High-temperature Absorption Cross Sections Research Articles

High-temperature absorption cross-sections and interference-immune sensing method for formaldehyde near 3.6-µm

Absorption cross-sections of formaldehyde (CH2O) were measured at elevated temperatures near 3.6 µm, a region of strong intensity due to the overlapping QQ5 branch of the v1 band and the PQ7 branch of the v2+v4 combination band. 1,3,5-trioxane was shock-heated in an argon bath gas to produce controlled mixtures of formaldehyde over a range of temperatures (900–1500 K) and pressures (0.4–4.8 atm). Spectrally-resolved absorption measurements of formaldehyde were obtained using a rapidly-tunable distributed-feedback interband cascade laser scanned at 40 kHz over 2778.1–2778.9 cm−1 behind reflected shock waves. The resulting absorption cross-sections were fit with functions reflecting the temperature and pressure dependence of the targeted features, enabling spectral reconstruction at any temperature and pressure within the range of the study. By exploiting the strong temperature and pressure sensitivity of the cross-sections, it is demonstrated that CH2O concentration, temperature, and pressure can be inferred from a measured absorbance spectrum. Cross-sections of common combustion species were also measured over the selected wavelength range to validate that the region is largely free of narrow-band interference. Further, the multi-parameter spectral fitting method is shown to correct for baseline interference and demonstrated in experiments involving transient gas properties.

Near-infrared Opacity of Late M Dwarfs and Hot Jupiters: The E3Π–X3Δ Transition of TiO

Abstract The TiO E 3Π–X 3Δ transition is an important opacity source in the near-infrared spectra of late M dwarfs. The 0–0 band of the 48Ti16O E 3Π–X 3Δ electronic transition has been reanalyzed using a recently published set of high-resolution, high temperature absorption cross sections. The analysis has been extended to higher J values and an improved set of spectroscopic constants were determined. Calculated high-precision line lists for the 0–0 and 0–1 bands are provided with line strengths based on a previously measured experimental lifetime.

The Reaction NCN + H2: Quantum Chemical Calculations, Role of 1NCN Chemistry, and 3NCN Absorption Cross Section.

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. Recently, in a combined shock tube and flame modeling study, the so far neglected reaction NCN + H2 and the related chemistry of the main product HNCN turned out to be significant for NO modeling under fuel-rich conditions. In this study, the reaction has been thoroughly revisited by detailed quantum chemical rate constant calculations both for the singlet 1NCN and triplet 3NCN pathways. Optimized geometries and vibrational frequencies of reactants, products, and transition states were calculated on B3LYP/aug-cc-pVQZ level with single-point energy calculations carried out against the optimized structures using CASPT2/aug-cc-pVQZ. The determined rate constants for the 1NCN + H2 reaction as well as the newly measured high temperature absorption cross section of 3NCN made a reevaluation of the shock tube data of the previous work necessary, finally revealing quantitative agreement between experiment and theory. Moreover, the new directly measured Doppler-limited absorption cross section data, σ(3NCN, λ = 329.1302 nm) = 2.63 × 109 × exp(-1.96 × 10-3 × T/K) cm2/mol (±23%, p = 0 bar, T = 870-1700 K), are in agreement with previously reported values based on detailed spectroscopic simulations. Hence, a long-standing debate about a reliable high temperature 3NCN absorption cross section has been resolved. Whereas 3NCN + H2 resembles a simple abstraction type reaction with the exclusive products HNCN + H, the singlet radical reaction is initiated by the insertion into the H-H bond. Up to pressures of 100 bar, the main products of the subsequent decomposition of the H2NCN intermediate are HNCN + H as well, with minor contributions of CN + NH2 toward higher temperatures. Although much faster than the triplet reaction, the singlet radical insertion is actually rather slow, due to the necessary reorganization of the HOMO electron density in 1NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of 1NCN and 3NCN chemistry. However, as the main reaction products in case of the H2 reaction are the same and as the population of the 1NCN in thermal equilibrium remains low, a properly weighted effective rate constant k(NCN + H2 → HNCN + H) = 2.62 × 104 × (T/K)2.78 × exp(-97.6 kJ/mol/RT) cm3 mol-1s-1(±30%, 800 K < T < 3000 K, p < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for 1NCN chemistry.

