Spectra Of Ethylene Research Articles

Multi-speciation in shock tube experiments using a single laser and deep neural networks

Chemical kinetic experiments involving the oxidation or pyrolysis of fuels can be complex, especially when multiple species are formed and consumed simultaneously. Therefore, a diagnostic strategy that enables fast and selective detection of multiple species is highly desirable. In this work, we present a mid-infrared laser diagnostic that can simultaneously detect multiple species in high-temperature shock-tube experiments using a single laser. By tuning the wavelength of the laser over 3038 – 3039.6 cm−1 wavelength range and employing a denoising model based on deep neural networks (DNN), we were able to differentiate the absorbance spectra of ethane, ethylene, methane, propane, and propylene. The denoising model is able to clean noisy absorbance spectra, and the denoised spectra are then split these into contributions from evolving species using multidimensional linear regression (MLR). To the best of our knowledge, this work represents the first successful implementation of time-resolved multispecies detection using a single narrow wavelength-tuning laser. To validate our methodology, we conducted pyrolysis experiments of ethane and propane. The results of our experiments showed excellent agreement with previous experimental data and chemical kinetic model simulations. Overall, our diagnostic strategy represents a promising approach for detecting multiple species in high-temperature transient environments.

Architecture for microcomb-based GHz-mid-infrared dual-comb spectroscopy

Dual-comb spectroscopy (DCS) offers high sensitivity and wide spectral coverage without the need for bulky spectrometers or mechanical moving parts. And DCS in the mid-infrared (mid-IR) is of keen interest because of inherently strong molecular spectroscopic signatures in these bands. We report GHz-resolution mid-IR DCS of methane and ethane that is derived from counter-propagating (CP) soliton microcombs in combination with interleaved difference frequency generation. Because all four combs required to generate the two mid-IR combs rely upon stability derived from a single high-Q microcavity, the system architecture is both simplified and does not require external frequency locking. Methane and ethane spectra are measured over intervals as short as 0.5 ms, a time scale that can be further reduced using a different CP soliton arrangement. Also, tuning of spectral resolution on demand is demonstrated. Although at an early phase of development, the results are a step towards mid-IR gas sensors with chip-based architectures for chemical threat detection, breath analysis, combustion studies, and outdoor observation of trace gases.

Fiber-based optical frequency comb at 3.3 µm for broadband spectroscopy of hydrocarbons [Invited

A 125 MHz fiber-based frequency comb source in the mid-infrared wavelength region is presented. The source is based on difference frequency generation from a polarization-maintaining Er-doped fiber pump laser and covers a spectrum between 2900 cm−1 and 3400 cm−1 with a simultaneous bandwidth of 170 cm−1 and an average output power up to 70 mW. The source is equipped with actuators and active feedback loops, ensuring long-term stability of the repetition rate, output power, and spectral envelope. An absorption spectrum of ethane and methane was measured using a Fourier transform spectrometer to verify the applicability of the mid-infrared comb to multispecies detection. The robustness and good long- and short-term stability of the source make it suitable for optical frequency comb spectroscopy of hydrocarbons.

Near infrared absorption cross sections for ethane broadened by hydrogen and nitrogen

Near infrared absorption spectra of ethane (C2H6) were recorded in the 1800–6200 cm−1 region by high resolution Fourier transform spectroscopy (Bruker IFS 125 HR) at the Canadian Light Source (CLS). Absorption cross sections were obtained for pure samples and with nitrogen or hydrogen as broadening gases as appropriate for astronomical environments. Corrections to the cross sections were made using data from the Pacific Northwest National Laboratory (PNNL) infrared database.

Transformation Properties under the Operations of the Molecular Symmetry Groups G36 and G36(EM) of Ethane H3CCH3

In the present work, we report a detailed description of the symmetry properties of the eight-atomic molecule ethane, with the aim of facilitating the variational calculations of rotation-vibration spectra of ethane and related molecules. Ethane consists of two methyl groups CH 3 where the internal rotation (torsion) of one CH 3 group relative to the other is of large amplitude and involves tunnelling between multiple minima of the potential energy function. The molecular symmetry group of ethane is the 36-element group G 36 , but the construction of symmetrised basis functions is most conveniently done in terms of the 72-element extended molecular symmetry group G 36 (EM). This group can subsequently be used in the construction of block-diagonal matrix representations of the ro-vibrational Hamiltonian for ethane. The derived transformation matrices associated with G 36 (EM) have been implemented in the variational nuclear motion program TROVE (Theoretical ROVibrational Energies). TROVE variational calculations are used as a practical example of a G 36 (EM) symmetry adaptation for large systems with a non-rigid, torsional degree of freedom. We present the derivation of irreducible transformation matrices for all 36 (72) operations of G 36 (M) (G 36 (EM)) and also describe algorithms for a numerical construction of these matrices based on a set of four (five) generators. The methodology presented is illustrated on the construction of the symmetry-adapted representations both of the potential energy function of ethane and of the rotation, torsion and vibration basis set functions.

