Mid-infrared polarization spectroscopy of C2H2: Non-intrusive spatial-resolved measurements of polyatomic hydrocarbon molecules for combustion diagnostics

Polarization spectroscopy in the mid-infrared (IRPS) has been applied to the detection of acetylene molecules making use of the asymmetric C-H stretching vibration at around 3 μm. The infrared laser pulses were produced through difference frequency generation in a LiNbO₃ crystal pumped by a Nd:YAG and dye laser system. By directly probing the ro-vibrational transitions with IRPS, sensitive detection of molecules with otherwise inaccessible electronic states was realized with high temporal and spatial resolution by using a pulsed laser and a cross-beam geometry. Detection sensitivities of 2 × 10¹³ molecules/cm³ (10 ppm in 70 mbar gas mixture) of C₂H₂ were achieved using the P(1 1) line of the (0 1 0(1 1)⁰)-(0 0 0 0⁰ 0⁰) band. The dependence of the IRPS signal on the pump laser fluence, acetylene mole fraction, and buffer gas pressure of Ar, N₂, H₂, and CO₂ has been studied experimentally. The investigation demonstrates the quantitative nature of IRPS for sensitive detection of polyatomic IR active molecules. In order to fully demonstrate the technique for combustion applications, nascent acetylene molecules were measured in a low pressure methane/oxygen flame.     

A photonic crystal fiber sensor based on differential optical absorption spectroscopy for mixed gases detection

A photonic crystal fiber sensor based on differential optical absorption spectroscopy for mixed gas detection is presented. In such sensor, hollow core photonic crystal fiber is utilized as gas cell and the feasibility for gas detection is verified by experiment. The components concentration of mixed gas NH₃ and C₂H₂ are measured and the detection sensitivity is 143 ppmv.     

Empirical low energy values for methane transitions in the 5852-6181 cm region by absorption spectroscopy at 81 K

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.62 and 1.71 μm (5852-6181 cm⁻¹) using a newly developed cryogenic cell and a series of distributed feedback (DFB) laser diodes. The minimum value of the measured line intensities is on the order of 3 × 10⁻²⁶ cm/molecule The investigated spectral range corresponds to the high energy part of the tetradecad dominated by the 2ν₃ band for which a theoretical treatment is not yet available. The positions and strengths at 81 K of 2187 transitions were obtained from the spectrum analysis. From the values of the line strength at liquid nitrogen and room temperatures, the low energy values of 845 transitions could be determined. The obtained results are discussed in relation with the previous work of Margolis and compared to the line list provided by the HITRAN database.     

The Proton NMR and Structures of Dimethyl and Diethyl Dichalcogenides

The proton NMR spectra of dimethyl dichalcogenides and diethyl dichalcogenides were measured. The experimental results do not show the evidence for the existence of rotational isomerism in each molecules.     

The density functional study of the stabilities,bond characters,static linear polarisabilities,and aromaticities of the ’in-out’ isomerism Hn-60@CnH60（n=70,72,74）

This paper uses the density functional theory to analyse the stabilities,bond characters,static linear polarisabilities,and aromaticities of the ’in-out’ isomerism H n-60 @C n H 60 (n=70,72,74).The binding energies,C-H bond energies,and energy gaps explore that the ’in-out’ isometric perhydrogenation of C n (n=70,72,74) can remarkably improve the stabilities.The static linear polarisabilies of H n-60 @C n H 60 (n=70,72,74) are indeed relative to their shapes,while they show almost nonaromatic character.This study can suggest that the ’in-out’ isometric perhydrogenation of fullerenes could lead to the invention of entirely novel potential hydrogen storage nanomaterials.     

The Raman and Infrared Spectra and Structures of Ethyl Chloride,Bromide and Iodide

The Raman, infrared and proton NMR spectra of the liquid molecules, C₂H₅-X(X[dbnd]Cl, Br, I) have been recorded. And the molecular structures have been studied on the basis of these spectra. Consequently a non-rigid structure in which a CH₂ group rotates freely with respect to the three hydrogen atoms of CH₃ group is established.     

