Validation of the Extended Simultaneous Kinetics and Ringdown Model by Measurements of the Reaction NH2 + NO

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.

A uniform flow-cavity ring-down spectrometer (UF-CRDS): A new setup for spectroscopy and kinetics at low temperature

The UF-CRDS (Uniform Flow-Cavity Ring Down Spectrometer) is a new setup coupling for the first time a pulsed uniform (Laval) flow with a continuous wave CRDS in the near infrared for spectroscopy and kinetics at low temperature. This high resolution and sensitive absorption spectrometer opens a new window into the phenomena occurring within UFs. The approach extends the detection range to new electronic and rovibrational transitions within Laval flows and offers the possibility to probe numerous species which have not been investigated yet. This new tool has been designed to probe radicals and reaction intermediates but also to follow the chemistry of hydrocarbon chains and PAHs which play a crucial role in the evolution of astrophysical environments. For kinetics measurements, the UF-CRDS combines the CRESU technique (French acronym meaning reaction kinetics in uniform supersonic flows) with the SKaR (Simultaneous Kinetics and Ring-Down) approach where, as indicated by its name, the entire reaction is monitored during each intensity decay within the high finesse cavity. The setup and the approach are demonstrated with the study of the reaction between CN (v = 1) and propene at low temperature. The recorded data are finally consistent with a previous study of the same reaction for CN (v = 0) relying on the CRESU technique with laser induced fluorescence detection.

Quantitative Mid-Infrared Cavity Ringdown Detection of Methyl Iodide for Monitoring Applications

Methyl iodide is a toxic halocarbon with diverse industrial and agricultural applications, and it is an important ocean-derived trace gas that contributes to the iodine burden of the atmosphere. Quantitative analysis of CH₃I is mostly based on gas chromatography coupled with mass spectrometry or electron capture detection (GC-MS/ECD) as of yet, which often limits the ability to conduct in situ high-frequency monitoring studies. This work presents an alternative detection scheme based on mid-infrared continuous wave cavity ringdown spectroscopy (mid-IR cw-CRDS). CH₃I was detected at the ^RR₂(15) rovibrational absorption transition at ṽ = 3090.4289 cm^-1; part of the corresponding v₄ vibration band has been measured with Doppler-limited resolution for the first time. A line strength of S(T = 295 K) = (545 ± 20) cm/mol, corresponding to a line center absorption cross-section σ_c(p = 0 bar) = (1.60 ± 0.06) × 10⁵ cm²/mol, and pressure-broadening coefficients γ_p(Ar) = (0.094 ± 0.002) cm^-1/bar and γ_p(N₂) = (0.112 ± 0.003) cm^-1/bar have been determined. The performance of the detection system has been demonstrated with a tank-purging experiment and has been directly compared with a conventional GC-MS/ECD detection system. Quantitative detection with high reproducibility and continuous sampling is possible with a current noise-equivalent limit of detection of 15 ppb at 20 mbar absorption-cell pressure and 70 s averaging time. This limit of detection is suitable for practical applications in the ppm mixing ratio level range such as workplace monitoring, leak detection, and process studies. Natural environmental abundances are much lower, therefore possibilities for future improvement of the detection limit are discussed.

Saturation dynamics and working limits of saturated absorption cavity ringdown spectroscopy

The decay transient dynamics and the optimum experimental conditions for reliable gas absorption measurements have been investigated using saturated CRDS (Sat-CRDS, SCAR).