Broadband coherent cavity-enhanced dual-comb spectroscopy

Cavity-enhanced spectroscopic instruments impact a broad range of scientific fields because they are sensitive, self-contained, and can be packaged for use by nonspecialists. Infrared-laser-based cavity-enhanced instruments are typically limited to measuring a few small molecules because they rely on narrowly tunable lasers that each measure a molecular transition. Broadband dual-comb spectroscopy (DCS) is a spectroscopic technique capable of simultaneously probing molecular absorption on thousands of narrowly spaced wavelengths using a single high-speed photodetector. With coherent averaging, DCS can measure multiple species, including large molecules, with a high signal-to-noise ratio (SNR). Here, we lock a high-finesse optical cavity to a broadband mode-locked frequency comb with an approach that enables a high SNR, broadband cavity-enhanced DCS with coherent averaging. With a 7.5 cm optical cavity, we attain a >12,000× path-length-enhancement factor, 60 nm bandwidth near 1660 nm (3.5% fractional bandwidth), and >27 dB SNR in 160 s. We measure the dense rovibrational spectrum of ethane in a background of overlapping H2O, CH4, and CO2 absorption. By combining the broadband, high-resolution, and single-detector nature of DCS with a compact optical cavity, we enable future self-contained instruments that are able to simultaneously detect a variety of molecules in realistic ambient conditions.

Theoretical Models of Low-Resolution Microwave Rotational Spectra of Ethane- and Propanethiol

Additive modeling of low-resolution microwave spectra of heteroisomeric substituted hydrocarbons produced theoretical spectra of ethanethiol and propanethiol in the range 0–2 THz with maxima at 465 ± 20 and 240 ± 20 GHz. More precise calculations in a narrow frequency band of these ranges used spectral line half-widths of 1.5, 0.8, and 0.5 MHz that modeled conditions in different layers of Earth’s troposphere. The strongest extrema of the low-resolution spectra of the studied molecules were found at 486 ± 5, 446 ± 5, and 436 ± 5 (ethanethiol) and at 257 ± 5, 239 ± 5, and 234 ± 5 GHz (propanethiol). Various aspects of the application of the results were discussed.

Raman spectrum of ethane in methane environment

AbstractChanges are investigated in the positions, half‐widths, and intensities of the Q‐branches of the main ethane Raman bands (ν3, 2ν6, 2ν2, 2ν8, and ν1) in the methane/ethane mixture depending on methane concentration at the fixed pressure of 25 bar. It was found that with increasing methane fraction, the above‐mentioned ethane Raman bands, with the exception of ν3, are shifted to the high wavenumbers, and the bands of 2ν8/ν1 Fermi doublet demonstrate a violation of the linear dependence of the intensity on the concentration. The data obtained will be useful for diagnostics of natural gas composition using Raman spectroscopy.

A small-volume PVTX system for broadband spectroscopic calibration of downhole optical sensors.

An instrument is presented that is capable of measuring the optical spectrum (long-wave ultraviolet through short-wave mid-infrared) of fluids under a range of temperature and pressure conditions from ambient pressure up to 138 MPa (20 000 psi) and 422 K (300 °F) using ∼5 ml of fluid. Temperature, pressure, and density are measured in situ in real-time, and composition is varied by adding volatile and nonvolatile components. The stability and accuracy of the conditions are reported for pure ethane, and the effects of temperature and pressure on characteristic regions of the optical spectrum of ethane are illustrated after correction for temperature and pressure effects on the optical cell path length, as well as normalization to the measured density. Molar absorption coefficients and integrated molar absorption coefficients for several vibrational combination bands are presented.

Cation States of Ethane: HEAT Calculations and Vibronic Simulations of the Photoelectron Spectrum of Ethane.

