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.

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 kOH+HO2 is determined to be (1.10 ± 0.12) × 10-10 cm3 molecule-1 s-1 at 296 K. Moreover, we explore the kinetics of this reaction under conditions in the presence of water, but no enhancement in the kOH+HO2 can be observed. This work as an independent experiment plays a crucial role in revisiting this prototypical radical-radical reaction.

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.

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.

Direct gas-phase formation of formic acid through reaction of Criegee intermediates with formaldehyde.

Ozonolysis of isoprene is considered to be an important source of formic acid (HCOOH), but its underlying reaction mechanisms related to HCOOH formation are poorly understood. Here, we report the kinetic and product studies of the reaction between the simplest Criegee intermediate (CH2OO) and formaldehyde (HCHO), both of which are the primary products formed in ozonolysis of isoprene. By utilizing time-resolved infrared laser spectrometry with the multifunctional dual-comb spectrometers, the rate coefficient kCH2OO+HCHO is determined to be (4.11 ± 0.25) × 10-12 cm3 molecule-1 s-1 at 296 K and a negative temperature dependence of the rate coefficient is observed and described by an Arrhenius expression with an activation energy of (-1.81 ± 0.04) kcal mol-1. Moreover, the branching ratios of the reaction products HCOOH + HCHO and CO + H2O + HCHO are explored. The yield of HCOOH is obtained to be 37-54% over the pressure (15-60 Torr) and temperature (283-313 K) ranges. The atmospheric implications of the reaction CH2OO + HCHO are also evaluated by incorporating these results into a global chemistry-transport model. In the upper troposphere, the percent loss of CH2OO by HCHO is found by up to 6% which can subsequently increase HCOOH mixing ratios by up to 2% during December-January-February months.

Dimeric Product of Peroxy Radical Self-Reaction Probed with VUV Photoionization Mass Spectrometry and Theoretical Calculations: The Case of C2H5OOC2H5.

Organic peroxy radicals (RO2) as key intermediates in tropospheric chemistry exert a controlling influence on the cycling of atmospheric reactive radicals and the production of secondary pollutants, such as ozone and secondary organic aerosols (SOA). Herein, we present a comprehensive study of the self-reaction of ethyl peroxy radicals (C2H5O2) by using advanced vacuum ultraviolet (VUV) photoionization mass spectrometry in combination with theoretical calculations. A VUV discharge lamp in Hefei and synchrotron radiation at the Swiss Light Source (SLS) are employed as the photoionization light sources, combined with a microwave discharge fast flow reactor in Hefei and a laser photolysis reactor at the SLS. The dimeric product, C2H5OOC2H5, as well as other products, CH3CHO, C2H5OH and C2H5O, formed from the self-reaction of C2H5O2 are clearly observed in the photoionization mass spectra. Two kinds of kinetic experiments have been performed in Hefei by either changing the reaction time or the initial concentration of C2H5O2 radicals to confirm the origins of the products and to validate the reaction mechanisms. Based on the fitting of the kinetic data with the theoretically calculated results and the peak area ratios in the photoionization mass spectra, a branching ratio of 10 ± 5% for the pathway leading to the dimeric product C2H5OOC2H5 is measured. In addition, the adiabatic ionization energy (AIE) of C2H5OOC2H5 is determined at 8.75 ± 0.05 eV in the photoionization spectrum with the aid of Franck-Condon calculations and its structure is revealed here for the first time. The potential energy surface of the C2H5O2 self-reaction has also been theoretically calculated with a high-level of theory to understand the reaction processes in detail. This study provides a new insight into the direct measurement of the elusive dimeric product ROOR and demonstrates its non-negligible branching ratio in the self-reaction of small RO2 radicals.

Kinetics and pressure-dependent HOx yields of the reaction between the Criegee intermediate CH2OO and HNO3.

