Identification and Self-Reaction Kinetics of Criegee Intermediates syn-CH3CHOO and CH2OO via High-Resolution Infrared Spectra with a Quantum-Cascade Laser

Detailed mechanism and kinetics of reactions of anti- and syn-CH3CHOO with HC(O)OH: infrared spectra of conformers of hydroperoxyethyl formate

The reaction of CH3CHOO with HC(O)OH has a large rate coefficient so that it might play a significant role in the formation of secondary organic aerosols (SOA) in the atmosphere. We investigated the detailed mechanism and kinetics of the reactions of Criegee intermediate anti- and syn-CH3CHOO with HC(O)OH with a step-scan Fourier-transform infrared spectrometer by recording time-resolved absorption spectra of transient species and end products produced upon irradiation at 308 nm of a flowing mixture of CH3CHI2/O2/HC(O)OH at 298 K and 60 Torr. Thirteen bands of hydroperoxyethyl formate [HC(O)OCH(CH3)OOH, HPEF], the hydrogen-transferred adduct of CH3CHOO and HC(O)OH, were observed. Careful analysis deconvoluted these bands into absorption of three conformers of HPEF: a transient HPEF (P2*/P3*), a more stable open-form HPEF (mainly P2), and a stable intramolecularly hydrogen-bonded HPEF (mainly P1). At a later period, the end-product formic acetic anhydride [CH3C(O)OC(O)H, FAA], a dehydrated product of HPEF, was observed; this end-product is the same as that observed in CH2OO + CH3C(O)OH. Theoretical calculations on the reaction pathway scheme were performed to elucidate these reaction paths. Syn-CH3CHOO + HC(O)OH produced conformers P2*/P3* initially, followed by conversion to conformers P2, whereas anti-CH3CHOO + HC(O)OH produced conformers P2 and P1 directly. We derived a rate coefficient for the reaction CH3CHOO + HC(O)OH to be k = (2.1 ± 0.7) × 10-10 cm3 molecule-1 s-1 at 298 K and 40-80 Torr; the rate coefficient appeared to show insignificant conformation-specificity. We also found that FAA was produced mainly from the dehydration of the open-form HPEF (P2) with a rate coefficient k = (1420 ± 70) s-1; the intramolecularly hydrogen-bonded HPEF (P1) is stable.

Kinetics of the Simplest Criegee Intermediate Reaction with Water Vapor: Revisit and Isotope Effect

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH2OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH2OO can be described as d[CH2OO]/dt = -kobs[CH2OO]; kobs = k0 + k1[water] + k2[water]2 + k3[water]3; k1 = (4.22 ± 0.48) × 10-16 cm3 s-1, k2 = (10.66 ± 0.83) × 10-33 cm6 s-1, k3 = (1.48 ± 0.17) × 10-50 cm9 s-1 at 298 K and 300 Torr with the respective Arrhenius activation energies of Ea1 = 1.8 ± 1.1 kcal mol-1, Ea2 = -11.1 ± 2.1 kcal mol-1, Ea3 = -17.4 ± 3.9 kcal mol-1. The contribution of the k3[water]3 term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H2O and D2O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k1) = k1(H2O)/k1(D2O) = 1.30 ± 0.32; similarly, KIE(k2) = 2.25 ± 0.44 and KIE(k3) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N2 buffer gas) with those under 600 Torr, a weak pressure effect of k3 was found. From quantum chemistry calculations, we found that the CH2OO + 3H2O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH2OO(H2O) + (H2O)2 and CH2OO(H2O)2 + H2O pathways will dominate in the three water reaction.