Experimental and kinetic modeling study of C2H4 oxidation at high pressure

A detailed chemical kinetic model for oxidation of C₂H₄ in the intermediate temperature range and high pressure has been developed and validated experimentally. New ab initio calculations and RRKM analysis of the important C₂H₃ + O₂ reaction was used to obtain rate coefficients over a wide range of conditions (0.003-100 bar, 200-3000 K). The results indicate that at 60 bar and medium temperatures vinyl peroxide, rather than CH₂O and HCO, is the dominant product. The experiments, involving C₂H₄/O₂ mixtures diluted in N₂, were carried out in a high pressure flow reactor at 600-900 K and 60 bar, varying the reaction stoichiometry from very lean to fuel-rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Under the investigated conditions the oxidation pathways for C₂H₄ are more complex than those prevailing at higher temperatures and lower pressures. The major differences are the importance of the hydroxyethyl (CH₂CH₂OH) and 2-hydroperoxyethyl (CH₂CH₂OOH) radicals, formed from addition of OH and HO₂ to C₂H₄, and vinyl peroxide, formed from C₂H₃ + O₂. Hydroxyethyl is oxidized through the peroxide HOCH₂CH₂OO (lean conditions) or through ethenol (low O₂ concentration), while 2-hydroperoxyethyl is converted through oxirane.     

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (II) The icosad region (1.49-1.30 μm)

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by differential absorption spectroscopy between 6717 and 7351 cm⁻¹ (1.49-1.36 μm) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral region corresponds to the very congested low energy part of the icosad for which the HITRAN database provides neither rovibrational assignments nor the lower state energies. The positions and strengths at 81 K of 9389 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10⁻²⁶ cm/molecule. From the variation of the line strength between 81 K and 296 K, the low energy values of a total of 4646 transitions were determined. They represent 79.4% and 68.4% of the total absorbance in the region at 81 and 296 K, respectively, and include 28 transitions assigned to the ν₂+4ν₄ band near 6765 cm⁻¹. The reliability of the method based on the association of lines with coinciding centers in the 81 K and 296 K spectra is discussed. The results of the present analysis have been combined with previously analyzed high energy part of the icosad dominated by the ν₂+2ν₃ band near 7510 cm⁻¹. The line list for the whole icosad (6717-7655 cm⁻¹) consists of 12 865 transitions at 81 K.     

Rotationally resolved electronic spectroscopy of a nonlinear carbon chain radical C6H4

Rotationally resolved electronic spectrum of the origin band in the 2A″-X2A″ transition of a nonlinear carbon chain radical C₆H₄⁺ has been recorded in the 604 nm region using cw cavity ring down spectroscopy. The radical was produced by a discharge through an acetylene-helium mixture in a supersonic planar expansion. The rotational structure has been analysed and precisely determined. A band having a-type prolate rotational structure has also been observed near 581 nm. By considering the results of ab initio calculations this band is assigned to a transition involving the excitation of the ν₁₂ fundamental in the upper 2A″ electronic state of the same C₆H₄⁺ isomer.     

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (I) The region of the 2ν3 band at 1.66 μm

In a recent contribution, (Gao B, Kassi S, Campargue A. Empirical low energy values for methane transitions in the 5852-6181 cm⁻¹ region by absorption spectroscopy at 81 K. J Mol Spectrosc 2009;253:55-63.), the low energy values of methane transitions between 1.71 and 1.62 μm were derived from the variation of the line intensities between 296 and 81 K. The line intensities at 81 K were retrieved from the high resolution absorption spectrum of methane recorded at liquid nitrogen temperature by direct absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. For the line intensities at 296 K, the values provided by the HITRAN database were used. As a consequence of the relatively high intensity cut off (4×10⁻²⁴ cm/molecule) of the HITRAN line list in the considered region, the lower energy values were derived for only 845 of the 2187 transitions measured at 81 K. In the present work, our line list was extended by the retrieval of many weak line intensities leading to a set of 3251 transitions. The minimum value of the measured line intensities (at 81 K) is on the order of 10⁻²⁶ cm/molecule. In relation with the project “Greenhouse Gases Observing Satellite” (GOSAT), a much more complete line list for CH₄ at 296 K has become available (intensity cut off of 4×10⁻²⁶ cm/molecule). By applying the two temperature method to our line intensities at 81 K and GOSAT intensities at 296 K, the lower energy values of 2297 transitions could be derived. These transitions represent 99.1% and 90.8% of the total absorbance in the region, at 81 and 296 K respectively. This line list provided as Supplementary Material allows then accounting for the temperature dependence of CH₄ absorption below 300 K. The investigated spectral range is dominated by the 2ν₃ band near 6005 cm⁻¹ which is of particular interest for atmospheric retrievals. The factor 2 narrowing of the Doppler linewidth from room temperature down to 81 K has allowed the resolution of a number of 2ν₃ multiplets and improving the line intensity retrievals. A detailed comparison with GOSAT and HITRAN line lists has revealed a number of possible improvements.     

Line broadening and shift coefficients of acetylene at 1550 nm

Pressure broadening and shift coefficients have been measured for the ν₁ + ν₃ band of acetylene, C₂H₂, broadened by N₂, H₂, D₂, air, and the noble gases at 295 K. Coefficients are reported for lines between 6470 and 6612 cm⁻¹ (1512-1546 nm). The pressure broadening coefficients are in general agreement with those reported for other vibrational bands, indicating that they are insensitive to vibrational excitation. The pressure shift coefficients, by contrast, are found to differ substantially among vibrational bands.     

