Coherent anti-Stokes Raman Spectroscopy Spectra Research Articles

Simultaneous temperature and exhaust-gas recirculation-measurements in a homogeneous charge-compression ignition engine by use of pure rotational coherent anti-Stokes Raman spectroscopy

Pure rotational coherent anti-Stokes Raman spectroscopy was used for the simultaneous determination of temperature and exhaust-gas recirculation in a homogeneous charge-compression ignition engine. Measurements were performed in a production-line four-cylinder gasoline engine operated with standard gasoline fuel through small optical line-of-sight accesses. The homogenization process of fresh intake air with recirculated exhaust gas was observed during the compression stroke, and the effect of charge temperature on combustion timing is shown. Single-pulse coherent anti-Stokes Raman spectroscopy spectra could not only be taken in the compression stroke but also during the gas-exchange cycle and after combustion. Consequently, the used method has been shown to be suitable for the investigation of two of the key parameters for self-ignition, namely temperature and charge composition.

Coherent Raman spectra of the ν1 mode of 10BF 3 and 11BF 3

High resolution (0.001 cm −1) coherent anti-Stokes Raman spectroscopy (CARS) was used to directly examine the ν 1 symmetric stretching mode of the planar symmetric D 3 h molecules 10BF 3 and 11BF 3. Simulations of the spectra were done using ν 1 rovibrational parameters deduced from published infrared hot-band and difference-band studies and the close similarity to the observed CARS spectra confirms the validity of the infrared constants. No significant perturbations by Fermi resonance or Coriolis interactions with nearby states are observed, in marked contrast to the case of sulfur trioxide, a similar D 3 h molecule recently studied. In the harmonic approximation, the 10BF 3 and 11BF 3 ν 1 Q-branches would be identical since the isotopic substitution is at the center of mass but, interestingly, the ν 1 stretching frequency for 11BF 3 is found to be 0.198 cm −1 higher than for the lighter 10BF 3 isotopomer. This counterintuitive result is reproduced almost exactly (0.200 cm −1) by ab initio calculations (B3LYP/cc-pVTZ) that included evaluation of cubic and quartic force constants and x ij anharmonicity constants. The ab initio computations also predict to within 1% the Δ B, Δ C changes in the rotational constants in going from the ground state to the v 1 = 1 vibrational level. The results illustrate nicely the complementary interplay of modern infrared, Raman, and ab initio methods in obtaining and analyzing rovibrational spectra.

Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed.

Thermometry for turbulent flames by coherent anti-Stokes Raman spectroscopy with simultaneous referencing to the modeless excitation profile

An optimal system for temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) in turbulent flames and flows is presented. In addition to a single-mode pump laser and a modeless dye laser, an echelle spectrometer with a cross disperser is used. This system permits simultaneous measurement of the N2 CARS spectrum and the broadband dye laser profile. A procedure is developed to use software to transform this profile into the excitation profile by which the spectrum is referenced. Simultaneous shot-to-shot referencing is compared to sequential averaged referencing for data obtained in flat flames and in room air. At flame temperatures, the resultant 1.5% imprecision is limited by flame fluctuations, indicating that the system may have a single-shot imprecision below 1%. At room temperature, the 3.8% single-shot imprecision is of the same order as the best values reported for dual-broadband pure-rotational CARS. Using the unique shot-to-shot excitation profiles, simultaneous referencing eliminates systematic errors. At 2000 and 300 K, the 95% confidence intervals are estimated to be +/- 20 and +/- 10 K, respectively.

Coherent Raman Spectra of the ν1 Mode of Carbon Suboxide

High-resolution (0.001 cm(-1)) coherent anti-Stokes Raman spectroscopy (CARS) has been used to study the nu1 symmetric CO stretching mode of the quasi-linear molecule carbon suboxide, C3O2. Q-branch transitions are seen that originate from the ground state and from thermally populated levels of the nu7 CCC bending mode, which is of unusually low frequency. The intensity variation of the Q-branch features on cooling to about 120 K in a jet expansion requires the reversal of the order of assignment given in a previous Raman study at low resolution. The identification of the nu1 sigma(g)+ <-- sigma(g)+ transition from the ground state is confirmed by the absence of J(odd) Q-branch lines in the resolved CARS spectrum. Analysis of this band in terms of a quasi-linear model gives a good fit to the observed transitions and leads to vibrational-rotational parameters (in cm(-1)) of nu1 = 2199.9773(12) and (B' - B'') = -2.044(6) x 10(-4). Other transitions originating from higher nu7 levels occur at only slightly lower wavenumber values and permit the calculation of the double minimum potential in the Q7 bending coordinate. The results indicate that the ground-state barrier to linearity (21.5 cm(-1)) increases by only 0.6 cm(-1) when the CO symmetric stretch is excited.

