Continuous-wave Cavity Ring-down Spectroscopy Research Articles

Continuous wave cavity ringdown spectroscopy incorporating with an off-axis arrangement, white noise perturbation, and optical re-injection

We present an ultra-sensitive continuous wave cavity ringdown spectroscopy (cw-CRDS) spectrometer to record high resolution spectra of reactive radicals and ions in a pulsed supersonic plasma. The spectrometer employs a home-made external cavity diode laser as the tunable light source, with its wavelength modulated by radio-frequency white noise. The ringdown cavity with a finesse of ∼105 is arranged with an off-axis alignment. The combination of the off-axis cavity and the white-noise perturbed laser yields quasi-continuum laser-cavity coupling without the need of mode matching. The cavity is further incorporated with an extra multi-pass cavity for optical re-injection of light reflected off the master cavity, which significantly increases the throughput power of the high-finesse cavity. A fast switchable semiconductor optical amplifier is used to modulate the cw laser beam to square wave pulses and to initialize timing controlled ringdown events, which are synchronized to the plasma pulses with an accuracy of ∼3 µs. The performance and potential of the cw-CRDS spectrometer are illustrated and discussed, based on the high resolution near-infrared spectroscopic detection of trace 13C13C radicals generated in a pulsed supersonic C2H2/Ar plasma with a pulse duration of ∼50 µs.

The spatial density distribution of H2O2 in the effluent of the COST-Jet and the kINPen-sci operated with a humidified helium feed gas

Abstract Cold atmospheric plasma jets operated with a helium feed gas containing small admixtures of water vapour are excellent sources of H2O2 for direct biomedical applications. However, H2O2 is typically distributed non-uniformly throughout the effluent region, meaning the dosage received by a patient or substrate is dependent on their positioning relative to the plasma source. This study presents the spatial distribution of absolute H2O2 number densities in the effluent of two popular plasma jets, the COST-Jet and the kINPen-sci plasma jet, when operated with a humidified helium feed gas. The measurements were performed using continuous wave cavity ring-down spectroscopy with a tunable, mid-infrared laser. The H2O2 number density measured close to the jet nozzle is 2.3 × 1014 cm−3 for the kINPen-sci plasma jet and 1.4 × 1014 cm−3 for the COST-Jet. The average number density of H2O2 in the effluent of the kINPen-sci plasma jet is a factor of two higher than in the effluent of the COST-Jet. The distribution of H2O2 in the COST-Jet effluent is initially highly uniform and suggests negligible mixing of H2O2 with the ambient air up to 15 mm from the jet nozzle, although it is rapidly diluted at further distances. In the case of the kINPen-sci plasma jet, the number density of H2O2 has a more pronounced radial distribution close to the nozzle, while the mixing with the ambient air is more gradual at further distances from the nozzle. It is evident that a detailed understanding of the H2O2 production in the plasma source, as well as of the transport of H2O2 to the substrate through the effluent, is required in order to optimise the intended effects. This work serves to highlight the difference of the distinct spatial distribution of H2O2 in the effluent of both types of plasma jets when considering their direct application in biomedicine.

The localised density of H2O2 in the effluent of a cold atmospheric pressure plasma jet determined by continuous-wave cavity ring-down spectroscopy

Although the research on cold atmospheric pressure plasma jets and their applications is steadily growing, several questions remain open regarding fundamental aspects of how reactive species, such as hydrogen peroxide (H2O2), are generated in cold atmospheric pressure plasma jets, and how the composition of reactive species can be tailored for a specific purpose. Accordingly, absolute and spatially resolved distributions of the densities of reactive species in the effluent of cold atmospheric pressure plasma jets are required. In this work, a time efficient way to determine the local distribution of gas phase H2O2 in the effluent of a cold atmospheric-pressure plasma jet using continuous-wave cavity ring-down spectroscopy at a wavelength of 8.12 μm is presented. By a combination of an axial scan and of several radial distributions, the localised density distribution of H2O2 in the effluent of the kINPen-sci plasma jet was obtained. Therefore, the effective absorption length was determined from the evolution of the radial distributions as a function of the distance from the nozzle, which was 1.6 mm close to the nozzle of the plasma jet, and increased to approximately 5 mm at a distance of 10 mm from the nozzle. The maximum density of approximately 2 ⋅ 1014 cm−3 was found in the centre of the effluent close to the nozzle. From the presented localised density distribution, it can be concluded that H2O2 is significantly generated within the plasma zone of the plasma jet. This work presents an important step towards the understanding of formation and consumption mechanisms of biomedically relevant species in the plasma zone and the effluent of a cold atmospheric pressure plasma jet.

