of 1.10 × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, indicating that the rate coefficient of the reaction OH + HO₂ would not be enhanced at the conditions in the presence of water.

<figure><img src="image_14.png" /><figcaption>**Figure 5.** Comparison of the experimental results of rate coefficient k<sub>OH+HO₂</sub> at 295–308 K. A summary of methods, conditions, and results of each experiment is listed in Table S2.</figcaption></figure>

**Figure 5** displays the comparison of the rate coefficient k<sub>OH+HO₂</sub> obtained from different experiments at near room temperature,<sup>6–14</sup> and the detailed descriptions of these experiments are summarized in Table S2. The rate coefficients k<sub>OH+HO₂</sub> are displayed in the range of (0.1–1.2) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, and the maximum discrepancy of 13.5σ for the k<sub>OH+HO₂</sub> is observed between the two more recent experiments which were respectively conducted by Assaf et al.<sup>6</sup> and Speak et al.<sup>7</sup> Owing to the high reactivity of OH radicals and indirect measurements of HO₂ radicals, many studies expended considerable effort on calibration and estimation of radical concentrations through various methods and employed multiple reactions to perform cross comparisons. Each experiment would have its systematic errors, and the additional uncertainty could be increased through the indirect determination based on kinetics of multiple reactions, because each reaction might be carried out using different systems in different groups.

In conclusion, we have accomplished the accurate measure-
ment of the kinetics of the OH + HO₂ radical–radical reaction
through direct determination of precursor and radical
concentrations in the reaction system of the 248 nm photolysis
of hydrogen peroxide. Employing synchronized two-color dual-
comb spectroscopy, multiple species, including H₂O₂, H₂O,
OH, and HO₂, can be directly quantified based on the
measured high-resolution infrared absorption spectra. Most
importantly, all key parameters such as the rate coefficient of
the reaction OH + H₂O₂ and absolute line strengths of H₂O₂, OH, and HO₂ were also measured using the same experimental system to reduce the systematic errors and to avoid unknown uncertainty. The water-free rate coefficient for the OH + HO₂ reaction was determined to be (1.10 ± 0.12) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 296 K, and no enhancement in the rate coefficient was observed in the experiments with the addition of water. This work as an independent experiment that was implemented based on direct quantitation of all key reactants and careful kinetic evaluations is important and essential for reassessing the rate coefficient of the reaction between OH and HO₂. Moreover, the proposed approach, which is capable of achieving direct measurement of precursor, radical, and product concentrations in the reactor, opens up opportunities for accurate kinetic studies of the complex radical–radical reactions.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.4c00494.

Description of previous investigations of the OH + HO₂ reaction, schematic of the experimental setup, difference absorbance spectra of OH and HO₂ with fitted Voigt curves, comparison of the measured and the simulated temporal profiles of OH and HO₂, a summary of experimental conditions of each experimental set, and a summary of different experiments determining the rate coefficient of the OH + HO₂ reaction (PDF)

Transparent Peer Review report available (PDF)

## AUTHOR INFORMATION

### Corresponding Author

Pei-Ling Luo - Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106319, Taiwan; orcid.org/0000-0002-5914-5234; Email: pllluo@gate.sinica.edu.tw

### Authors

I-Yun Chen - Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106319, Taiwan; Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

Che-Wei Chang - Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106319, Taiwan; Molecular Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, 11529 Taipei, Taiwan; International Graduate Program of Molecular Science and Technology, National Taiwan University, 10617 Taipei, Taiwan

Christa Fittschen - University Lille, CNRS, UMR 8522, PC2A-Physicochimie des Processus de Combustion et de l'Atmosphère, F-59000 Lille, France; orcid.org/0000-0003-0932-432X

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.jpclett.4c00494

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This project is supported by National Science and Technology Council, Taiwan (Grant No. 112-2112-M-001-068). C.

Fittschen and P.-L. Luo thank NSTC and MESR for financial support through PHC Orchid project (Grant No. 112-2927-I-001-503/No. 48265 NH) and CNRS for financial support through IEA (Project No. 00619).

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