Improved species concentration measurements using a species‐specific weighting procedure on rotational CARS spectra

AbstractRotational coherent anti‐Stokes Raman spectroscopy (CARS) is used for measurements of temperatures and relative species concentrations. The evaluation of a rotational CARS spectrum is normally performed using a least‐squares fitting algorithm to find the best‐fit theoretical spectrum in a library of spectra calculated at different temperatures and relative concentrations. A general problem is that species with weak spectral features have a minor influence on the results in the standard evaluation procedure. A species‐specific weighting (SSW) procedure enhancing the influence of weak spectral features in the spectral fitting algorithm has therefore been developed. The SSW procedure was tested on the evaluation of low concentrations of O2 in mixtures with N2, and it is shown that both the accuracy and precision of single‐shot spectra concentration measurements are improved by use of the SSW evaluation procedure. Also, a comparison with previously used weighting procedures shows that this approach is superior. The application of the technique to oxygen concentration measurements in the product gas of a fuel‐lean flame is demonstrated. In addition, a test was made using the developed weighting procedure for thermometry, by applying spectral weighting to the rotational spectrum originating from the first thermally excited vibrational state. The precision of the temperature measurements was then slightly improved. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Dual-broadband rotational CARS thermometry in the product gas of hydrocarbon flames

Coherent anti-Stokes Raman spectroscopy (CARS) is an established technique for gas-phase thermometry in combustion. In this work, rotational CARS in the dual-broadband approach has been investigated for flame thermometry in the product gas of ethylene/air flames in a wide range of equivalence ratios (0.5 < ϕ < 2.5). The nitrogen lines dominated the rotational CARS spectra in the whole equivalence range, but for fuel-lean flames also oxygen lines gave significant spectral contributions. Therefore, the temperature evaluation was based on spectral fitting of both nitrogen and oxygen. Product gas species such as carbon dioxide, water, carbon monoxide, and hydrogen contribute to the non-resonant susceptibility of the gas and influence the nitrogen and oxygen linewidths through collisions. In this work, the main focus was on the influence of these collisions on the nitrogen and oxygen Raman linewidths and consequently the evaluated temperature in the product gas of flames. In the range of studied equivalence ratios with various amounts of different product gas species, the evaluated temperature was raised up to 45 K when including broadening from CO 2, H 2O, and CO on the measured nitrogen and oxygen lines. It was also concluded that H 2O was the main contributor in all non-sooting flames. The influence of the non-resonant susceptibility in the spectral evaluation is discussed, and its coupling to the inclusion of the line broadening from product gas species as well. It has thus been shown that line-broadening effects from product gas species are highly important for quantitative rotational CARS thermometry in flames.

Coherent anti-stokes Raman spectroscopy studies of vapour phase niobium pentachloride

The coherent anti-stokes Raman spectroscopy (CARS) technique was used for the first time to determine vibrational spectra of high temperature niobium pentachloride (NbCl 5). Vapour phase CARS spectra of NbCl 5 were obtained from the sample at temperatures 578 K, 623 K and 673 K for υ 1 mode and 573 K for υ 2 mode, respectively. The vibrational frequencies measured were 394 cm −1 for υ 1 mode and 312 cm −1 for υ 2 mode, respectively.

CARS thermometry in high pressure rocket combustors

Coherent anti-Stokes Raman spectroscopy (CARS) has been used to investigate high pressure cryogenic liquid oxygen/gaseous hydrogen (LOX/GH 2) flames at pressures up to 6.5 MPa. The method employs two synchronized broadband CARS systems to probe simultaneously H 2 and H 2O. Temperature is deduced from the H 2 and H 2O CARS profiles while the H 2O concentration is deduced from the modulation from the resonant the H 2O resonant signal superimposed on the nonresonant background signal. Details of the modeling of the CARS spectra at high pressure and the data reduction are discussed. Problems issued from the presence of liquid phase are also investigated. The results demonstrate the applicability and usefulness of CARS for investigating cryogenic combustion under representative rocket model combustor regimes where the pressure range is above the supercritical condition ( P>5.04 MPa).