A Mid-Infrared Quantum Cascade Laser Ultra-Sensitive Trace Formaldehyde Detection System Based on Improved Dual-Incidence Multipass Gas Cell

Formaldehyde (HCHO) is a tracer of volatile organic compounds (VOCs), and its concentration has gradually decreased with the reduction in VOC emissions in recent years, which puts forward higher requirements for the detection of trace HCHO. Therefore, a quantum cascade laser (QCL) with a central excitation wavelength of 5.68 μm was applied to detect the trace HCHO under an effective absorption optical pathlength of 67 m. An improved, dual-incidence multi-pass cell, with a simple structure and easy adjustment, was designed to further improve the absorption optical pathlength of the gas. The instrument detection sensitivity of 28 pptv (1σ) was achieved within a 40 s response time. The experimental results show that the developed HCHO detection system is almost unaffected by the cross interference of common atmospheric gases and the change of ambient humidity. Additionally, the instrument was successfully deployed in a field campaign, and it delivered results that correlated well with those of a commercial instrument based on continuous wave cavity ring-down spectroscopy (R2 = 0.967), which indicates that the instrument has a good ability to monitor ambient trace HCHO in unattended continuous operation for long periods of time.

High-sensitivity methane monitoring based on quasi-fundamental mode matched continuous-wave cavity ring-down spectroscopy

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) is an important technical means to monitor greenhouse gases in atmospheric environment. In this paper, a CW-CRDS system is built to meet the needs of atmospheric methane monitoring. The problem of mode matching is explained from the perspective of transverse mode and longitudinal mode, and the influence of laser injection efficiency on measurement precision is further analyzed. The results of cavity ring-down time measurement show that the measurement precision is higher when the laser is coupled with the fundamental mode. In the experiment, DFB laser is used to calibrate the system with standard methane concentration, and the measurement residual is less than ±4 × 10−4 μs−1. The methane concentration in the air is monitored in real time for two days. The results show the consistency of the concentration changes over the two days, which further demonstrates the reliability of the system for the measurement of trace methane. By analyzing the influence of mode matching, it not only assists the adjustment of the optical path, but also further improves the sensitivity of the system measurement.

Rate Constant and Branching Ratio for the Reactions of the Ethyl Peroxy Radical with Itself and with the Ethoxy Radical

The self-reaction of the ethyl peroxy radical (C2H5O2) (1) has been studied using laser photolysis coupled to a selective time-resolved detection of two different radicals by continuous-wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared range: C2H5O2 was detected in the Ã-X̃ electronic transition at 7596 cm–1 and HO2 was detected in the 2ν1 vibrational overtone transition at 6638.2 cm–1. Radicals were generated from pulsed 351 nm photolysis of the C2H6/Cl2 mixture in the presence of O2. The reaction can proceed via a radical propagation channel, leading to two C2H5O radicals (1a) or to stable products (1b/1c). Because C2H5O radicals react subsequently with O2, leading to HO2, which in turn reacts rapidly with C2H5O2, knowledge of the branching ratio is indispensable for retrieving the rate constant. A strong disagreement exists about the rate constant and the branching ratio of (1) between the recommendation of the IUPAC (Atkinson et al., Atmos. Chem. Phys. 2006, 3625) or JPL (Publication 19-5, 2020) and a recent, more direct measurement (Noell et al., J. Phys. Chem. A 2010, 6983). The rate constant for the self-reaction has been found here to be k1 = (1.0 ± 0.2) × 10–13 cm3 s–1 with the branching fraction of the C2H5O radical channel being α = (0.31 ± 0.06), being in contradiction with the recommendations, but confirming the most recent measurement and indicating that the current recommendation for this reaction should be revised. The reaction of C2H5O2 with the ethoxy radical (C2H5O) can also be involved in the reaction mechanism, and its rate constant is also extracted from modeling. It is found to be k9 = (7 ± 1.5) × 10–12 cm3 s–1, two times slower than the only available measurement from Noell et al. J. Phys. Chem. A 2010, 6983.