Infrared Characterization of the Products of the Reaction between the Criegee Intermediate CH3CHOO and HCl

The rapid reactions between Criegee intermediates and hydrogen halides play important roles in atmospheric chemistry, particularly in the polluted urban atmosphere. Employing a step-scan Fourier transform spectrometer, we recorded infrared absorption spectra of transient species and end products of the reaction CH3CHOO + HCl in a flowing mixture of CH3CHI2/HCl/O2/N2 irradiated at 308 nm. Bands at 1453.6, 1383.7, 1357.9, 1323.8, 1271.8, 1146.2, 1098.2, 1017.5, 931.5, and 847.0 cm-1 were observed and assigned to the anti-conformer of chloroethyl hydroperoxide (anti-CEHP or anti-CH3CHClOOH). In addition, absorption bands of H2O and acetyl chloride [CH3C(O)Cl, at 1819.1 cm-1] were observed; some of them were produced from the secondary reactions of CH3CHClO + O2 → CH3C(O)Cl + HO2 and OH + HCl → H2O + Cl, according to temporal profiles of H2O and CH3C(O)Cl. These secondary reactions are conceivable because the nascent formation of CH3CHClO + OH via decomposition of internally excited CEHP was predicted by theory, and both HCl and O2 are major species in the system. The nascent formation of CH3CHClO + OH appears to be more important than that of CH3C(O)Cl + H2O, consistent with theoretical predictions. By adding methanol to deplete some anti-CH3CHOO, we observed only anti-CEHP with a reduced proportion; this observation indicates that the conversion from syn-CEHP, expected to be produced from syn-CH3CHOO + HCl, to anti-CEHP is facile. We also estimated the overall rate coefficient of the reaction syn-/anti-CH3CHOO + HCl to be kHCl = (2.7 ± 1.0) × 10-10 cm3 molecule-1 s-1 at ∼70 Torr and 298 K; this rate coefficient is about six times the only literature value kHClsyn = (4.77 ± 0.95) × 10-11 cm3 molecule-1 s-1 reported for syn-CH3CHOO + HCl by Liu et al., indicating that anti-CH3CHOO reacts with HCl much more rapidly than syn-CH3CHOO.

Infrared Characterization of the Products and Rate Coefficient of the Reaction between Criegee Intermediate CH2OO and HNO3

The reactions of Criegee intermediates with HNO₃ are important in the polluted urban atmosphere because of their large rate coefficients and the significant concentration of HNO₃. Employing a step-scan Fourier-transform spectrometer, we recorded infrared spectra of transient species and end products in the reaction CH₂OO + HNO₃ upon irradiation of a flowing mixture of CH₂I₂/HNO₃/N₂/O₂ at 308 nm. Eight bands at 1686, 1426, 1348, 1294, 1052, 965, 891, and 825 cm^-1 were assigned to the absorption of the adduct nitrooxymethyl hydroperoxide (NMHP, NO₃CH₂OOH). Additional products from two dissociation channels were observed. Four bands at 1709, 1325, 1276, and 886 cm^-1 were assigned to H₂C(O)ONO₂ (with coproduct OH), produced from the fission of the O-O bond of internally hot NMHP (NMHP*). Simultaneous detection of H₂CO (1746 cm^-1), NO₂ (1617 cm^-1), and HO₂ (1392 and 1098 cm^-1) indicated a direct cleavage of the N-OC and C-OO bonds of NMHP*. The relative yields of these three channels in pressure range 10-150 Torr were estimated. At 10 Torr, the absorption of internally excited HNO₃ near 885 and 1320 cm^-1 was also detected at an early stage of the reaction. We investigated also the rate coefficient of the reaction CH₂OO + HNO₃ by probing the temporal profiles of the formation of NMHP and NO₂ under total pressures of 40 and 70 Torr at 298 K. The rate coefficient k_HNO3 = (2.4 ± 0.4) × 10^-10 cm³ molecule^-1 s^-1 is less than half the only literature value, (5.4 ± 1.0) × 10^-10 cm³ molecule^-1 s^-1, reported by Foreman et al. (Angew. Chem. Int. Ed. 2016, 55, 10419-10422).