Quantum control of coherent anti-Stokes Raman processes

Using femtosecond pulse-shaping techniques, we are able to control several useful parameters of coherent anti-Stokes Raman spectroscopy (CARS). By shaping both the pump and the Stokes pulses with an appropriate spectral phase function, we eliminated the nonresonant CARS background. In addition, we designed pulses which selectively excite one of two neighboring Raman levels, even when both are well within the excitation spectrum. High-resolution CARS spectra were recorded in spite of the broad femtosecond pulse spectrum.

Simultaneous vibrational and pure rotational coherent anti-stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames.

The potential of measuring temperature and multiple species concentrations (N2, O2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS spectra, O2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique for probing of additional species are discussed.

Real time ultrafast spectroscopy of shock front pore collapse

Shock-wave induced nanopore collapse (average diameter 100 nm) at 4.2 GPa in a 3-μm-thick poly-methyl methacrylate (PMMA) layer is measured in real time using coherent anti-Stokes Raman spectroscopy (CARS). Pore collapse is monitored via CARS transitions of a dye probe embedded in the porous medium. A pore collapse time constant of 3 ns in PMMA is in poor agreement with hydrodynamic pore collapse models but in excellent agreement with a viscoplastic model that uses the “shock viscosity” determined from the PMMA viscoelastic response to shock. The shock viscosity is more than 12 orders of magnitude smaller than the ordinary viscosity. A downstream gauge of polycrystalline anthracene monitors changes in the steeply rising shock front (&amp;lt;25 ps rise time) after passing through the porous medium or a scattering medium with 100-nm-diam scatterers. The anthracene is a two-dimensional (2D) shock gauge that provides a time sequence of CARS spectra S(t,λ). The 2D gauge is shown to be capable of discriminating between a shock front that gradually rises with time constant tr or a bunch of steeply rising shocklets with an arrival time spread equal to tr. The transmitted shock front is shown to consist of a bunch of steep shocklets with an arrival time spread of 550 ps.

Comparison of gas temperatures measured by coherent anti-Stokes Raman spectroscopy (CARS) of O_2 and N_2

We investigate the accuracy of temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) of O(2) and use measurements taken with N(2) CARS and a thermocouple for comparison. Scanning vibrational CARS spectra of O(2) and N(2) were recorded over a broad range of temperatures: between 294 K and 1900 K in air that was heated in a tube furnace and at approximately 2450 K in a fuel-lean CH(4)-O(2)-N(2) flame. Temperatures were derived from least-squares fits of simulated and experimental spectra. Both the fundamental vibrational band and the first hot vibrational band were included in fitting. In the case of the tube furnace, the N(2) and the O(2) CARS temperature measurements agreed to within 3%, and results were similar with the thermocouple; in the flame the agreement was to within 1%. We conclude that, for cases in which O(2) is present in sufficient concentrations ( approximately 10% or greater), the accuracy of O(2) thermometry is comparable with that of N(2).

N2 CARS Thermometry within the Outer Jacket of a Metal Halide Lamp

We applied high-resolution coherent anti-Stokes Raman spectroscopy (CARS) to measure the gas temperature at a number of points within the nitrogen-filled outer jacket of a 360 W metal halide lamp. A comparison between the measured rotationally resolved N2 CARS spectra with spectral simulations collected between 500 and 1000 K allowed the temperature at each point in the lamp to be determined. In addition to demonstrating the diagnostic power of the CARS technique in a high-intensity discharge lamp, a comparison of the measured temperatures to calculated axial and radial temperature profiles was achieved. Good agreement between the theoretical and experimental temperature profiles was attained.

