X2Π System Research Articles

Multi-speciation and ignition delay time measurements of ammonia oxidation behind reflected shock waves

Ammonia (NH3) oxidation was studied using emission and laser absorption diagnostics in a shock tube. Experiments were carried out using stoichiometric mixtures of NH3 and O2 highly dilute in Ar seeded with CO2 at post-reflected-shock conditions from 1700–2500K near 2atm. Five optical diagnostics in the infrared (IR), visible, and ultraviolet (UV) were implemented to measure NH3 (before and after shock-heating), NO, electronically excited amino radicals (NH2∗), electronically excited hydroxyl radicals (OH∗), and temperature during oxidation. To mitigate mixture uncertainty due to NH3 adsorption, a scanned-wavelength laser absorption diagnostic near 10.4μm was developed to measure initial NH3 concentration. A two-color, fixed-wavelength laser absorption diagnostic near 225 nm was developed to measure quantitative NO and NH3 time-histories. A detailed spectroscopic model of the NO A2Σ+←X2Π system was used to provide NO absorption cross-sections at both wavelengths, and NH3 absorption cross-sections were measured at the conditions of interest. Emission from NH2∗ near 600 nm was collected to provide qualitative insight into radical formation prior to ignition, and emission from OH∗ near 308 nm was used to determine ignition delay times (IDTs). A two-color, fixed-wavelength laser absorption diagnostic at 4.17μm and 4.19μm was developed to measure temperature using small quantities of seeded CO2. Measured time-histories are compared against predictions from two kinetic models. Experimental results and model predictions show several discrepancies, both in terms of timescales and absolute quantities. Sensitivity analysis shows that the reactions H + O2⇌ O + OH, NH3 + M ⇌ NH2 + H + M, and NH3 + H ⇌ NH2 + H2 are dominant across all conditions, and that NH3 pyrolysis reactions in general drive much of the chemistry. Direct, low-scatter rate constant measurements of the NH3 + M ⇌ NH2 + H + M reaction and additional multi-speciation measurements of NH3 pyrolysis and oxidation at combustion-relevant temperatures and pressures are recommended.Novelty and significance statementThe novelty of this work is exemplified most strongly by the use of UV laser absorption diagnostics to measure NH3 and NO during high-temperature oxidation of NH3. Simultaneous measurements of temperature via CO2 laser absorption at two wavelengths and light emission from NH2∗ and OH∗ are also novel. Simultaneous, time-resolved, quantitative measurements of NH3, NO, and temperature and qualitative measurements of NH2∗ and OH∗ light emission are of significant utility as a model validation-suited data set. Additionally, the data presented herein and the associated findings provide evidence of current chemical kinetic model deficiencies pertaining to NH3 chemistry and suggest avenues for model improvement.

The emission spectroscopy of the astrophysically significant B2Σ−–X2Π system of CD: further investigation

The band has been identified and measured in the laboratory spectrum of the CD molecule for the first time. The emission from the discharge was observed with a high accuracy dispersive optical spectroscopy technique using a plane-grating spectrograph and recorded by a photomultiplier tube. In total, 99 ro-vibronic frequencies were measured with an absolute accuracy of about cm. In the following work, we present the results of the combined analysis of the current experimental data and the (0−0) [Szajna et al., J. Mol. Spectrosc. 324, 48–52 (2016)] and (0−0) [Herzberg and Johns, Astrophys. J. 158, 399–417 (1969)] bands of CD. The resulting data, compared to those from the earlier measurements, present more accurate molecular constants for the v = 0 vibrational level of the and states. The parameters of the Λ-doubling (p, q) of the (v = 0) level were obtained for the first time. The current observations and analysis have become a basis to discuss the possible detection of the CD radical in the interstellar medium by the , + and spectral lines of the band in the 3880 Å region.

Энергетические и радиационные характеристики системы A-=SUP=-2-=/SUP=-Sigma-=SUP=-+-=/SUP=--X-=SUP=-2-=/SUP=- радикала OH: неэмпирические расчеты и неадиабатическое моделирование

A quantum-chemical model for non-adiabatic simulation of the energetic and radiative properties of the A2Σ+ − X2Π system of the OH radical is presented. The model parameters (potential energy functions and electronic matrix elements of the spin-orbit and the electron-rotation interactions) are obtained by means of ab initio calculation. The reliability of the performed calculations is confirmed by comparing ab initio estimates with experimental values for the spin-orbit splitting, the Λ-doubling parameters of the X2Π state, and the γ-doubling of the A2Σ+ state. The frequencies and Einstein coefficients for rovibronic transitions between the states A2Σ+ and X2Π are simulated in a wide range of rovibrational excitations. The obtained values are in quantitative agreement with the data available from literature.