Development of an in situ analysis system for methane dissolved in seawater based on cavity ringdown spectroscopy.

This paper reports the development of a compact in situ real-time concentration analysis system for methane dissolved in seawater by using a continuous-wave cavity ringdown spectroscopy (CRDS) technique. The miniaturized design of the system, including optical resonance cavity and control and data acquisition-analysis electronics, has a cylindrical dimension of 550 mm in length and 100 mm in diameter. Ringdown signal generation, data acquisition and storage, current driver, and temperature controller of the diode laser are all integrated in the miniaturized system circuits, with an electrical power consumption of less than 12 W. Fitting algorithms of the ringdown signal and spectral line are implemented in a digital signal processor, which is the main control chip of the system circuit. The detection sensitivity for methane concentration can reach 0.4 ppbv with an approximate averaging time of 240 s (or 4 min). Comparing the system's measurement of ambient air against a high-quality commercial CRDS instrument has demonstrated a good agreement in results. In addition, as a "proof of concept" for measuring dissolved methane, the developed instrument was tested in an actual underwater environment. The results showed the potential of this miniaturized portable instrument for in situ gas sensing applications.

The absorption spectrum and absolute absorption cross sections of acetylperoxy radicals, CH3C(O)O2 in the near IR

The Ã-X˜ electronic transition of acetylperoxy radicals (CH3C(O)O2) in the near-infrared was measured at 67hPa synthetic air in the spectral ranges from 6094 cm−1 to 6180 cm−1 and 6420 cm−1 to 6600 cm−1. CH3C(O)O2 radicals were generated by the pulsed photolysis of a acetaldehyde/Cl2/O2 mixture at 351 nm and subsequently measured by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). The absorption cross sections of eight discrete absorption lines were determined relative to the absorption cross section of HO2, which has previously been reported. The strongest absorption cross section was found at 6510.73 cm−1 and was determined to be (4.9 ± 2.5) × 10−20 cm2.

Random vibration-driven continuous-wave CRDS system for calibration-free gas concentration measurement.

Random vibrations were employed to pick up each monochromatic component in a continuous-wave cavity ringdown spectroscopy (CRDS) system using a bichromatic laser source. Light frequencies were selected within flat portions of an absorption profile to suppress the jitter in laser frequency during measurements. An interference effect caused by cavity length variations was suppressed by optimizing the initial fit point for each ringdown transient. The difference in exponential decay rates of two frequencies determined the gas mole fraction, and no calibration of empty cavity losses was necessary. Experiments on varying humidity were conducted, and the results agreed with the readings of a commercial hygrometer.

A Wide-Range and Calibration-Free Spectrometer Which Combines Wavelength Modulation and Direct Absorption Spectroscopy with Cavity Ringdown Spectroscopy.

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, τ0, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring τ0 in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 μm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm−1 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

Monitoring of ambient methane and carbon dioxide concentrations based on wavelength modulation-direct absorption spectroscopy

Wavelength modulation-direct absorption spectroscopy (WM-DAS) integrates the advantages of measuring absolute absorbance profile from calibration-free direct absorption spectrum (DAS) with the enhanced noise rejection and high sensitivity of wavelength modulation spectrum (WMS). This method can be used to precisely recover the crucial absorbance profile via the extraction of the characteristic frequency of the modulated transmitted light. In this paper, the WM-DAS method with non-calibration and high signal-to-noise ratio is integrated with a Herriott cell (about 128 m). Under the condition of atmospheric pressure and room temperature, the absorptance functions of two spectral lines of CO<sub>2</sub> (6330.821 cm<sup>–1</sup>) and CH<sub>4</sub> (6046.964 cm<sup>–1</sup>) in air are measured, and their standard deviations of spectral fitting residual are 5.6 × 10<sup>–5</sup> and 7 × 10<sup>–5</sup>, respectively. Subsequently, the concentration of CO<sub>2</sub> and CH<sub>4</sub> in air are monitored on-line by the WM-DAS method integrated with the Herriott cell, and compared with those by the highly sensitive continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results show that the measured results of the long optical path WM-DAS method are consistent with those by the CW-CRDS method, and the linear correlation between the two methods is above 0.99. The detection limit of CO<sub>2</sub> and CH<sub>4</sub> by the WM-DAS method are 170 ppb and 1.5 ppb respectively, which are slightly higher than those by the CW-CRDS. However, the measurement speed by WM-DAS is much higher than that by CW-CRDS, and possesses the advantages of simpler operation, lower environmental requirements, long-term stability, etc.