Rotational coherent anti-Stokes Raman spectroscopy (CARS) in nitrogen at high pressures (0.1-44 MPa): experimental and modelling results

Pure rotational coherent anti-Stokes Raman scattering (CARS) has for the first time been investigated for pressures up to 44 MPa in nitrogen gas at room temperature. An atomic filter consisting of a heated cell with sodium vapour was successfully applied for suppression of stray light originating from the narrowband CARS pump laser beam. With increasing pressure the rotational CARS spectrum is smoothed gradually, and above around 10 MPa spectral lines are no longer resolvable. Experimental data were compared using Raman linewidths calculated with three different models: the energy corrected sudden scaling law (ECS), the modified exponential gap model (MEG), and a semi-classical ab initio model. All three models resulted in bad spectral fits when a linear scaling with pressure was employed. By using additional scale factors for the Raman linewidths the quality of the spectral fits and also the temperature accuracy were significantly improved. The resulting scale factors indicate a non-linear pressure dependence of the linewidths, and visualize a narrowing of the spectral envelope at the highest pressure, i.e. 44 MPa. The results indicate shortcomings in the present isolated line models and also emphasize the need for new experimental data on pure rotational Raman linewidths at high density. Copyright (C) 2000 John Wiley and Sons, Ltd. (Less)

Femtosecond CARS on H2

Abstract Time-resolved non-resonant CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in the gas phase with femtosecond time resolution. The experimental transients show detailed beating structures, reflecting the rotational dynamics of all thermally populated ground state J levels with high fidelity. Modelling of the experimental data yields temperatures and pressures with high resolution. The good agreement with values determined by frequency-domain experiments demonstrates the suitability of this method for the determination of J-dependent collisional-broadening and line-shifting coefficients in gaseous mixtures at high pressures and temperatures, where frequency-domain CARS spectra are difficult to model.

Article

In-situ real time CARS (coherent anti-Stokes Raman spectroscopy) spectra of the nu1 mode of SF6 were recorded in a discharge tube with varying delay times between the discharge pulse and the CARS detection pulse. Theoretical CARS spectra were calculated for the nu1 mode of SF6 at various vibrational temperatures. Based on relationships between line width, frequency shift, and temperature, a best fit method was developed to extract the temperatures from experimental line shapes instead of peak intensities. The kinetic behaviour of the vibrational temperature of SF6 during the discharge pulse was thus obtained. This method is useful for examination of chemical and physical processes in which the total population in a specific vibrational mode changes. PACS No.:33.20f

Annular phase-matched dual-pump coherent anti-Stokes Raman spectroscopy system for the simultaneous detection of nitrogen and methane

The concentration and pressure dependence of dual-pump coherent anti-Stokes Raman spectroscopy (CARS) signals from nitrogen and methane was investigated. CARS spectra were acquired from a gas cell at pressures of 0.007 to 2.24 MPa and methane concentrations of 0.5 to 50%. The square root of the methane signal intensity divided by the nitrogen signal intensity was found to have a near-linear dependence on methane concentration at all pressures investigated. The pressure dependence of this integrated intensity ratio decreased with increasing pressure and became negligible at the highest pressures tested. The shot-to-shot variation at concentrations determined from single-laser-shot measurements was less than 7%. Single-laser-shot CARS spectra of nitrogen and methane were obtained from the cylinder of a firing direct-injection natural gas engine.

Estimation of spatial averaging of temperatures from coherent anti-Stokes Raman spectroscopy

Spatial averaging is a potential problem in the application of coherent anti-Stokes Raman spectroscopy (CARS) to combustion diagnostics when varying temperatures and composition are present at spatial scales smaller than the typical 1-2-mm spatial resolution of CARS. The observed CARS spectrum is then a mixture of the hot and cold components. We show that simulated, spatially averaged spectra, generated by the incoherent addition of intensities, can be significantly different from those obtained by the coherent addition of the electric field amplitudes of the component spectra. The analyses of these simulated, spatially averaged CARS spectra demonstrate that the use of theoretical CARS spectra, generated by the addition of intensities of the hot and cold components of a binary gas mixture, can lead to errors in the estimated flame temperatures.

Determination of the absolute Raman cross section of methyl

Methyl (CH 3) is generated by photodissociation of methyliodide (CH 3I) applying an excimer laser operating at 248 nm. The CH 3 is detected using coherent anti-Stokes Raman spectroscopy (CARS). The CH 3 density is deduced by monitoring simultaneously the reduction of the CARS signal of the parent molecule CH 3I. CARS spectra of CH 3 ( v = 3005 cm −1) are recorded and the rotational temperature is evaluated. The CARS signal intensities are compared with those of deuterium and methane in order to obtain an absolute calibration of the Raman cross section. The measurements yield a Raman cross section of dσ/ dΩ = 7 × 10 −31 cm 2/ sr at λ P = 532 nm .