Temperature measurements in a swirling impinging flame by planar laser-induced fluorescence

This article presents the results of approbation of the method for registering temperature distributions based on the planar laser-induced fluorescence of a hydroxyl radical (OH) when the band (1-0) of the A2Σ+–X2Π system is excited. The thermometry is based on the recording the ratio of the radiation intensity of the band (2-0) and the bands (0-0), (1-1). Numerical modelling of fluorescence spectra is performed using the LASKIN program for the most frequent excitation lines Q2(7), Q1(8), R1(14), P1(2). The temperature field of a swirling flame, impinging on a flat cold surface, for H/d = 1, 2 and 3 calibres (where H is the distance between the jet nozzle and the surface, d is the outlet diameter of the nozzle) is obtained. The results of the work demonstrate that when the transition Q1(8) is excited, the ratio of the intensity of fluorescence signals for the band (2-0) and the bands (0-0), (1-1) provides a high sensitivity to temperature and is not significantly affected by fluorescence quenching. The report also concludes that this method can be implemented using single pulsed laser illumination and is effective for the detecting the position of flow recirculation zones and registering hot heat release zones with the combustion products.

10 kHz 2D thermometry in turbulent reacting flows using two-color OH planar laser-induced fluorescence.

10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate ∼5mJ/pulse at 282 nm and 286 nm to excite the Q1(5) and Q1(14) transitions of the A2Σ+←X2Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.

Transition dipole moments and probabilities of the CCl radicals

This work studies the transition dipole moments and properties of 12 electronic states. These states are X2Π, A2Δ, B2Σ+, C2Π, 12Σ−, 22Σ−, a4Σ−, b4Π, 14Σ+, 14Δ, 24Π, and 24Σ−; they originate from the first dissociation limit of the carbon monochloride radical. The potential energy curves were calculated using the complete active space self-consistent field method, followed by the valence internally contracted multireference configuration interaction approach. The B2Σ+ state has a double well and C2Π a single barrier. The radiative lifetimes of the vibrational levels are in the order of 10−7 s for the A2Δ state, and 10−7 and 10−5 s for the first and second wells of the B2Σ+ state, respectively, suggesting that the spontaneous emissions originated from the two states can easily occur. The radiative lifetimes are long for C2Π, b4Π, 14Δ, 14Σ+, and 24Π states and the Einstein A coefficients are small for the emissions originated from these states, indicating that the spontaneous emissions generated from these states are difficult to measure through spectroscopy. The 12Σ−, 22Σ−, and 24Σ− states are repulsive. Furthermore, the spectroscopic parameters and vibrational levels were evaluated. For the A2Δ – X2Π and B2Σ+ – X2Π systems, the Einstein A coefficients of the vibronic emissions are large, indicating that the emissions from the two systems can be easily detected by spectroscopy. The spectral range of the spontaneous emissions was evaluated for certain transitions.

The emission spectroscopy of the [formula omitted] system of the CD radical

The emission spectrum of the 0-0,1-1 and 2-2 bands of the C2Σ+-X2Π system in the CD molecule have been obtained using a stainless steel hollow-cathode lamp with two anodes and filled with a mixture of He buffer gas and CD4. The emission from the discharge was observed with high accuracy dispersive optical spectroscopy using a plane-grating spectrograph and recorded by a photomultiplier tube. In total, 428 transition frequencies belonging to the three bands were precisely measured and rotationally analyzed. The current data were elaborated with the help of precise ro-vibrational and equilibrium ground state parameters reported by Zachwieja et al. (2012) from investigation of the A2Δ-X2Π transition. The spin-rotation interaction constants γv and γD,v of the C2Σ+,v=0,1,2 levels were obtained for the first time. Moreover, the main equilibrium parameters of the third excited state were obtained as follows: Te=31,825.944(12) cm−1, ωe=2078.803(27) cm−1, ωexe=59.558(12) cm−1, Be=7.86810(23) cm−1, αe=0.25315(58) cm−1 and γe=-0.01518(28) cm−1.