Mechanism and kinetics of the reaction of the Criegee intermediate CH2OO with acetic acid studied using a step-scan Fourier-transform IR spectrometer

Acetic acid, CH₃C(O)OH, plays an important role in the acidity of the troposphere. The reactions of Criegee intermediates with CH₃C(O)OH have been proposed to be a potential source of secondary organic aerosol in the atmosphere. We investigated the detailed mechanism and kinetics of the reaction of the Criegee intermediate CH₂OO with CH₃C(O)OH. The time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH₂I₂/O₂/CH₃C(O)OH at 298 K were recorded using a step-scan Fourier-transform infrared spectrometer. The decrease in the intensity of the bands of CH₂OO was accompanied by the appearance of bands near 886, 971, 1021, 1078, 1160, 1225, 1377, 1402, 1434, and 1777 cm^-1, assigned to the absorption of hydroperoxymethyl acetate [CH₃C(O)OCH₂OOH, HPMA], the hydrogen-transferred adduct of CH₂OO and CH₃C(O)OH. Two types of conformers of HPMA, an open form and an intramolecularly hydrogen-bonded form, were identified. At a later reaction period, bands of the open-form HPMA became diminished, and new bands appeared at 930, 1045, 1200, 1378, 1792, and 1810 cm^-1, assigned to formic acetic anhydride [CH₃C(O)OC(O)H, FAA], a dehydrated product of HPMA. The intramolecularly hydrogen-bonded HPMA is more stable. From the temporal profiles of HPMA and FAA, we derived a rate coefficient k = (1.3 ± 0.3) × 10^-10 cm³ molecule^-1 s^-1 for the reaction CH₂OO + CH₃C(O)OH to form HPMA and a rate coefficient k = 980 ± 40 s^-1 for the dehydration of the open-form HPMA to form FAA. Theoretical calculations were performed to elucidate the CH₂OO + CH₃C(O)OH reaction pathway and to understand the distinct reactivity of these two forms of HPMA.

Synchronized Two-Color Time-Resolved Dual-Comb Spectroscopy for Quantitative Detection of HOx Radicals Formed from Criegee Intermediates

Criegee intermediates, derived from ozonolysis of alkenes and recognized as key species in the production of nonphotolytic free radicals, play a crucial role in atmospheric chemistry. Here, we present a spectrometer based on synchronized two-color time-resolved dual-comb spectroscopy, enabling simultaneous spectral acquisitions in two molecular fingerprint regions near 2.9 and 7.8 μm. Upon flash photolysis of CH₂I₂/O₂/N₂ gas mixtures, multiple reaction species, involving the simplest Criegee intermediates (CH₂OO), formaldehyde (CH₂O), hydroxyl (OH) and hydroperoxy (HO₂) radicals are simultaneously detected with microsecond time resolution. The concentration of each molecule can be determined based on high-resolution rovibrational absorption spectroscopy. With quantitative detection and simulation of temporal concentration profiles of the targeted molecules at various conditions, the underlying reaction mechanisms and pathways related to the formation of the HO_x radicals, which can be generated from decomposition of initially energized and vibrationally excited Criegee intermediates, are explored. This approach capable of achieving multispectral measurements with simultaneously high spectral and temporal resolutions opens up the opportunities for quantification of transient intermediates and products, thus, enabling elucidation of complex reaction mechanisms.

Experimental and Computational Studies of Criegee Intermediate syn-CH3CHOO Reaction with Hydrogen Chloride

Hydrogen chloride (HCl) contributes substantially to the atmospheric Cl; both species could affect the composition of Earth's atmosphere and the fate of pollutants. Here, we present the kinetics study for syn-CH₃CHOO reaction with HCl using experimental measurement and theoretical calculations. The experiment was conducted in a flow tube reactor at a pressure of 10 Torr and temperatures ranging from 283 to 318 K by using the OH laser-induced fluorescence (LIF) method. Transition-state theory and quantum chemistry calculations with QCISD(T) were used to calculate the rate coefficients. Weak negative temperature dependence was observed with a measured activation energy of -(2.98 ± 0.12) kcal mol^-1 and a calculated zero-point-corrected barrier energy of -3.29 kcal mol^-1. At 298 K, the rate coefficient was measured to be (4.77 ± 0.95) × 10^-11 cm³ s^-1, which was in reasonable agreement with 2.2 × 10^-11 cm³ s^-1 from the theoretical calculation.