The reaction of Criegee intermediates with nitric acid (HNO3) plays an important role for removal of Criegee intermediates as well as in oxidation of atmospheric HNO3 because of its fast reaction rate. Theoretical prediction suggests that the product branching ratios of the reaction of the simplest Criegee intermediate CH2OO with HNO3 are strongly pressure dependent and the CH2OO may be catalytically converted to OH and HCO radicals by HNO3. The direct quantification of HOx radicals formed from this reaction is hence crucial to evaluate its atmospheric implications. By employing mid-infrared multifunctional dual-comb spectrometers, the kinetics and product yields of the reaction CH2OO + HNO3 are investigated. A pressure independent rate coefficient of (1.9 ± 0.2) × 10-10 cm3 molecule-1 s-1 is obtained under a total pressure of 6.3-58.6 Torr at 296 K. The product branching ratios are derived by simultaneous determination of CH2OO, formaldehyde (CH2O), OH and HO2 radicals. At the total pressure of 12.5 Torr, the yield for the formation of NO2 + CH2O + HO2 is 36% and only 3.2% for OH + CH2(O)NO3, whereas the main remainder may be thermalized nitrooxymethyl hydroperoxide (NMHP, NO3CH2OOH). Additionally, the fractional yields of both the OH and HO2 product channels are decreased by a factor of roughly 2 from 12 to 60 Torr, indicating that there is almost no catalytic conversion of CH2OO to the OH radicals in the presence of HNO3.

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 HOx 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.

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 kHCl = (4.8 ± 0.4) × 10-11 cm3 molecule-1 s-1, is comparable to the only literature value kHCl = (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.

Profiling the Resident and Infiltrating Monocyte/Macrophages during Rejection following Kidney Transplantation.

Immune tolerance research is essential for kidney transplantation. Other than antibody and T cell-mediated immune rejection, macrophage-mediated innate immunity plays an important role in the onset phase of transplantation rejection. However, due to the complexity of the kidney environment as well as its diversity and low abundance, studies pertaining to monocyte/macrophages in kidney transplantation require further elucidation. In this study, kidney samples taken from healthy human adults and biopsy specimens from patients undergoing rejection following kidney transplantation were analysed and studied. By conducting a single-cell RNA analysis, the type and status of monocyte/macrophages in kidney transplantation were described, in which monocyte/macrophages were observed to form two different subpopulations: resident and infiltrating monocyte/macrophages. Furthermore, previously defined genes were mapped to all monocyte/macrophage types in the kidney and enriched the differential genes of the two main subpopulations using gene expression databases. Considering that various cases of rejection may be of the monocyte/macrophage type, the present data may serve as a reference for studies regarding immune tolerance following kidney transplantation.

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.

Fast molecular fingerprinting with a coherent, rapidly tunable dual-comb spectrometer near 3 µm.

Molecular spectroscopy in the mid-infrared spectral range is expected to present fingerprints of chemical species that provide a unique identification with adequate sensitivity. Dual-comb spectroscopy, a promising successor to conventional Fourier-transform spectroscopy, enables rapid molecular fingerprinting with high-resolution and high-precision. Here, we report gas-phase transmittance and dispersion spectra by employing a rapidly tunable dual-comb spectrometer based on electro-optic frequency combs near 3 µm. The tunable spectral range >550 cm-1 and adjustable optical resolutions from a few hundred MHz to a few GHz were demonstrated. With the capabilities of wavelength agility and easily adjustable comb-mode spacing, the spectra of nitrous oxide and its isotopes were measured with Doppler-limited resolution. Furthermore, the N2-broadened spectra of acetylene were recorded to derive the collisional broadening coefficient. Fast acquisitions with adequate spectral resolution were also achieved within sub-microseconds. Both precision spectral metrology and fast molecular sampling could be implemented by employing the high-flexible dual-comb spectrometer.

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

The reaction of the simplest Criegee intermediate CH2OO with NO2 is considered to be important in atmospheric chemistry because of its prospective contribution to the decay of CH2OO and the additional source of NO3, hence its effects on the NOx and HOx cycles. The reported rate coefficients of this reaction varied by a factor of 5. Employing a cw quantum-cascade laser with wavelength near 11 µm coupled with Herriott mirrors to probe CH2OO sensitively, we investigated in detail the reaction CH2OO + NO2 under total pressure 5.9-9.7 Torr at 298 K and reported a rate coefficient kNO2 = (1.0 ± 0.2) × 10-12 cm3 molecule-1 s-1, about one-seventh the value determined with direct measurements of CH2OO by Weltz et al. This smaller rate coefficient implies that the title reaction contributes to the atmospheric source of NO3 and the decay of CH2OO much less than previously conceived.