Two-color volumetric laser-induced fluorescence for 3D OH and temperature fields in turbulent reacting flows.

Single-shot, two-color, volumetric laser-induced fluorescence was demonstrated for three-dimensional (3D), tomographic imaging of the structural properties of the OH radical and temperature field in a turbulent hydrogen-air flame. Two narrowband laser sources were tuned to the Q1(5) and Q1(14) transitions of the (1,0) band in the A2Σ←X2Π system and illuminated a volumetric region of the flame. Images from eight unique perspectives collected simultaneously from each of the two transitions were used to reconstruct overlapping OH fields with different Boltzmann fractions and map the 3D temperature distribution with nanosecond precision. Key strategies for minimizing sources of error, such as detector sensitivity and spatial overlap of the two fields, are discussed.

Absolute CF2 density and gas temperature measurements by absorption spectroscopy in dual-frequency capacitively coupled CF4/Ar plasmas

Broadband ultraviolet absorption spectroscopy has been used to determine the CF2 radical density in dual-frequency capacitively coupled CF4/Ar plasmas, using the CF2 Ã1B1←X̃1A1 system of absorption spectrum. The rotational temperature of ground state CF2 and excited state CF was also estimated by using Ã1B1←X̃1A1 system and B2Δ−X2Π system, respectively. The translational gas temperature was deduced from the Doppler width of the Ar*(3P2) and Ar*(3P0) metastable atoms absorption line by using the tunable diode laser absorption spectroscopy. The rotational temperatures of the excited state CF are about 100 K higher than those of ground state CF2, and about 200 K higher than the translational gas temperatures. The dependences of the radical CF2 density, electron density, electron temperature, rotational temperature, and gas temperature on the high frequency power and pressure have been analyzed. Furthermore, the production and loss mechanisms of CF2 radical and the gas heating mechanisms have also been discussed.

Theoretical study of the spectroscopy and radiative transition properties of NS

The low-lying electronic states (X2Π, a4Π, b4Σ−, B2Π, 12Φ, A2Δ and B′2Σ+) of the NS molecule are computed at the AV5Z/CASSCF/MRCI+Q+DK level of theory. The potential energy curves are presented for these states; the corresponding spectroscopic constants are reported. Electronic transition moment functions, Einstein coefficients, Franck–Condon factors and radiative lifetimes for the B2Π–X2Π, B′2Σ+–X2Π and A2Δ–X2Π system have been calculated. The F–C factors of the B2Π are in good agreement with the corresponding RKR data. The B′2Σ+ state possesses a barrier in the potential energy curve, located at about 2.16Å. The height of the barrier is predicted to be 7890cm−1.

An analytic model of the stereodynamics of rotationally inelastic molecular collisions

We develop an analytic model of vector correlations in rotationally inelastic atom–diatom collisions and test it against the much examined Ar–NO (X2Π) system. Based on the Fraunhofer scattering of matter waves, the model furnishes complex scattering amplitudes needed to evaluate the polarization moments characterizing the quantum stereodynamics. The analytic polarization moments are found to be in an excellent agreement with experimental results and with close-coupling calculations available at thermal energies. The model reveals that the stereodynamics is governed by diffraction from the repulsive core of the Ar–NO potential, which can be characterized by a single Legendre moment.

OH A2Σ+ –X2Π band ratios observed in the mesosphere by OSIRIS

In this study, we present spectra of the mesospheric OH A2Σ+ –X2Π band system, including the 0–0, 1–1, and 1–0 bands, as observed by OSIRIS (Optical Spectrograph and Infrared Imaging System). Spectral components due to Rayleigh-scattered sunlight, lower thermospheric dayglow emission features, and baffle scatter have been removed to isolate the OH emission signature. The observed spectra are compared with model spectra assembled using rotational emission rate factors for solar resonance fluorescence (g-factors) plus prompt emission of the OH A2Σ+ –X2Π band system from solar Lyman-α photodissociation of water. The observed band ratios are in good agreement with the model values. The altitude variation of the 0–0 band, relative to the 1–1 band, is in agreement with model predictions based on vibrational energy transfer from OH A2Σ+ ν′ = 1 to OH A2Σ+ ν′ = 0. This detailed understanding of the OH A2Σ+ –X2Π system is critical for the successful application of OH observations to the determination of mesospheric OH densities and water vapor concentrations.PACS Nos.: 33.20.Lg, 33.20.Tp, 33.70.Fd, 92.60.hc, 92.60.hw