High-accuracy ab initio calculations have been carried out on ethane and its radical cation. With the HEAT-345(Q) scheme, adiabatic ionization potentials of 11.52 and 11.57 eV are determined for the X̃ (2)Eg and à (2)A1g states, respectively, with an uncertainty of ±0.015 eV. Also considered in this report are linear and quadratic vibronic coupling involving both states. With this simple vibronic model, the photoelectron spectrum of ethane was simulated in the 11-15 eV region using linear and full quadratic Jahn-Teller coupling Hamiltonians, and with up to 70 billion direct product basis functions in a high-performance computing environment. Although the linear vibronic coupling model adequately reproduces the spectral envelope, the quadratic vibronic treatment results in much better agreement with the observed spectrum.

Ethane thermometry using rotational coherent anti-Stokes Raman scattering (CARS)

The complete rotational CARS (coherent anti-Stokes Raman scattering) spectrum of ethane (C2H6) has for the first time been recorded instantaneously under high spectral resolution, and the potential for thermometry has been investigated. Experiments were performed in the temperature range 292–650K in pure ethane and in binary mixtures with nitrogen. A polarization technique was used to suppress the non-resonant contribution to the CARS signal. The ethane RCARS spectra show both S- and R-branch lines, which are more closely spaced than for the well-known nitrogen spectrum and located at much smaller Raman shifts. The peak signal strength was found to be around 240 times lower for ethane than for nitrogen (at 292K). Two main approaches for ethane thermometry are evaluated, which both show high potential. The first is a method in which a spectrum with unknown temperature is fitted using a library of experimental spectra recorded at various temperatures. The second is a method based on ratios of integrated signals in different spectral regions. A theoretical model for simulation of theoretical spectra is under development.

The high-resolution FTR spectrum of ethane between 3170 and 3430 cm−1

The high resolution FTIR spectrum of ethane has been reinvestigated in the region near 3μm, where three perpendicular and one parallel infrared active bands are found.The main perpendicular band is ν4+ν10. It is shown that the perpendicular partners of the combinations ν4+2ν8 and ν4+2ν11 are coupled with each other in such a way that only one resultant mode can interact effectively with ν4+ν10, becoming detectable by intensity stealing. The third perpendicular band is found to be the hot (2ν4+ν10)−ν4, lying at about 33cm−1 below the cold ν4+ν10, because of the strong anharmonicity of the torsional mode ν4, and then well separated cold and hot transitions occur in this region. It is suggested that they could be exploited in the temperature monitoring in atmospheric and planetary spectra.The complexity and the irregular rotation–torsion structure makes an analysis of this perpendicular system not practicable, but a considerable body of new line assignments, especially for the hot transitions, have been found.We have been able to analyze the parallel band, in spite of the anomalous torsional structure, and some peculiar torsional effects show that the vibrational upper state must be the infrared active ν2+ν9+ν12(A2u).A total of 304 transitions of this active parallel band were fitted with a rms deviation of 0.0097cm−1 to derive band parameters, and interacting parameters from the perturbers 3ν4+ν11∓1+ν12∓1 and ν11±1+ν9∓1+ν12±1/ν11±1+ν9±1+ν12∓1.

Self- and air-broadened cross sections of ethane (C2H6) determined by frequency-stabilized cavity ring-down spectroscopy near 1.68 µm

The absorption spectrum of ethane was measured by frequency-stabilized cavity ring-down spectroscopy over the wave number range 5950–5967cm−1. Spectra are reported for both pure ethane acquired at pressures near 3Pa and mixtures of ethane in air at pressures ranging from 666Pa to 101.3kPa. Absorption cross sections are reported with a spectrum sampling period of 109MHz and frequency resolution of 200kHz. Atmospheric pressure cross sections agree fairly well with existing cross sections determined by FTS in nitrogen, but there are significant variations in cross sections at lower pressures. Source identification of fugitive methane emissions using spectroscopic measurements of the atmospheric ethane-to-methane ratio is also discussed.

NMR of Short-Chain Hydrocarbons in Nematic and Smectic A Liquid Crystals

The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.

Size variation of infrared vibrational spectra from molecules to hydrogenated diamond nanocrystals: a density functional theory study.