High precision calibration of spectral parameters of CO at 1567 nm based on wavelength modulation-direct absorption spectroscopy

<sec> Direct absorption spectrum (DAS) can be used to measure the molecular absorptivity function and determine the spectral parameters of the gas by fitting the measured absorptivity function. Wavelength modulation-direct absorption spectroscopy (WM-DAS) is based on DAS and combines with the idea of harmonic analysis in wavelength modulation spectrum (WMS). The measurement accuracy of absorptivity function can be effectively improved by using Fourier transform. In this paper, the absorptivity function of CO R5–R11 near infrared weak absorption line at 1567 nm is accurately reproduced by using the WM-DAS method combined with long optical path gas absorption cell at room temperature and low pressure. The standard deviation of the fitting residual reaches 3 × 10<sup>–5</sup>, and then the spectral parameters such as collision broadening, Dicke narrowing and speed-dependent collision broadening coefficients are measured in high precision.</sec><sec> These parameters are compared with the results from the high sensitivity continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results show that the signal-to-noise ratio of the absorptivity function measured by CW-CRDS is about 2.5 times that of the long-path WM-DAS, and the measured spectral parameters are highly consistent. The relative errors of the collision broadening coefficients obtained by using the Voigt profiles of the two methods are less than 1%. At the same time, the detection limit of CO at 1567 nm based on the WM-DAS method is about 80 ppb, and the corresponding absorption coefficient is 2 × 10<sup>–10</sup> cm<sup>–1</sup>, which is slightly higher than that from the CW-CRDS method. However, the WM-DAS method has the advantages of fast measurement speed, simple system and low cost, and is expected to provide a new method of measuring the weak absorption lines.</sec>

Influence of Spatial Inhomogeneity of Detector Temporal Responses on the Spectral Fidelity in Continuous Wave Cavity Ringdown Spectroscopy.

Due to their advantages of having a wide bandwidth, low cost, and being easy to obtain, traditional photodetectors (PDs) are being widely applied in measurements of transient signals. The spatial inhomogeneity of such PD temporal responses was measured directly to account for the PD spatial effect of decay rate due to poor alignment in continuous wave cavity ringdown spectroscopy (CW-CRDS) experiments. Based on the measurements of three PDs (i.e., model 1611 (Newport), model 1811 (Newport), and model PDA10CF-EC (Thorlabs)), all the temporal responses followed a tendency of declining first and then rising, and steady platforms existed for the last two PDs. Moreover, as we expected, the closer the PD center was, the faster the response. On the other hand, the initial shut-off amplitude generally reached a larger value for a faster temporal response. As a result, the spatial effect can strongly influence the spectral line shape and value, which will introduce more errors into the precise measurements of spectral parameters using the CRDS technique if this effect is not considered. The defined effective detection area (EDA) of the PDs, which was close to the active area given by manufacturers, was the key parameter that should be paid more attention by researchers. Therefore, the PD should be aligned perfectly to make sure that the EDA covers the laser spot completely.

Multicomponent gas detection based on concise CW-cavity ring-down spectroscopy with a bow-tie design.

Concise open-path continuous-wave cavity ring-down spectroscopy (CW-CRDS) with a bow-tie cavity structure is demonstrated in the single- and dual-optical-path experiments for multicomponent gas detection, e.g., greenhouse gas concentration evaluation in ambient air. Owing to its features of optical feedback suppression and small free spectral range (FSR), the bow-tie configuration shows its special advantages in the realization of both a compact arrangement and two counter-propagating non-interference optical paths. The minimum of the Allan deviation reaches 1.6×10-10 cm-1 for an integration time of 100s, corresponding to the noise equivalent absorption coefficient of 1.6×10-9 cm-1 Hz-1/2. The detection sensitivity of methane is deduced to be 0.9 ppbv with its absorption cross section of 1.48×10-20 cm2/molecule in the 512 decays averaging mode. A wavelength-correction method is proposed to reduce by about 30% the uncertainty in the measurements caused by the deviation in the wavelength resonance between incident laser and ring-down cavity. The concentrations of greenhouse gases in ambient air are measured by the open-path CW-CRDS with the uncertainties of 0.02, 100, and 10 ppmv for CH4, H2O, and CO2, respectively.