Shifts in the A2Σ+–X2Π band systems of 16OH, 18OH, 16OD and 18OD

We calculate and present the shifts that are induced by predissociation of the band origins in the A2Σ+–X2Π system of the hydroxyl radical 16OH, using a sum rule method in which the projection of the predissociating interaction onto a complete set of states is replaced by a single linear response. We present also the corresponding shifts for the isotopomers 18OH, 16OD and 18OD.

Collisional Transfer between Λ-Doublet Levels of OH(X2Π3/2, v = 1, j = 3.5, and 6.5) in Collisions with He, Ar, N2, and HNO3

Using an infrared−ultraviolet double resonance method, we have measured rate coefficients at room temperature for the transfer of OH radicals between the Λ-doublet levels associated with the j = 3.5 and j = 6.5 rotational levels of the X2Π, Ω = 3/2, v = 1 vibronic state in collisions with He, Ar, N2, and HNO3. OH radicals were generated by 266 nm pulsed laser photolysis of HNO3 and promoted to selected Λ-doublet levels in j = 3.5 and j = 6.5 using a pulsed infrared laser tuned to the appropriate line in the (1, 0) infrared fundamental band of OH. The evolution of population in each Λ-doublet component of the selected rotational line was then observed using time-delayed laser-induced fluorescence in the (1, 1) band of the A2Σ+−X2Π system. The measured rate coefficients (kΛ) show a strong dependence on collision partner with kΛ(He) < kΛ(Ar) < kΛ(N2) ≪ kΛ(HNO3) and a significant dependence on rotational level, with the values of kΛ larger for j = 3.5 than for j = 6.5, especially for HNO3 as the collision par...

Electronic Absorption Spectra of C4O- and C4S- in Neon Matrixes

The electronic spectra of the anions C4O- and C4S- have been observed by absorption spectroscopy in a neon matrix using mass selection. The symmetries of the states are determined on the basis of spectroscopic evidence and ab initio calculations. Two absorption systems of C4O- are assigned to the A2Σ+ ← X2Π and B2Π ← X2Π electronic transitions, with band origins near 875 and 442 nm, respectively. The B2Π ← X2Π system of C4S-, with the origin near 568 nm, has also been observed. After photobleaching, the 3Σ- ← X3Σ- electronic transition of C4S is observed around 447 nm.

Two-line laser-induced fluorescence imaging of vibrational temperatures in a NO-seeded flame

Two-dimensional temperature fields are measured in lean and sooting flames by means of two-color laser-induced fluorescence (LIF) imaging that uses seeded NO. Vibrational thermometry is performed by the probing of different vibrational ground-state levels. Spectral properties of the excited transitions within the A (2)?(+)-X (2)? system are well known from previous studies. The energy difference of 1974 cm(-1) between the (0, 0)Q(1) + P(21)(33.5) and the (0, 2)O(12)(5.5) lines offers great sensitivity in the temperature range that is relevant for combustion processes. Excitation is possible by use of a tunable KrF excimer laser on its fundamental (248-nm) and Raman shifted (in H(2), 225-nm) wavelengths. An excitation scheme for instantaneous two-line measurements by use of a single laser is developed. The possibility of single-shot measurements is discussed.

Optical Analysis of Pulse Combustion Using Shadowgraph and Planar CH-LIF Imaging Technique

Planar imaging of laser-induced fluorescence of CH radical is made to examine combustion processes in a valveless pulse combustor. An excimer-pumped dye laser tuned to a wavelength of 387 nm is used to excite the R1N″=6 line of (0,0) band of the B2Σ−−X2Π system of CH radical, and an image-intensified CCD camera system is used to detect the (0,1) band emission at around 435 nm. According to the CH-LIF images, it is found that the progress in combustion during a pulsation period is expressed by the enlargement and breakup of the earlobe-shaped flame front along the outline of a pair of large-scale eddies of fresh mixture.