Quantitative measurements of broad-band mid-infrared absorption spectra of formaldehyde, acetaldehyde, and acetone at combustion-relevant temperatures near 5.7 µm

Temperature-dependent absorption cross sections of formaldehyde, acetaldehyde, and acetone are presented for their CO stretching bands near 5.7 µm. The measurement methodology employs a broad-scan, rapid-tuning EC-QCL in conjunction with shock tube facilities and can provide quantitative spectroscopic information with an average spectral resolution of 0.6 cm−1 at a tuning rate in excess of 30 cm−1/ms. The absorption spectrum of formaldehyde in the ν2 vibrational band is reported at 998 K and 3.8 atm, showing reasonable agreement with spectral simulations based on HITRAN 2016. The residuals are ascribed to the emergence of hotbands and high-J rovibrational transitions at elevated temperatures. The first high-temperature absorption cross section measurements of acetaldehyde and acetone are reported near atmospheric pressure at combustion-related temperatures up to 1000 K, with strong temperature broadening observed in the measured spectra. The results presented are archived in the Stanford ShockGas-IR database and are expected to be useful in future spectroscopic studies.

A two-wavelength ethylene-absorption temperature diagnostic

Recent high-temperature absorption cross-section measurements of ethylene have revealed spectral features with strong sensitivity to temperature. Motivated by these observations, this work presents a novel two-wavelength laser absorption diagnostic for inferring temperature from the ethylene spectrum. Two CO2 gas laser wavelengths, 10.532 µm and 10.591 µm, were systematically chosen for their cross-section strengths and the temperature sensitivity of their absorbance ratio. Cross-sections at each wavelength and absorbance ratio measurements were collected over conditions 418–1671 K, 0.3–4.3 atm in a shock tube. The absorbance ratio was found to be insensitive to pressure between 1.1 atm and 4.3 atm, and a correlation was developed to describe the ratio as a function of temperature. At lower pressures, the correlation incorporates an additional pressure factor. The diagnostic was used to study the shock tube pyrolysis of ethane at 1330 K and 2.2 atm, and further applications are suggested.

A multi-wavelength speciation framework for high-temperature hydrocarbon pyrolysis

A framework for the study of high-temperature hydrocarbon pyrolysis is presented. The proposed framework details a multi-wavelength speciation method using convex optimization, a review of complementary fixed-wavelength laser diagnostics, and a database of high-temperature absorption cross-sections to enable use of the framework for 11 different species. The proposed speciation method involves a vectorized Beer-Lambert system solved under solution constraints using convex optimization. In support of the proposed method, a review of pertinent laser diagnostics and a database of high-temperature absorption cross-sections are also provided. The database includes measured absorption cross-sections of methane (CH4), ethylene (C2H4), propene (C3H6), isobutene (i-C4H8), 1-butene (1-C4H8), benzene (C6H6), toluene (C7H8), and four jet fuels (samples of Jet A, JP5, JP8, and Gevo ATJ) at the wavelengths of 3.1758 µm, 3.17595 µm, 3.283 µm, 3.392 µm, 3.410 µm, 10.532 µm, 10.675 µm, 10.89 µm, 10.958 µm, 10.962 µm, and 11.345 µm within the range of 300–1600 K and 0.25–4 atm. The database is comprised of over 1000 high–temperature absorption cross-section measurements from new shock tube experiments, new Fourier transform infrared spectrometer (FTIR) experiments, and an aggregated dataset from the literature. The convex speciation method and cross-section database provided will facilitate laser absorption studies of hydrocarbon pyrolysis used in the development of chemical kinetic mechanisms.

Time-Resolved Broadband Cavity-Enhanced Absorption Spectroscopy behind Shock Waves.

A fast and sensitive broadband absorption technique for measurements of high-temperature chemical kinetics and spectroscopy has been developed by applying broadband cavity-enhanced absorption spectroscopy (BBCEAS) in a shock tube. The developed method has effective absorption path lengths of 60-200 cm, or cavity enhancement factors of 12-40, over a wavelength range of 280-420 nm, and is capable of simultaneously recording absorption time profiles over an ∼32 nm spectral bandpass in a single experiment with temporal and spectral resolutions of 5 μs and 2 nm, respectively. The accuracy of the kinetic and spectroscopic measurements was examined by investigating high-temperature reactions and absorption spectra of formaldehyde behind reflected shock waves using 1,3,5-trioxane as a precursor. The rate constants obtained for the thermal decomposition reactions of 1,3,5-trioxane (to three formaldehyde molecules) and formaldehyde (to HCO + H) agreed well with the literature data. High-temperature absorption cross sections of formaldehyde between 280 and 410 nm have been determined at the post-reflected-shock temperatures of 955, 1265, and 1708 K. The results demonstrate the applicability of the BBCEAS technique to time- and wavelength-resolved sensitive absorption measurements at high temperatures.