First detection of a key intermediate in the oxidation of fuel + NO systems: HONO

This paper reports the first online gas phase detection of absolute concentrations of HONO under engine relevant conditions, during the oxidation of an alkane in the presence of NOx. The detection was achieved at laboratory scale thanks to the coupling of a jet-stirred reactor to a continuous-wave Cavity Ring-Down Spectroscopy cell. The evidence of the formation of HONO was obtained by comparing measured cw-CRDS spectra with a literature one. The formation of HONO was simultaneously observed with the appearance of another nitrogen compound: NO2. This confirms that HONO could also be formed from NO2 under engine conditions.

Wavelength-scanned cavity ring down spectroscopy based on Fourier transform

Continuous wave cavity ring down spectroscopy (CW-CRDS) method with using cavity length scanning is ideal for accurately characterizing the low pressure spectra and measuring the small spectral parameters (such as the Dicke narrowing coefficient and the speed dependent collision broadening coefficient). However, the laser of any wavelength can be coupled to the cavity due to the cavity scan, so the spectral noise caused by the laser wavelength fluctuations cannot be ignored. This noise is non-uniformly distributed in the spectrum (especially on both wings on the spectral line) and is difficult to eliminate even with long-term averaging. Unlike the complex laser frequency locking techniques or the optical frequency combs or the better lasers, in this paper, a simple, easy to operate, fast wavelength-scanned CRDS method is proposed based on Fourier transform. The laser wavelength is continuously tuned across the absorption line to measure the periodic ring-down time. A reconstruction algorithm is developed to precisely recover the absorbance by extracting the characteristic frequencies of the periodic ring-down time after the Fourier transform. An etalon, instead of the wavelength meter, is used to calibrate the relative laser wavelength. This method effectively eliminates the non-uniform spectral noise caused by laser wavelength fluctuation in traditional CW-CRDS and significantly improves the measurement accuracy of spectral line parameters (especially line parameters in complex line shapes, such as speed dependent Voigt line shape) at low pressure. In addition, the measuring system, in which no wavelength meter is used, is simpler, more economical than CW-CRDS. The smaller residuals of the Galatry profile fit to the measured CO transitions at R(5) 6371.299 cm<sup>–1</sup> and R(6) 6374.406 cm<sup>–1</sup> show that the noise on both wings of the spectra, caused by laser wavelength fluctuation, is effectively reduced and the spectral SNR is then improved. The measured N<sub>2</sub> perturbed collision broadening coefficient of the Voigt profile fit for CO is consistent with that from the classical CW-CRDS method and is in good agreement with the HITRAN2016 database. The measured N<sub>2</sub> perturbed Dicke narrowing coefficient of the Rautian and Galatry profile and speed dependent collision broadening coefficient of the speed dependent Voigt profile have very good linear relationship with pressure, and have smaller uncertainties than the results from the CW-CRDS method.

Numerical methods of mode selection in continuous-wave cavity ring-down spectroscopy