Vibrational Structure of the B—X System of Isotopic Ago Molecules

Spectra of 107Ag18O and 107Ag16O molecules have been obtained in a low-pressure arc in oxygen atmosphere, and recorded with medium dispersion. Vibrational assignments for the bands of B 2π —X2π system were verified by the study of the oxygen isotope effect, and the vibrational constants were obtained for the states involved in transitions.

Analysis of the 0–0 Band of theC2Σ+–X2Π Band System in the12CH and13CH Isotopic Radicals

The emission spectrum of the 0–0 band of theC2Σ+–X2Π transition for12CH and13CH isotopic molecules have been obtained using a Geissler tube. The spectra have been photographed under high resolution and calibrated against Th standard lines. The reduction of the spectra for the individual bands has been performed via a nonlinear least-squares fit with the effective Hamiltonian of Brown. New values of the molecular constants for upper state have been determined. The 0–0 band of theC2Σ+–X2Π system in the13CH radical has been obtained and analyzed for the first time.

Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence

Two-dimensional laser-induced fluorescence measurements (2-D LIF) of the hydroxyl radical distribution in a laminar, premixed methane/air flat flame at various pressures were performed. Instantaneous and time averaged concentration fields were determined for three different pressures (1, 5, 20 bar ). The 2-D LIF data were set to an absolute scale by a calibration with a novel, single pulse one-dimensional UV absorption measurement made simultaneously with only one intensified CCD camera. To compensate effects of beam steering a spherical lens was inserted into the sheet path after the flame. The measurements are compared with results from numerical calculations. PACS: 07.65; 42.30.Va; 82.40.Py Combustion processes are characterized by the complex interaction between different transport processes and numerous chemical reactions. To study the various processes and to validate numerical calculations, non-intrusive in-situ measurements of concentration and temperature are necessary. For this purpose several laser-based spectroscopic methods have been developed and successfully applied in recent years [1–3]. As most technical combustion systems are turbulent, multidimensional single-pulse diagnostics is required. Two-dimensional laser-induced fluorescence (2-D LIF) is a powerful technique to measure many of the chemically important species, and temperature in a flame, with high spatial and temporal resolution with single pulse capability [4]. For 2-D LIF a laser beam is formed into a light sheet by several lenses. The light sheet intersects the flow field in a twodimensional plane and excites the molecules or atoms. The occurring fluorescence is collected with a two-dimensional detector [5, 6] such as an image intensified CCD camera [7]. Concentration fields of OH radicals are of special interest owing to its important role in combustion chemistry and for comparison with results from reduced reaction schemes. Although qualitative results are sometimes valuable, often quantitative data are necessary. However, quantification is not a trivial problem, especially at higher pressures. Usually other spectroscopic methods, e.g. absorption spectroscopy [8–12] have to be used to calibrate LIF data. In the present paper we propose to combine planar LIF measurements with absorption measurements at any height above the burner surface (1-D absorption) on a single pulse basis. The calibration of the LIF signals at any axis in the plane by a separate determination of the absorption along that axis reduces strongly the influence of an unknown quenching rate, since absorption measurements are less influenced by quenching. One could consider the proposed method to be just a 1-D absorption measurement. However, the LIF measurements are needed not only to determine the length of the absorption axis but also the distribution of the OH radicals along the absorption path. 1 High pressure burner The flame under study was a methane/air flat flame stabilized on a water cooled sinter plate of 20 mm diameter. This burner, constructed following drawings of O.N.E.R.A. allows steady combustion within a pressure range between 1 and 40 bar . Gas velocity and mixture composition were controlled by mass flow controllers. Thereby mixing ratios λ (air to fuel ratio) between 0.5 and 1.3 are possible though most measurements were done for λ= 1 and λ= 1.1. Optical access is realized by four suprasil windows orthogonal to each other. Some major advantages of this type of burner are the very high stability of the flame, which allows time integrated measurements, and the fact that similar devices are available at several other groups like O.N.E.R.A. and DLR Stuttgart. So data of these research groups can be compared directly. 2 Laser and detection system To detect OH radicals using 2-D LIF several detection schemes have been published. Advantages and disadvantages of these schemes are discussed by Ketterle et al. [13]. The absorption of the (3–0) band in the A2Σ–X2Π system [14] is too weak to be measured with pulsed laser systems. Excitation