Coherent anti-Stokes Raman Spectroscopy Measurements Research Articles

Coherent anti-Stokes Raman spectroscopy measurement of ethylene in combustion.

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) has been developed for spatially and temporally resolved simultaneous measurement of temperature and mole fraction of most major species in ethylene-air flames. This paper describes a method to infer coherent anti-Stokes Raman spectroscopy complex susceptibility distributions of the ν3 band of ethylene from WIDECARS spectra measured in heated mixtures of ethylene and air, and to use such distributions to fit experimental WIDECARS spectra in an ethylene-air flame. The method is used to measure mole fraction ethylene in a dual-mode supersonic combustor burning premixed ethylene and air with single-laser-shot precision (one standard deviation) of ±0.0025 (absolute).

Characterization of single coal particle combustion within oxygen-enriched environments using high-speed OH-PLIF

This work presents first-of-its-kind high-speed planar laser-induced fluorescence measurements of the hydroxyl radical in the boundary layer of single coal particles. Experiments were performed in a laminar flow reactor providing an oxygen-enriched exhaust gas environment at elevated temperatures. Single coal particles in a sieve fraction of 90–125 µm and a significant amount of volatiles (36 wt%) were injected along the burner’s centerline. Coherent anti-Stokes Raman spectroscopy measurements were taken to characterize the gas-phase temperature. Time-resolved imaging of the OH distribution at 10 kHz allowed identifying reaction and post-flame zones and gave access to the temporal evolution of burning coal particles. During volatile combustion, a symmetric diffusion flame was observed around the particle starting from a distance of ~150 µm from the particle surface. For subsequent char combustion, this distance decreased and the highest OH signals appeared close to the particle surface.

Development of two-beam femtosecond/picosecond one-dimensional rotational coherent anti-Stokes Raman spectroscopy: Time-resolved probing of flame wall interactions

Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman spectroscopy (CARS) is developed utilizing a two-beam phase-matching approach for one-dimensional (1D) measurements demonstrated in an impinging jet burner to probe time-resolved head on quenching (HOQ) of a methane/air premixed flame at Φ=1.0 and Reynolds number=5000. Single-laser-shot 1D temperature profiles are obtained over a distance of at least 4mm by fitting the pure-rotational N2 CARS spectra to a spectral library calculated from a time-domain CARS code. An imaging resolution of ∼61μm is obtained in the 1D-CARS measurements. The acquisition of single-shot 1D CARS measurements, as opposed to traditional point-wise CARS techniques, enables new spatially correlated conditional statistics to be determined, such as the position, magnitude, and fluctuations of the instantaneous temperature gradient. The temperature gradient increases as the flame approaches the metal surface, and decreases during quenching. The standard deviation of the temperature gradient follows the same trend as the temperature gradient, increasing as the flame front approaches the surface, and decreasing after quenching.

Dual-Pump Coherent Anti-Stokes Raman Spectroscopy Measurements in a Dual-Mode Scramjet

In this paper, the authors describe dual-pump coherent anti-Stokes Raman spectroscopy (CARS) measurements of mixing and combustion in a direct-connect scramjet combustor operating at equivalent flight Mach typical of the ramjet–scramjet transition, in the scram mode. Measurements were performed in the University of Virginia’s scramjet test facility in which the air is heated by electrical resistance heaters. The CARS technique is used to acquire temporally and spatially resolved measurements of temperature and species mole fraction. Measurements were at four planes: one upstream of an fuel injector and three downstream. Contour plots of mean flow and standard deviation statistics are presented for cases with and without reaction of the fuel. The vibrational temperature at the exit of the facility nozzle, and in the freestream of the scramjet combustor, is elevated compared with the rotational temperature; the air– vibrational temperature is the same as the facility stagnation temperature. There are spatial nonuniformities of temperature exiting the heater. The mixing of the fuel jet from the single ramp wall injector and the growth of the plume downstream is shown. The flame is attached at the injector and propagates from the wall adjacent to the fuel plume, around the periphery of the plume, before engulfing the whole plume. Computational fluid dynamics modeling shows that the flow can be predicted well using hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes methods with relatively simple subgrid models, provided facility effects are properly accounted for.