New FIR laser lines from optically pumped C2H3F, C2H3Cl, C2H3Br, C2H3CN, C2H5F, CH2CF2, HCOOH and CH3Br

Twenty-seven new cw far infrared laser lines with wavelengths between 137 and 988m have been observed from optically pumping C₂H₃F, C₂H₃Cl, C₂H₃Br, C₂H₅F, C₂H₃CN, CH₂CF₂, HCOOH and CH₃Br with a CO₂ laser. The wavelengths of these FIR laser lines were determined together with their optimum pressures and relative intensities.     

ESR of Gamma Irradiated C2H204.2H2O Single Crystal

The gamma-irradiated samples of vacuumed powder and single crystal of C₂H₂O₄.2H₂O have been investigated by ESR method. the spectra of these single crystal and unvacuumed powder are attributed to R C HOH radical, but the peaks belonging to this radical in the gamma irradiated powder sample are not observed in the spectrum after having been vacuumed. After a and g tensor values of this radical have been calculated, it was observed to be anisotropic. At low temperature, a considerable change in the spectrum has not been observed.     

On the isotope effect of Tc in squaric acid (H2C4O4)

The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on T_c is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H₂SQ) is stretched to 2R(D₂SQ) the phase transition temperature 7^H _c of H₂SQ coincides with T^D _c of D₂SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.     

High-sensitivity CRDS absorption spectroscopy of acetylene between 5851 and 6341 cm−1

The absorption spectrum of acetylene has been recorded at room temperature (297 K) using high-sensitivity cavity ring-down spectroscopy (α_min ～ 5×10^?11 cm^?1) in the 5851 and 6341 cm^?1 interval corresponding to a region of very weak absorption. A list of about 10,700 absorption features with estimated absolute line intensities was constructed. The smallest intensities are of the order of 5×10^?29 cm molecule^?1. The line list includes about 2500 absorption lines of ethylene present at the ppm level in the acetylene sample and identified on the basis of a high-resolution Fourier transform spectrum specifically recorded. A total of more than 2700 lines of ¹²C₂H₂ were rovibrationally assigned in comparison with accurate predictions provided by a global effective operator model. Overall, the present effort adds about 2260 new assignments to the set of about 500 assigned transitions available in the literature. The new assignments correspond to 45 new bands and 17 already-known bands, for which additional J lines were assigned. Spectroscopic parameters were derived for the upper vibrational levels from a band by band fit of the line positions (typical root mean square deviation values are of the order of 0.001 cm^?1). A few of the analysed bands were found to be affected by rovibrational perturbations, which are discussed. The new data will be valuable to refine the parameters of the global effective Hamiltonian and dipole moments of ¹²C₂H₂.     

Atomic-potential parameters for H2 and D2: quantum corrections in the calculation of second-virial coefficients

First-order quantum corrections were introduced into the computation of the second-virial coefficients of H₂ and D₂. The quantum effects, for the studied two light molecules, are considerable even at the room temperature and become prominent at low temperatures. Atomic potentials, incorporating the quadrupole interactions, were employed in the calculations. Optimum atomic-potential parameters ε_H, σ_H, ε_D and σ_D were obtained from the nonlinear least-squares fit of the experimental second-virial coefficients. The fitted virial coefficients cover the temperature ranges of 173-423 and 153- for H₂ and D₂, respectively.     

A hybrid variational–perturbational nuclear motion algorithm

A hybrid variational–perturbational nuclear motion algorithm based on the perturbative treatment of the Coriolis coupling terms of the Eckart–Watson kinetic energy operator following a variational treatment of the rest of the operator is described. The algorithm has been implemented in the quantum chemical code DEWE. Performance of the hybrid treatment is assessed by comparing selected numerically exact variational vibration-only and rovibrational energy levels of the C₂H₄, C₂D₄, and CH₄ molecules with their perturbatively corrected counterparts. For many of the rotational–vibrational states examined, numerical tests reveal excellent agreement between the variational and even the first-order perturbative energy levels, whilst the perturbative approach is able to reduce the computational cost of the matrix-vector product evaluations, needed by the iterative Lanczos eigensolver, by almost an order of magnitude.     

Infrared investigation of benzene to 30 GPa transformation to a polymer

Abstract

After a pressure cycle to 30 GPa, benzene transforms to a white solid which can be recovered at ambient. Results obtained from infrared spectroscopy and other physico-chemical methods show that the recovered solid is a highly cross-linked polymer. The molecular structure of benzene is very likely completely altered by the transformation.