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. The spectra were recorded with a tunable cw external-cavity quantum-cascade laser (EC-QCL) system coupled with a Herriott multipass absorption cell. Over one thousand lines were assigned to determine the rovibrational parameters of the ν6 band. Rotational parameters A', B', and C' and higher-order parameters ΔJ, ΔJK, ΔK, δJ, and δK were determined precisely. Rotational perturbations for Ka' = 3, Ka' = 6, and Ka' ≥ 11 on high-J levels were observed. In particular, for Ka' = 6, frequency shifts as a function of J' showed an avoided crossing; the shifts were analyzed in terms of a Coriolis interaction between modes ν6 and ν8 of CH2OO. According to the distinct temporal profiles, lines of a hot band near 899.5 cm-1 were also identified.

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

The Criegee intermediates, carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play important roles in atmospheric chemistry. The two conformers of CH3CHOO exhibit distinct reactivity toward several atmospheric species, but a distinct conformer-specific probe is challenging because ultraviolet and infrared absorption bands of syn- and anti-CH3CHOO overlap at low-resolution. Employing a quantum-cascade laser and a Herriott cell, we recorded the O-O stretching bands of CH2OO and syn-CH3CHOO in region 880-932 cm-1 at resolution 0.0015 cm-1. In addition to completely resolved vibration-rotational lines of CH2OO extending over 50 cm-1, some spectral lines associated with hot bands were identified. Spectral lines solely due to syn-CH3CHOO were also identified. Probing these lines, we determined the rate coefficient for the self-reaction of syn-CH3CHOO to be kself = (1.6 ± 0.60.5) × 10-10 cm3 molecule-1 s-1, about twice that of CH2OO.

Mid-infrared dual-comb spectroscopy with electro-optic modulators.

Absorption spectroscopy of fundamental ro-vibrational transitions in the mid-infrared region provides a powerful tool for studying the structure and dynamics of molecules in the gas phase and for sensitive and quantitative gas sensing. Laser frequency combs permit novel approaches to perform broadband molecular spectroscopy. Multiplex dual-comb spectroscopy without moving parts can achieve particularly high speed, sensitivity and resolution. However, achieving Doppler-limited resolution in the mid-infrared still requires overcoming instrumental challenges. Here we demonstrate a new approach based on difference-frequency generation of frequency-agile near-infrared frequency combs that are produced using electro-optic modulators. The combs have a remarkably flat intensity distribution, and their positions and line spacings can be freely selected by simply dialing a knob. Using the proposed technique, we record, in the 3-µm region, Doppler-limited absorption spectra with resolved comb lines within milliseconds, and precise molecular line parameters are retrieved. Our technique holds promise for fast and sensitive time-resolved studies of, for example, trace gases.

Precision frequency metrology of helium 2(1)S(0)→2(1)P(1) transition.

We report a precision frequency measurement of the (4)He 2(1)S(0)→2(1)P(1) transition at 2058 nm. The saturated absorption spectroscopy is performed in a rf discharge sealed-off cell with a volume Bragg grating-based Tm:Ho:YLF laser. The absolute transition frequency measured using a fiber optical frequency comb is 145 622 892 822 (183) kHz with a relative uncertainty of 1.3×10(-9). Our result is ten times more precise than current best theoretical calculations and is in reasonable agreement with the calculated values. However, the ionization energy of the 2(1)P(1) state, derived from our result and other precisely measured transitions, shows a discrepancy of approximately 3.5σ with the most precise atomic theory. We have also determined the isotope shift between (3)He and (4)He to be 4248.7 (5.3) MHz, which is more precise than the previous measurement by one order of magnitude.