Measurements of temperature and hydroxyl radical generation/decay in lean fuel–air mixtures excited by a repetitively pulsed nanosecond discharge

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H2-air, CH4-air, C2H4-air, and C3H8-air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel–air flow in the reactor, initially at T0=500K and P=100torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28kV, discharge gap 10mm, estimated pulse energy 1.25mJ/pulse), operated in burst mode at 10kHz pulse repetition rate. In most measurements, burst duration is limited to 50 pulses, to preclude plasma-assisted ignition. The discharge uniformity in air and fuel–air flows is verified using sub-ns-gated images (employing an intensified charge-coupled device camera). Temperatures measured at the end of the discharge burst are in the range of T=550–600K, using both OH LIF and CARS, and remain essentially unchanged for up to 10ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H2-air is in good agreement with kinetic model predictions. Based on CARS measurement, vibrational nonequilibrium is not a significant factor at the present conditions.Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C2H4-air, C3H8-air, and CH4 is highest in lean mixtures. In H2-air, OH concentration is nearly independent of the equivalence ratio. In C2H4-air and C3H8-air, unlike in CH4-air and in H2-air, transient OH-concentration overshoot after the discharge is detected. In C2H4-air and C3H8-air, OH decays after the discharge on the time scale of ∼0.02–0.1ms, suggesting little accumulation during the burst of pulses repeated at 10kHz. In CH4-air and H2-air, OH concentration decays within ∼0.1–1.0ms and 0.5–1.0ms, respectively, showing that it may accumulate during the burst.The experimental results are compared with kinetic modeling calculations using plasma/fuel chemistry model employing several H2-air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H2-air, CH4-air, and C2H4-air developed by A. Konnov provide the best overall agreement with OH measurements. In C3H8-air, none of the hydrocarbon chemistry mechanisms agrees well with the data. The results show the need for development of an accurate, predictive low-temperature plasma chemistry/fuel chemistry kinetic model applicable to fuels C3 and higher.

Vibrational and rotational CARS measurements of nitrogen in afterglow of streamer discharge in atmospheric pressure fuel/air mixtures

The use of nonequilibrium plasma generated by nanosecond discharges to ignite fuel/air mixtures, known as transient plasma ignition (TPI), has been shown to effectively reduce ignition delay and improve engine performance relative to spark ignition for combustion engines. While this method is potentially useful for many engine applications, at present the underlying physics are poorly understood. This work uses coherent anti-Stokes Raman spectroscopy (CARS) to measure the rotational and vibrational excitation of nitrogen molecules in the discharge afterglow in a variety of fuel/air mixtures outside the limits of combustion in order to elucidate the thermal behaviour of TPI. The time evolution of relative populations of vibrationally excited states of nitrogen in the electronic ground state are reported for each gas mixture; it is shown that generation of these vibrationally excited states is inefficient during the discharge in air but that generation occurs at a high rate roughly 5 µs following the discharge; with the addition of fuels vibrationally excited states are observed during the discharge but an increase in population is still seen at 5 µs. Possible mechanisms for this behaviour are discussed. In addition, rotational temperature increases of at least 500 K are reported for all gas mixtures. The effect of this temperature increase on ignition, reaction rates, and thermal energy pathways are discussed.

Nonlinear CARS measurement of nitrogen vibrational and rotational temperatures behind hypervelocity strong shock wave

The hypervelocity strong shock waves are generated, when the space vehicles reenter the atmosphere from space. Behind the shock wave radiative and non-equilibrium flow is generated in front of the surface of the space vehicle. Many studies have been reported to investigate the phenomena for the aerospace exploit and reentry. The research information and data on the high temperature flows have been available to the rational heatproof design of the space vehicles. Recent development of measurement techniques with laser systems and photo-electronics now enables us to investigate the hypervelocity phenomena with greatly advanced accuracy. In this research strong shock waves are generated in low-density gas to simulate the reentry range gas flow with a free-piston double-diaphragm shock tube, and CARS (Coherent Anti-stokes Raman Spectroscopy) measurement method is applied to the hypervelocity flows behind the shock waves, where spectral signals of high space/time resolution are acquired. The CARS system consists of YAG and dye lasers, a spectroscope, and a CCD camera system. We obtain the CARS signal spectrum data by this special time-resolving experiment, and the vibrational and rotational temperatures of N2 are determined by fitting between the experimental spectroscopic profile data and theoretically estimated spectroscopic data.

Rotational CARS for simultaneous measurements of temperature and concentrations of N2, O2, CO, and CO2 demonstrated in a CO/air diffusion flame

Rotational coherent anti-Stokes Raman spectroscopy (CARS) has over the years demonstrated its strong potential to measure temperature and relative concentrations of major species in combustion. A recent work is the development and experimental validation of a CO2 model for thermometry, in addition to our previous rotational CARS models for other molecules. In the present work, additional calibration measurements for relative CO2/N2 concentrations have been made in the temperature range 294–1246K in standardized CO2/N2 mixtures. Following these calibration measurements, rotational CARS measurements were performed in a laminar CO/air diffusion flame stabilized on a Wolfhard–Parker burner. High-quality spectra were recorded from the fuel-rich region to the surrounding hot air in a lateral cross section of the flame. The spectra were evaluated to obtain simultaneous profiles of temperature and concentrations of all major species; N2, O2, CO, and CO2. The potential for rotational CARS as a multi-species detection technique is discussed in relation to corresponding strategies for vibrational CARS.