In continuous-wave cavity ring-down spectroscopy (CW-CRDS), the measurement sensitivity is seriously affected by the multimode excitations in the ring-down cavity. The using of an intracavity aperture is a common way to restrain the excitation of high-order modes, thus leading the laser power to additionally lose and the signal-to-noise ratio to degrade. In this paper, two numerical methods, named “trigger threshold method” and “curve fitness method”, are proposed for selecting the mode in which the decays excited by the high-order modes can be removed. The laser coupling efficiency between the incident laser and the oscillating fundamental or high-order modes is studied in a misadjusted ring-down cavity. It is found that with a misadjusted ring-down cavity, the laser energy is partially coupled into the high-order modes, and the coupling energy increases with the extent of the cavity misadjustment increasing. In this case, the ring-down decaying traces excited by these high-order modes are different from and much shorter than those excited by the fundamental mode, which are respectively called “bad decays” and “good decays” in this paper. Both the fundamental mode and the high-order modes can reach the threshold in the case of low triggering threshold selection and result in the components of both good and bad decays in the output ring-down curves. When the trigger threshold rises, the bad decays are effectively restrained by the deficient coupling into the high-order modes. Thus raising the trigger threshold is an effective method to restrain bad decays for the mode selection. Another approach is to consider the time spent on turning off the laser injection since the fitting goodness of good decays is better than that of bad decays. In this paper this characteristic is also used to separate the good decays from the bad ones. These two methods are demonstrated in the CW-CRDS experiments. The results show that the sensitivity of the CW-CRDS instrument can be greatly improved by one order of magnitude in the trigger threshold method with the minimum of Allan deviations gradually approaching to a constant, while the acquisition rate of the ring-down decays slows down with the increase of the trigger threshold. The results also explain the relationship between single sampling and averaged sampling, which presents an answer to the question about the sequence choice between averaging and fitting. A numerical model is proposed to estimate the probability of good decays versus the trigger threshold, which can be used to choose appropriate trigger threshold for CW-CRDS experiment. The applicable conditions and the limitations of these two methods in CW-CRDS for trace gas detection are also discussed in the paper.

Optical axis maladjustment sensitivity in a triangular ring resonator.

This paper estimates and measures the influences of mismatch and misalignment on continuous-wave cavity ring-down spectroscopy. We describe the theoretical differences in maladjustment effects on power transfer under different resonant cavity configurations and present a method for reducing the impact of maladjustment. Finally, we demonstrate a method for measuring the effect of maladjustment on power transfer when a Gaussian beam is matched to a triangular ring resonator.

The sensitizing effects of NO2 and NO on methane low temperature oxidation in a jet stirred reactor

Abstract The oxidation of neat methane (CH 4 ) and CH 4 doped with NO 2 or NO in argon has been investigated in a jet-stirred reactor at 107 kPa, temperatures between 650 and 1200 K, with a fixed residence time of 1.5 s, and for different equivalence ratios (Φ), ranging from fuel-lean to fuel-rich conditions. Four different diagnostics have been used: gas chromatography (GC), chemiluminescence NO x analyzer, continuous wave cavity ring-down spectroscopy (cw-CRDS) and Fourier transform infrared spectroscopy (FTIR). In the case of the oxidation of neat methane, the onset temperature for CH 4 oxidation was above 1025 K, while it is shifted to 825 K with the addition of NO 2 or NO, independently of equivalence ratio, indicating that the addition of NO 2 or NO highly promotes CH 4 oxidation. The consumption rate of CH 4 exhibits a similar trend with the presence of both NO 2 and NO. The amount of produced HCN has been quantified and a search for HONO and CH 3 NO 2 species has been attempted. A detailed kinetic mechanism, derived from POLIMI kinetic framework, has been used to interpret the experimental data with a good agreement between experimental data and model predictions. Reaction rate and sensitivity analysis have been conducted to illustrate the kinetic regimes. The fact that the addition of NO or NO 2 seems to have similar effects on promoting CH 4 oxidation can be explained by the fact that both species are involved in a reaction cycle interchanging them and whose result is 2CH 3 + O 2 = 2CH 2 O + 2H. Additionally, the direct participation of NO 2 in the NO 2 + CH 2 O = HONO + HCO reaction has a notable accelerating effect on methane oxidation.

Stretching our understanding of C3: Experimental and theoretical spectroscopy of highly excited nν1 + mν3 states (n ≤ 7 and m ≤ 3).

We present the high resolution infrared detection of fifteen highly vibrationally excited nν1 + mν3 combination bands (n ≤ 7 and m ≤ 3) of C3 produced in a supersonically expanding propyne plasma, of which fourteen are reported for the first time. The fully resolved spectrum, around 3 μm, is recorded using continuous wave cavity ring-down spectroscopy. A detailed analysis of the resulting spectra is provided by ro-vibrational calculations based on an accurate local ab initio potential energy surface for C3 (X̃1Σg+). The experimental results not only offer a significant extension of the available data set, extending the observed number of quanta v1 to 7 and v3 to 3, but also a vital test to the fundamental understanding of this benchmark molecule. The present variational calculations give remarkable agreement compared to experimental values with typical accuracies of ∼0.01% for the vibrational frequencies and ∼0.001% for the rotational parameters, even for high energy levels around 10 000 cm-1.