Kinetics of CH2OO and syn-CH3CHOO reaction with acrolein

The kinetics for the reactions of CH2OO and syn-CH3CHOO with acrolein, a typical unsaturated aldehyde in the atmosphere, were studied in a flash photolysis flow reactor using the OH laser-induced fluorescence (LIF) method. The bimolecular reaction rate coefficients were measured at temperatures ranging from 281 to 318 K, and pressures ranging from 5 to 200 Torr. No obvious dependence of the rate coefficients on pressure was observed under the current experimental conditions. Both reactions exhibit negative temperature-dependence, with an activation energy of (-1.70 ± 0.19) and (-1.47 ± 0.24) kcal mol-1 for CH2OO and syn-CH3CHOO reacting with acrolein, derived from the Arrhenius equation. At 298 K, the measured rate coefficients for CH2OO/syn-CH3CHOO + acrolein reactions are (1.63 ± 0.19) × 10-12 cm3 s-1 and (1.17 ± 0.16) × 10-13 cm3 s-1, respectively. The rate coefficient of the former reaction is in reasonable agreement with a recent theoretical result.

Infrared characterization of the products and the rate coefficient of the reaction between Criegee intermediate CH2OO and HCl

Reactions between Criegee intermediates and hydrogen halides might be significant, particularly in the polluted urban atmosphere, because of their large rate coefficients. Employing a Fourier-transform spectrometer in a step-scan mode or a continuous-scan mode, we recorded infrared spectra of transient species and end products in a flowing mixture of CH₂I₂/HCl/N₂/O₂ irradiated at 308 nm. Five bands near 823.2, 1061.1, 1248.4, 1309.2, and 1359.6 cm^-1 were observed and assigned to the gauche-conformer of chloromethyl hydroperoxide (CMHP, CH₂ClOOH). At a later time of the reaction, absorption bands of H₂O and formyl chloride (CHClO) at 1782.9 cm^-1 were observed; these species were likely produced from the secondary reactions of CH₂ClO + O₂→ CHClO + HO₂ and OH + HCl → H₂O + Cl according to temporal profiles of CMHP, H₂O, and CHClO; formation of CH₂ClO + OH via decomposition of internally excited CMHP was predicted by theory and both HCl and O₂ are major species in the system. We investigated also the rate coefficient of the reaction CH₂OO + HCl on probing CH₂OO with a continuous-wave infrared quantum-cascade laser absorption system under total pressure 5.2-8.2 torr at 298 K. The rate coefficient k_HCl = (4.8 ± 0.4) × 10^-11 cm³ molecule^-1 s^-1, is comparable to the only literature value k_HCl = (4.6 ± 1.0) × 10^-11 cm³ molecule^-1 s^-1 reported by Foreman et al.

Long-wave mid-infrared time-resolved dual-comb spectroscopy of short-lived intermediates

In this Letter, an electro-optic dual-comb spectrometer with a central tunable range of 7.77-8.22 µm is demonstrated to perform transient absorption spectroscopy of the simplest Criegee intermediate (CH₂OO), a short-lived species involved in many key atmospheric reactions, and its self-reaction product via comb-mode-resolved spectral sampling at microsecond temporal resolution. By combining with a Herriott-type flash photolysis cell, CH₂OO can be probed with a detection limit down to ∼1×10¹¹moleculescm^-3. Moreover, pressure broadening of CH₂OO absorption lines can be studied with spectrally interleaved dual-comb spectroscopy. This Letter holds promise for high-resolution precision measurements of transient molecules, especially for the study of large molecules in complex systems.

Dynamics of the reaction CH2I + O2 probed via infrared emission of CO, CO2, OH and H2CO