Pulse shaping for mode-selective ultrafast coherent Raman spectroscopy of highly scattering solids

We report on mode-selective ultrafast coherent anti-Stokes Raman spectroscopy (CARS) of a powder of sodium dipicolinate. We produce a pair of stretched laser pulses with precisely adjusted matching chirp rates. We use the pulses for frequency-resolved excitation of coherent molecular vibrations in this highly scattering medium. The induced oscillations are probed with the third delayed ultrashort laser pulse. Since the attained spectral width of the pump-Stokes excitation band is on the order of the spacing between the Raman lines, time-resolved CARS measurements reveal single-mode as well as double-mode coherence decay dynamics, depending on the timing between the pump and Stokes pulses. For a fixed probe pulse delay, the sweeping of the arrival time for one of the preparation pulses maps out the CARS spectrum of the analyte.

Experimental investigation of the HNF flame structure

This paper presents the results of an experimental investigation of the flame structure of hydrazinium nitroformate (HNF) at pressures of 0.06 and 0.13 MPa. Laser diagnostics, including laser Doppler anemometry (LDA) and coherent anti-Stokes Raman spectroscopy (CARS), has been used to obtain the velocity and temperature profiles above the burning surface of an HNF pellet. Great effort has been made to measure velocity and temperature in the flame regions close to the burning surface (primary reaction zone and nonluminous zone). As a by-product of the LDA and CARS measurements, we found that these regions contain a dispersed phase with aerosols and gaseous reactants being irregularly released from bubbles formed at the foam layer. Aerosols with size larger than approximately 0.1–0.2 μm have been found in the region between the foam layer and the primary reaction zone. At larger distances from the surface, the aerosols shrink to a size below 0.1 μm until they are fully consumed in the beginning of the brightly luminous flame. It appears that at a fixed location above the surface one intermittently finds final products or gaseous reactants with the balance between the two shifting toward the hot products with increasing distance from the surface.

Broadband Coherent Anti-Stokes Raman Spectroscopy Characterization of Polymer Thin Films

Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of polymer thin film materials. A simple modification to a 1 kHz broad bandwidth sum frequency generation (SFG) spectrometer permits acquisition of CARS spectra for polymer thin films less than 100 nm thick, a dimension relevant to organic electronic device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance-enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained under these conditions consistently demonstrate enhanced signal-to-noise ratio compared to the spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The dielectic properties of the substrate have a dramatic effect on the detected signal intensity. For ultrathin films, the strongest signals are obtained from fused silica surfaces. Similar to surface-enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS, no surface roughening is necessary.

Nonequilibrium vibrational excitation of H2 in radiofrequency discharges: A theoretical approach based on coherent anti-Stokes Raman spectroscopy measurements

Vibrational and rotational experimental temperatures of molecular hydrogen obtained by coherent anti-Stokes Raman spectroscopy in radiofrequency inductive plasmas have been analyzed and interpreted in terms of vibration, electron, dissociation-recombination, and attachment kinetics by using a sophisticated kinetic model recently developed. The analysis clarifies the role of atomic hydrogen in affecting the vibrational content of the molecules. Theoretical plasma composition and population and electron energy distributions are presented as a function of the recombination coefficient γH of atomic hydrogen on the surfaces. The agreement between theoretical and experimental results is achieved for recombination coefficients consistent with those found in the literature.

Strong S1−S2 Vibronic Coupling and Enhanced Third Order Hyperpolarizability in the First Excited Singlet State of Diphenylhexatriene Studied by Time-Resolved CARS

A strong S1−S2 vibronic coupling effect is observed in the time-resolved coherent anti-Stokes Raman spectroscopy (CARS) spectra originating from the first excited singlet state of diphenylhexatriene. As determined from picosecond CARS measurements, the excited state spectrum appears on a subpicosecond time scale. An extremely high excited state hyperpolarizability |γ|excited state ≅ 3 ×10-31 is derived from a CARS line shape analysis and is attributed to the increased delocalization after excitation in accordance with semiempirical calculations of bond lengths. We observe two strongly frequency-broadened vibrations being upshifted with respect to the CC double bond stretching region of the ground state and assign them to the totally symmetric CC stretching motion of the chain. Both frequencies depend on the solvent polarizability, giving evidence of strong S1−S2 vibronic coupling in the lowest excited singlet states. We discuss a model of S1−S2 vibronic coupling via an asymmetric low frequency mode. According to this model a double-well potential for the respective vibrational coordinate is generated in the first excited singlet state, resulting in two frequencies originating from the same type of vibration.