High-temperature measurements of methane and acetylene using quantum cascade laser absorption near 8 μm

The mid-infrared wavelength region near 8μm contains absorption bands of several molecules such as water vapor, hydrogen peroxide, nitrous oxide, methane and acetylene. A new laser absorption sensor based on the ν4 band of methane and the ν4+ν5 band of acetylene is reported for interference-free, time-resolved measurements under combustion-relevant conditions. A detailed line-selection procedure was used to identify optimum transitions. Methane and acetylene were measured at the line centers of Q12 (1303.5cm−1) and P23 (1275.5cm−1) transitions, respectively. High-temperature absorption cross sections of methane and acetylene were measured at peaks (on-line) and valleys (off-line) of the selected absorption transitions. The differential absorption strategy was employed to eliminate interference absorption from large hydrocarbons. Experiments were performed behind reflected shock waves over a temperature range of 1200–2200K, between pressures of 1–4atm. The diagnostics were then applied to measure the respective species time-history profiles during the shock-heated pyrolysis of n-pentane.

Temporally and spectrally resolved UV absorption and laser-induced fluorescence measurements during the pyrolysis of toluene behind reflected shock waves

Toluene is frequently used as fluorescence tracer in high-temperature combustion applications. A quantitative analysis of laser-induced fluorescence (LIF) signals requires the knowledge of photophysical properties and decomposition kinetics. Using spectrally and temporally resolved ultraviolet absorption and LIF measurements, we studied the spectral properties of toluene and its pyrolysis products behind shock waves between 810 and 1,755 K. Transient absorption spectra were acquired between 220 and 300 nm. The temporal behavior of the absorption at 266 nm was compared to simulations based on literature kinetics models of toluene pyrolysis and available high-temperature absorption cross-sections of toluene, benzyl radicals, and C7H6 as a product from benzyl decomposition. Experiment and simulation agree well at the beginning of the pyrolysis process, whereas for longer reaction times deviations occur presumably due to the build-up of high molecular weight species, which contribute to the observed absorption but have unknown spectral properties. Additionally, LIF emission spectra were recorded following 266-nm excitation at selected reaction times. From measurements up to 1,220 K, the relative fluorescence quantum yield of toluene was derived, extending existing data to higher temperatures. Products from toluene pyrolysis were found to be the major contributors to the LIF signal at higher temperatures.

VUV-absorption cross section of CO2at high temperatures and impact on exoplanet atmospheres

Ultraviolet (UV) absorption cross sections are an essential ingredient of photochemical atmosphere models. Exoplanet searches have unveiled a large population of short-period objects with hot atmospheres, very different from what we find in our solar system. Transiting exoplanets whose atmospheres can now be studied by transit spectroscopy receive extremely strong UV fluxes and have typical temperatures ranging from 400 to 2500 K. At these temperatures, UV photolysis cross section data are severely lacking. Our goal is to provide high-temperature absorption cross sections and their temperature dependency for important atmospheric compounds. This study is dedicated to CO2, which is observed and photodissociated in exoplanet atmospheres. We performed these measurements for the 115 - 200 nm range at 300, 410, 480, and 550 K. In the 195 - 230 nm range, we worked at seven temperatures between 465 and 800 K. We found that the absorption cross section of CO2 is very sensitive to temperature, especially above 160 nm. Within the studied range of temperature, the CO2 cross section can vary by more than two orders of magnitude. This, in particular, makes the absorption of CO2 significant up to wavelengths as high as 230 nm, while it is negligible above 200 nm at 300 K. To investigate the influence of these new data on the photochemistry of exoplanets, we implemented the measured cross section into a 1D photochemical model. The model predicts that accounting for this temperature dependency of CO2 cross section can affect the computed abundances of NH3, CO2, and CO by one order of magnitude in the atmospheres of hot Jupiter and hot Neptune.

High-temperature measurements of VUV-absorption cross sections of CO2and their application to exoplanets