Infrared spectra of hydrogenated diamond nanocrystals of one nanometer length are calculated by ab initio methods. Positions of atoms are optimized via density functional theory at the level of the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) using 3-21G basis states. The frequencies in the vibrational spectrum are analyzed against reduced masses, force constants and intensities of vibration. The spectrum can be divided into two regions depending on the properties of the vibrations or the gap separating them. In the first region, results show good matching to several experimentally obtained lines. The 500 cm−1 broad-peak acoustical branch region is characterized by pure C–C vibrations. The optical branch is centered at 1185 cm−1. Calculations show that several C–C vibrations are mixed with some C–H vibrations in the first region. In the second region the matching also extends to C–H vibration frequencies that include different modes such as symmetric, asymmetric, wagging, scissor, rocking and twisting modes. In order to complete the picture of the size dependence of the vibrational spectra, we analyzed the spectra of ethane and adamantane. The present analysis shows that acoustical and optical branches in diamond nanocrystals approach each other and collapse at 963 cm−1 in ethane. Variation of the highest reduced-mass-mode C–C vibrations from 1332 cm−1 of bulk diamond to 963 cm−1 for ethane (red shift) is shown. The analysis also shows the variation of the radial breathing mode from 0 cm−1 of bulk diamond to 963 cm−1 for ethane (blue shift). These variations compare well with experiment. Experimentally, the above-mentioned modes appear shifted from their exact positions due to overlap with neighboring modes.

Nuclear magnetic resonance study of alkane conformational statistics

NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl-4(')-cyanobiphenyl (5CB) and Merck ZLI 1132 (1132) are investigated over a wide temperature range. The ratios of dipolar couplings of ethane to propane are constant over the entire temperature range. Assuming that this constancy applies to the butane conformers facilitates the separation of probability from order parameter. This separation allows the investigation of conformational distribution without the need of invoking any model for the anisotropic intermolecular potential. The results give an order matrix that is consistent with that predicted from model potentials that describe the orientational potential in terms of short-range size and shape effects. The isotropic intermolecular potential contribution to the trans-gauche energy difference E(tg) is found to be temperature dependent with the values and variation in agreement with that found when the same results are analyzed using the chord model for anisotropic interactions [A. C. J. Weber and E. E. Burnell, Chem. Phys. Lett. 506, 196 (2011)]. The fit obtained for 9 spectra in 5CB (63 dipolar couplings) has an RMS difference between experimental and calculated dipolar couplings of 2.7 Hz, while that for the 16 spectra in 1132 (112 couplings) is 6.2 Hz; this excellent fit with nine adjustable parameters suggests that the assumption of equal temperature dependencies of the order parameters for ethane, propane, and each conformer of butane is correct. Also the fit parameters (E(tg) and the methyl angle increase) obtained for 1132 and 5CB agree. The results indicate that the chord model, which was designed to treat hydrocarbon chains, is indeed the model of choice for these chains. The temperature variation of E(tg) provides a challenge for theoreticians. Finally, even better fits to the experimental dipolar couplings are obtained when the energy in the Boltzmann factor is used for scaling ethane to butane results. However, in this case the values obtained for E(tg) differ between 1132 and 5CB.

Toward the understanding of the high resolution infrared spectrum of C 2H 6 near 3.3 μm

The Fourier transform infrared spectrum of ethane between 2860 and 3060 cm −1 has been re-investigated under high resolution at 229 K. The infrared absorption in this region is due mainly to the CH stretching fundamentals ν 5 (parallel band) and ν 7 (degenerate perpendicular band), and to the parallel combination system ν 8 + ν 11 ( A 4s, A 3s). All the relevant perturbation mechanisms affecting the observed absorption patterns have been clarified. In particular, the main perturbers of the ν 7 state are identified to be the degenerate vibrational combination states ν 8 + ν 11 ( l-type interaction) and ν 3 + 2 ν 4 + ν 8 (Fermi-type interaction). Because of the last interaction, the K″Δ K = −6 transitions occur with intensities comparable to both the infrared active fundamental ν 7 and the almost dark combination ν 3 + 2 ν 4 + ν 8. The parallel combination system ν 8 + ν 11 ( A 4s, A 3s) is overlapped and heavily perturbed by the nearby parallel system ν 4 + ν 11 + ν 12 ( A 4s, A 3s), whose K-structure is spread by the strong z-Coriolis interaction of its two vibrational components. In this work, 95 new transitions to the perturbers of ν 7 have been assigned. They belong mostly to the degenerate vibrational states ν 8 + ν 11 ( E 1d) and ν 3 + 2 ν 4 + ν 8 ( E 1d), and to the parallel system ν 8 + ν 11 ( A 4s, A 3s). A least squares fit calculation, limited to the ν 7 degenerate fundamental and its degenerate perturbers ν 8 + ν 11, ν 3 + 2 ν 4 + ν 8, ν 4 + ν 11 + ν 12, and ν 3 + 3 ν 4 + ν 12 was performed. From the results of this fit, we created a line-by-line database containing the molecular parameters for 4969 transitions in these five bands of 12C 2H 6. Finally, we identified the degenerate combination band ν 2 + ν 8 (62 observed transitions) to be the main perturber ( x, y-Coriolis-type interaction) of the parallel fundamental ν 5.