Self-consistent microwave field and plasma discharge simulations for a moderate pressure hydrogen discharge reactor

A self-consistent two-dimensional model of the electromagnetic field and the plasma in a hydrogen discharge system has been developed and tested in comparison to experimental measurements. The reactor studied is a 25 cm diameter resonant cavity structure operating at 2.45 GHz with a silica belljar of 10 cm diameter and 17 cm height contained within the microwave cavity. The inside of the belljar where the discharge occurs contains a substrate holder of 5 cm diameter that is used to hold substrates for diamond deposition. The electromagnetic field model solves for the microwave fields using a finite difference time-domain solution of Maxwell’s equations. The plasma model is a three energy mode (gas, molecular vibration, and electron) and nine species (H2, H, H(n=2), H(n=3), H+, H2+, H3+, H−, electron) model which accounts for non-Boltzmann electron distribution function and has 35 reactions. Simulated characteristics of the reactor in two dimensions include gas temperature, electron temperature, electron density, atomic hydrogen molar fraction, microwave power absorption, and microwave fields. Comparisons of the model are made with close agreement to several experimental measurements including coherent anti-Stokes Raman Spectroscopy measurement of H2 temperature versus position above the substrate, Doppler broadening optical emission spectroscopy (OES) measurements of H temperature versus pressure, actinometry measurements of the relative H atom concentration, Hα OES intensity measurements versus position, and microwave electric field measurements. The parameter range studied includes pressures of 2500–11 000 Pa, microwave powers of 300–2000 W, and three vertical positions of the substrate holder.

Multiplex CARS measurements in supersonic H2/air combustion

and O2 multiplex coherent anti-stokes Raman spectroscopy (CARS) employing a single dye laser has been explored to simultaneously determine the temperature and concentrations of H2 and O2 in a hydrogen-fueled supersonic combustor. Systematic calibrations were performed through a well-characterized H2/air premixed flat-flame burner. In particular, temperature measurement was accomplished using the intensity ratio of the H2 S(5) and S(6) rotational lines, whereas extraction of the H2 and O2 concentrations was obtained from the H2 S(6) and O2 Q-branch, respectively. Details of the calibration procedure and data reduction are discussed. Quantification of the supersonic mixing and combustion characteristics applying the present technique has been demonstrated to be feasible. The associated detection limits as well as possible improvements are also identified.

Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene

Time-resolved coherent antistokes Raman spectroscopy (CARS) spectra from an excited singlet state of diphenylhexatriene (DPH) are reported. As determined from the CARS measurements, the excited-state population increases on a sub-picosecond time scale after excitation in the 1A g–1B u absorption band. We observe two strongly upshifted and anomalously broadened resonances in the double-bond stretching region. According to our semi-empirical calculations of vibrational frequencies, they can be assigned to polyenic p-chain modes of the 1B u and 2A g states, respectively. Frequency shifts as well as broadenings are extremely solvent-dependent, indicating a strong influence on the excited-state conformations. Our time-resolved measurements support a mixed excited singlet state with drastically different geometries of the local minima.

Coherent Anti-Stokes Raman Spectroscopy measurements of temperature fluctuations in turbulent natural gas-fueled piloted jet diffusion flames

Temperature fluctuations have been measured in a turbulent, natural gas-fueled, piloted jet diffusion flame with a fuel jet exit Reynolds number of 9700, using broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) nitrogen thermometry with a best spatial resolution of 0.9 mm. Radial profiles of mean temperature and root mean square (rms) temperature fluctuations have been acquired and temperature probability density functions (pdfs) have been constructed for streamwise distances in the range 4.2 ≤ x d ≤ 66.7 . Comparison with thermocouple measurements shows very good agreement (within 50 K) in regions of moderate temperature gradients (jet core and jet tails), whereas in the steeper gradient jet flank areas spatial averaging leads to CARS mean temperatures lower by as much as 60 K for x d ≥ 25 and by 150 K for x d = 8.3 . The largest rms temperature fluctuations (640 K) occur at radial locations corresponding to about 70% of the full jet height, for all streamwise distances. Comparison with numerical predictions that employ a k-ε model for turbulence and a constrained equilibrium model for chemistry along with a presumed pdf shape (β function) for the mixture fraction, shows very good agreement, for x d ≥ 16.7 , between computed and measured peak and centerline mean temperatures and rms temperature fluctuations, and fair agreement for x d = 8.3 . The thermal jet widths are underpredicted for solx d ≥ 25 . Measured pdfs attain a variety of shapes, from nearly symmetric around the centerline and bimodal near the average reaction zone location, to nearly uniform in parts of the jet flanks and, finally, to triangular at the jet tails. In addition, measured centerline pdfs evolve from triangular to nearly Gaussian as x d increases. The agreement between predicted and measured pdf shapes is excellent at the centerline and is very good in other parts of the jet flame, with the possible exception of bimodal shapes around the average reaction zone location, attributed to inherent limitations in the beta function representation.