The reaction CH2I + O2 has been widely employed recently for the production of the simplest Criegee intermediate CH2OO in laboratories, but the detailed dynamics of this reaction have been little explored. Infrared emission of several products of this reaction, initiated on irradiation of CH2I2 and O2 (∼8 Torr) in a flowing mixture at 308 or 248 nm, was recorded with a step-scan Fourier-transform spectrometer; possible routes of formation were identified according to the observed vibrational distribution of products and published theoretical potential-energy schemes. Upon irradiation at 308 nm, Boltzmann distributions of CO (v ≤ 5, J ≤ 19) with an average vibrational energy of 32 ± 3 kJ mol-1 and OH (v ≤ 3, J ≤ 5.5) with an average vibrational energy of 29 ± 4 kJ mol-1 were observed and assigned to the decomposition of HCOOH* to form CO + H2O and OH + HCO, respectively. The broadband emission of CO2 was simulated with two vibrational distributions of average energies (91 ± 4) and (147 ± 8) kJ mol-1 and assigned to be produced from the decomposition of HCOOH* and methylene bis(oxy), respectively. Upon irradiation of samples at 248 nm, the emission of OH and CO2 showed similar distributions with slightly greater energies, but the distribution of CO (v ≤ 11, J ≤ 19) became bimodal with average vibrational energies of (23 ± 4) and (107 ± 29) kJ mol-1, and branching (56 ± 5) : (44 ± 5). The additional large-v component is assigned to be produced from a secondary reaction HCO + O2 to form CO + HO2; HCO is a coproduct of OH. The branching between CO and OH is (50 ± 5) : (50 ± 5) at 308 nm and (64 ± 5) : (36 ± 4) at 248 nm, consistent with the mechanism according to which an additional channel to produce CO opens at 248 nm. Highly internally excited H2CO was also observed. With O2 at 16 Torr, the extrapolated nascent internal distributions are similar to those with O2 at 8 Torr except for a slight quenching effect.

Simultaneous determination of transient free radicals and reaction kinetics by high-resolution time-resolved dual-comb spectroscopy

Quantitative determination of multiple transient species is critical in investigating reaction mechanisms and kinetics under various conditions. Dual-comb spectroscopy, a comb-laser-based multi-heterodyne interferometric technique that enables simultaneous achievement of broadband, high-resolution, and rapid spectral acquisition, opens a new era of time-resolved spectroscopic measurements. Employing an electro-optic dual-comb spectrometer with central wavelength near 3 µm coupled with a Herriott multipass absorption cell, here we demonstrate simultaneous determination of multiple species, including methanol, formaldehyde, HO₂ and OH radicals, and investigate the reaction kinetics. In addition to quantitative spectral analyses of high-resolution and tens of microsecond time-resolved spectra recorded upon flash photolysis of precursor mixtures, we determine a rate coefficient of the HO₂ + NO reaction by directly detecting both HO₂ and OH radicals. Our approach exhibits potential in discovering reactive intermediates and exploring complex reaction mechanisms, especially those of radical-radical reactions.

Kinetic Studies for the Reaction of syn-CH3CHOO with CF3CH═CH2

Hydrofluoroolefins (HFOs, C_xF_2x+1CH═CH₂) have great potential to replace hydrofluorocarbons (HFCs) as refrigerants. Here the kinetics for the reaction of syn-CH₃CHOO with CF₃CH═CH₂ (HFO-1243zf), the simplest of HFOs, have been studied in a flash photolysis flow reactor at a total pressure of 50 Torr, by using the OH laser-induced fluorescence (LIF) method. The bimolecular reaction rate coefficients were measured at temperatures ranging from 283 to 318 K. A weak positive temperature dependence was observed, with an activation energy of 1.41 ± 0.12 kcal mol^-1. At 298 K, the measured rate coefficient was (2.42 ± 0.51) × 10^-14 cm³ s^-1, in the vicinity of the previously reported upper limit value for the reaction of CH₂OO with CF₃CH═CH₂.

Temperature- and pressure-dependent rate coefficient measurement for the reaction of CH2OO with CH3CH2CHO

The rate coefficients of the CH₂OO + CH₃CH₂CHO reaction were studied at temperatures and pressures in the range of 283–318 K and 5–200 Torr.

Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH3CHOO

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH₃CHOO, has two conformers, syn- and anti-conformers. Syn-CH₃CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH₃CHOO was probed in real time with UV absorption spectroscopy at 278–318 K and 100–700 torr. We used water vapor as the scavenger of anti-CH₃CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH₃CHOO (at 300 torr), which increases from (67 ± 15) s⁻¹ at 278 K, (146 ± 31) s⁻¹ at 298 K, to (288 ± 81) s⁻¹ at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol⁻¹ and a weak pressure dependence for 100–700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH₃CHOO at higher pressures, which are more relevant to atmospheric conditions.