Jupiter’s stratospheric hydrocarbons and temperatures after the July 2009 impact from VLT infrared spectroscopy

Aims. Thermal infrared imaging and spectroscopy of the July 19, 2009 Jupiter impact site has been used to identify unique features of the physical and chemical atmospheric response to this unexpected collision. Methods. Images and high-resolution spectra of methane, ethane and acetylene emission (7−13 μm) from the 2009 impact site were obtained by the Very Large Telescope (VLT) mid-infrared camera/spectrometer instrument, VISIR. An optimal estimation retrieval algorithm was used to determine the atmospheric temperatures and hydrocarbon distribution in the month following the impact. Results. Ethane spectra at 12.25 μm could not be explained by a rise in temperature alone. Ethane was enhanced by 1.7−3.2 times the background abundance on July 26, implying production as the result of shock chemistry in a high C/O ratio environment, favouring an asteroidal origin for the 2009 impactor. Small enhancements in acetylene emission were also observed over the impact site. However, no excess methane emission was found over the impact longitude, either with broadband 7.9-μm imaging 21 h after the impact, or with center-to-limb scans of strong and weak methane lines between 7.9 and 8.1 μm in the ensuing days, indicating either extremely rapid cooling in the initial stages, or an absence of heating in the upper stratosphere (p 10 mbar), though this solution is highly dependent on the spectral properties of stratospheric debris. The enhanced ethane emission was localised over the impact streak, and was diluted in the ensuing weeks by redistribution of heated gases by zonal flow and mixing with the unperturbed jovian air. Conclusions. The different thermal energy deposition profiles, in addition to the highly reducing (C/O > 1) environment and shallow impactor angle, suggest that (a) the 2009 plume and shock-fronts did not reach the sub-microbar altitudes of the Shoemaker-Levy 9 plumes; and (b) models of a cometary impact are not directly applicable to the unique impact circumstances of July 2009.

Infrared Spectra of Simple Methylidyne, Methylidene, and Insertion Complexes Generated in Reactions of Laser-Ablated Rhodium Atoms with Halomethanes and Ethane

Small rhodium carbene complexes, CX2═RhX2, CXH═RhX2, and CH2═RhX2, are identified in the product matrix infrared spectra from reactions of laser-ablated Rh atoms with tetra-, tri-, and dihalomethanes. Evidence for Rh carbynes, XC≡RhX3, is also found in the tetrahalomethane spectra. Calculated Rh−C bond lengths of 1.788−1.820 and 1.730−1.740 A are appropriate for double and triple bonds, respectively. While only the CH3−RhF insertion primary product is observed in the CH3F spectra, weak RhH and RhD diatomic molecule absorptions suggest that the C−H activation product CH2F−RhH, containing a Rh−H bond, also forms during reaction, but subsequently dissociates. The analogous CH3CH2−RhH insertion product is identified in the ethane spectra, in line with the recently discovered CH3−RhH. The observation of RhH and RhD without dihydride or methylidene absorptions in the product spectra suggests that only the insertion complex is generated in the Rh + C2H6 reactions. The present results reconfirm the trend that hig...

Infrared absorption cross sections for ethane (C 2H 6) in the 3 μm region

Infrared absorption cross sections for ethane have been measured in the 3 μm spectral region from spectra recorded using a high-resolution FTIR spectrometer (Bruker IFS 125/HR). Results are presented for pure ethane gas from spectra recorded at 0.004 cm −1 resolution and for mixtures with dry synthetic air from spectra obtained at 0.015 cm −1 resolution (calculated as 0.9/MOPD using the Bruker definition of resolution), at a number of temperatures and pressures appropriate for atmospheric conditions. Intensities were calibrated using three ethane spectra (recorded at 278, 293, and 323 K) taken from the Pacific Northwest National Laboratory (PNNL) IR database.