Experimental Setup of a Laser Diagnostics System for a High-Temperature Solar Receiver/Reactor

A solar receiver/reactor has been designed specifically to study high-temperature gas phase chemical reactions using a laser based metrology. It is a cavity-type receiver, lined with stabilized ZrO2, and operated at temperatures up to 2000 K. The gas temperature is measured in situ using the coherent anti-Stokes Raman spectroscopy (CARS) of N2. Optical access for the CARS measurement is accomplished via two side windows, each subtending a 118-mrad cone angle at the center of the cavity, providing enough clearance for the input laser beams and the output signal carrying the temperature information. Two endothermic processes were used for the initial evaluation of this method: the NH3 dissociation into N2 and H2, and the CO2-reforming of CH4 into synthesis gas. The process flow was directly exposed to high solar fluxes in addition to infrared radiation emitted by the hot reactor walls. The laser-based metrology performed satisfactorily in spite of the presence of the intense radiation field. This paper describes in detail the technical aspects of the experimental setup, presents examples of spectra and temperature measurements, and discusses practical problems encountered during experimentation.

CARS TEMPERATURE MEASUREMENTS IN A SUPERSONIC RAMJET COMBUSTION CHAMBER

The temperature distribution in the flowfield of a two-dimensional hydrogen jet in a supersonic air stream has been experimentally investigated. Hydrogen was injected parallel to the freestream air through the base of a strut centered in the combustion chamber. This strut, formed like a symmetrical wedge, also served as a flameholder. The aim of this work was the measurement of temperatures in a turbulent supersonic hydrogen-fueled ramjet for better understanding of combustion phenomena and improvement of theoretical predictions. Coherent Anti-Stokes Raman Spectroscopy (CARS) was used as a diagnostic tool for temperature mapping in the combustion chamber. Temperature profiles were determined at three different axial locations by translating the CARS probe-volume vertically across the combustor. Experimental parameters were the total inlet temperature of mainstream-air and the mass flow of injected hydrogen. Schlieren photographs and video pictures of the 306-nm-OH-intensity have been used to get a qualitative understanding of the flowfield and to determine the locations of interest for CARS measurements. Due to the nonlinear dependence of CARS on temperature and the highly turbulent flowfield, time averaging would lead to serious measurement errors. Thus, single pulse CARS measurements have been recorded using nitrogen as a probe molecule.

Dual-broadband rotational cars measurements in an IC engine

This is the first report of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) measurementsin an internal combustion (IC) engine. Single-shot, dual-broadband rotational CARS (DB-RCARS) spectra were recorded both prior to ignition and in the postcombustion gases. From these spectra, both temperature and relative oxygen concentrations were evaluated. The pressure was registered simultaneously with the CARS measurements in the spark-ignition engine burning natural gas and air. Prior to ignition, normally at temperatures below 1000 K and pressures below 2 MPa, a rotational CARS spectrum is very temperature sensitive, and the technique can be used for temperature measurements with high accuracy. Evaluated temperatures show a negligible dependence on uncertainties in parameters such as the nonresonant susceptibility of the gas and the slit width. Moreover, no collisional narrowing of the lines has to be taken into account. The relative standard deviation of evaluated temperatures and of relative oxygen concentrations from single-shot measurements were as low as 1, and 1.4–1.9%, respectively. In the postcombustion gases at temperatures above 2000 K and pressures above 1.5 MPa, the nonresonant CARS background gave a large contribution to the total spectrum. In this temperature and pressure range, the evaluated values of temperature and nonresonant susceptibility are not independent, and the nonresonant susceptibility had to be fixed at a precalculated value to get a reliable temperature evaluation. The avantages and disadvantages of rotational CARS in comparison with vibrational CARS for IC engine measurements are discussed.