This work provides temperature dependent unimolecular rates of syn-CH₃CHOO at higher pressures.

Unimolecular Reaction Rate Measurement of syn-CH3CHOO

The unimolecular reactions of Criegee intermediates (CIs) are thought to be one of the significant sources of atmospheric OH radicals. However, stark discrepancies exist in the unimolecular reaction rate of the methyl-substituted CI CH₃CHOO, typically from ozonolysis of alkenes such as trans-2-butene, between the results of ozonolysis of alkene experiments and the up-to-date theoretical calculations. That no further progress has been made since the method that directly produces CIs in the laboratory was developed is mostly attributed to the existence of two conformers, syn- and anti-CH₃CHOO, and the methodological limitations of sensitive conformer-specific detection. We report a conformer-specific measurement of the unimolecular reaction rate of syn-CH₃CHOO by using a high-repetition-rate laser-induced fluorescence method. At 298 K, the observed value of 182 ± 66 s^-1 is in good agreement with recent theoretical calculations.

Temperature-Dependent Rate Coefficient for the Reaction of CH3SH with the Simplest Criegee Intermediate

The kinetics of the reaction of the simplest Criegee intermediate CH₂OO with CH₃SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17)   ×  10^-12 cm³ s^-1. The results exhibit a weak negative temperature dependence: the activation energy E_a ( k = Ae^{- E_a/ RT}) is -1.83 ± 0.05 kcal mol^-1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6  ×  10^-12 cm³ s^-1 at 298 K; E_a = - 2.80 kcal mol^-1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH₂OO with CH₃SH are compared to the reactions of CH₂OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH₂OO or CH₃SH.

Detailed mechanism and kinetics of the reaction of Criegee intermediate CH2OO with HCOOH investigated via infrared identification of conformers of hydroperoxymethyl formate and formic acid anhydride

Infrared spectra of hydroperoxymethyl formate indicate that the open-form conformer decomposes to formic-acid anhydride, but the hydrogen-bonded one does not.

Rate coefficient of the reaction CH2OO + NO2 probed with a quantum-cascade laser near 11 μm

Employing a cw quantum-cascade laser coupled with Herriott mirrors to probe CH₂OO, we report a rate coefficient k = (1.0 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for the reaction CH₂OO + NO₂ at 298 K, which is much smaller than literature values.

Absolute Infrared Absorption Cross Section of the Simplest Criegee Intermediate Near 1285.7 cm-1

The ν₄ fundamental of the simplest Criegee intermediate, CH₂OO, has been monitored with high-resolution infrared (IR) transient absorption spectroscopy under total pressures of 4-94 Torr. This IR spectrum provides an unambiguous identification of CH₂OO and is potentially useful to determine the number density of CH₂OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH₂OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 and 1286.4 cm^-1 (covering the Q branch), and then we converted this integrated absorbance to the absolute integral IR cross section of CH₂OO (for the Q branch); its absolute value is (3.7 ± 0.6) × 10^-19 cm·molecule^-1 or 2.2 ± 0.4 km·mol^-1. The whole rotational band (P, Q, and R branches) can be adequately simulated by using the precise spectroscopic parameters from the literature, yielding the absolute integral IR cross section (full ν₄ band) to be 19.2 ± 3.5 km·mol^-1. For a practical detection of CH₂OO, this work also reports the peak cross section as a function of total pressure (4-94 Torr O₂). At low pressure (≤4 Torr), where the pressure broadening is insignificant, the absorption cross section of the highest peak is (6.2 ± 0.9) × 10^-18 cm²·molecule^-1 (at the system line width of 0.004 cm^-1 fwhm).

High-resolution vibration–rotational spectra and rotational perturbation of the OO-stretching (ν6) band of CH2OO between 879.5 and 932.0 cm−1

We report the observation of a rotationally resolved ν₆ band associated with the OO-stretching mode of the simplest Criegee intermediate, CH₂OO, in the range of 879.5–932.0 cm⁻¹ (11.37–10.73 μm) at an optical resolution of 0.0015 cm⁻¹.