UV absorption cross sections are an essential ingredient of photochemical atmosphere models. Exoplanet searches have unveiled a large population of short-period objects with hot atmospheres, very different from what we find in our solar system. Transiting exoplanets whose atmospheres can now be studied by transit spectroscopy receive extremely strong UV fluxes and have typical temperatures ranging from 400 to 2500 K. At these temperatures, UV photolysis cross section data are severely lacking. Aims. Our goal is to provide high-temperature absorption cross sections and their temperature dependency for important atmospheric compounds. This study is dedicated to CO2, which is observed and photodissociated in exoplanet atmospheres. We also investigate the influence of these new data on the photochemistry of some exoplanets. We performed these measurements for the 115 - 200 nm range at 300, 410, 480, and 550 K. In the 195 - 230 nm range, we worked at seven temperatures between 465 and 800 K. We implemented the measured cross section into a 1D photochemical model. For wavelengths > 170 nm, the wavelength dependence of ln(cross-section_CO2(wavelength, T)x1/Qv(T)) can be parametrized with a linear law. Thus, we can interpolate cross-section_CO2(wavelength, T) at any temperature between 300 and 800 K. Within the studied range of temperature, the CO2 cross section can vary by more than two orders of magnitude. This, in particular, makes the absorption of CO2 significant up to wavelengths as high as 230 nm. The absorption cross section of CO2 is very sensitive to temperature. The model predicts that accounting for this temperature dependency of CO2 cross section can affect the computed abundances of NH3, CO2, and CO by one order of magnitude in the atmospheres of hot Jupiter and hot Neptune. This effect will be more important in hot CO2-dominated atmospheres.

NCO Quantitative Measurement in Premixed Low Pressure Flames by Combining LIF and CRDS Techniques

NCO is a short-lived species involved in NO(x) formation. It has never been quantitatively measured in flame conditions. In the present study, laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) were combined to measure NCO radical concentrations in premixed low-pressure flames (p = 5.3 kPa). NCO LIF excitation spectrum and absorption spectrum (using CRDS) measured in a stoichiometric CH(4)/O(2)/N(2)O/N(2) flame were found in good agreement with a simulated spectrum using PGOPHER program that was used to calculate the high-temperature absorption cross section of NCO in the A(2)Σ(+)-X(2)Π transition around 440.479 nm. The relative NCO-LIF profiles were measured in stoichiometric CH(4)/O(2)/N(2)O/N(2) flames where the ratio N(2)O/O(2) was progressively decreased from 0.50 to 0.01 and in rich CH(4)/O(2)/N(2) premixed flames. Then, the LIF profiles were converted into NCO mole fraction profiles from the absorption measurements using CRDS in a N(2)O-doped flame.

Thermal Decomposition of NCN3 as a High-Temperature NCN Radical Source: Singlet−Triplet Relaxation and Absorption Cross Section of NCN(3Σ)

The potential of the thermal decomposition of cyanogen azide (NCN3) as a high-temperature cyanonitrene (NCN) source has been investigated in shock tube experiments. Electronic ground-state NCN(3Σ) radicals have been detected by narrow-bandwidth laser absorption at overlapping transitions belonging to the Q1 branch of the vibronic 3Σ+−3Π subband of the vibrationally hot 3Πu(010)−3Σg−(010) system at = 30383.11 cm(-1) (329.1302 nm). High-temperature absorption cross sections σ have been directly measured at total pressures of 0.2−2.5 bar, log[σ/(cm2 mol(-1))] = 8.9−8.3 × 10(-4) × T/K (±25%, 750 < T < 2250 K). At these high temperatures, NCN(3Σ) formation is limited by a slow electronic relaxation of the initially formed excited NCN(1Δ) radical rather than thermal decomposition of NCN3. Measured temperature-dependent collision-induced intersystem crossing (CIISC) rate constants are best represented by kCIISC/(cm3 mol(-1) s(-1)) = (1.3 ± 0.5) × 1011 exp[−(21 ± 4) kJ/mol/RT] (740 < T < 1260 K). Nevertheless, stable NCN concentration plateaus have been observed, showing that NCN3 is an ideal precursor for NCN kinetic experiments behind shock waves.

Direct measurements of the reaction H + CH2O → H2+ HCO behind shock waves by means of Vis–UV detection of formaldehyde

AbstractThe combination of sensitive detection of formaldehyde by 174 nm absorption and use of ethyl iodide as a hydrogen atom source allowed direct measurements of the reaction H + CH2O → H2+ HCO behind shock waves. The rate constant was determined for temperatures from 1510 to 1960 K to bek2= 6.6 × 1014exp(−40.6 kJ mol−1/RT) cm3mol−1s−1(Δ logk2= ± 0.22)Considering the low uncertainty ink2, which accounts both for experimental and mechanism‐induced contributions, this result supports the upper range of previously reported, largely scattered high temperature rate constants. Vis–UV light of 174 nm was generated by a microwave N2discharge lamp. At typical reflected shock wave conditions of 1750 K and 1.3 atm, as low as 33 ppm formaldehyde could be detected. High temperature absorption cross sections of CH2O and other selected species have been determined. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 374–386, 2002