Coherent anti-Stokes Raman Spectroscopy Thermometry Research Articles

Application of a backside-illuminated charge-coupled-device camera for single-pulse coherent anti-Stokes Raman spectroscopy N_2 thermometry

The application of an unintensified backside-illuminated CCD for the acquisition of broadband single-pulse coherent anti-Stokes Raman spectroscopy (CARS) spectra is demonstrated. This CCD shows a quantum efficiency 5 times higher than a front-illuminated CCD and offers significant advantages compared with intensified linear photodiode array detectors generally used for single-pulse CARS thermometry. It overcomes the main drawbacks of the intensified linear photodiode array detector in single-pulse CARS N(2) spectroscopy: nonlinearity, limited dynamic range, and image persistence. A method for extending the dynamic range is demonstrated in a highly turbulent flame.

Rotationally inelastic rates over a wide temperature range based on an energy corrected sudden–exponential-power theoretical analysis of Raman line broadening coefficients and Q branch collapse

We study the ability of the energy corrected sudden (ECS) scaling law associated with a hybrid exponential-power (EP) fitting law for the basis rate constants to model the rotational dependence of isotropic Raman linewidths. We determine the temperature dependence of the hybrid law (EP) parameters and give several applications for various molecular collisional systems over a wide temperature range. In particular, we find that the ECS–EP law gives a very good description of both rotational and temperature dependences of the Raman line broadening coefficients of N2 perturbed by H2O contrary to the ECS–P law. For all the collisional systems studied the ECS–EP law is found very suitable to describe the line broadening coefficients. We give a complete set of ECS–EP parameters for these collisional systems which play an important role in coherent anti-stokes Raman spectroscopy thermometry. We also test the ability of the ECS–EP law to predict accurate collapsed Q branch at high density. An application performed on the Raman Q branch of pure N2 at high density demonstrates that the ECS–EP law properly models the rates of state-to-state rotational energy transfer. The overall collisional line shift obtained from the collapsed Q branch is in perfect agreement with low pressure measurements on isolated lines.

CVD diamond deposition processes investigation: CARS diagnostics/modeling

We have applied Coherent Anti-Stokes Raman Spectroscopy (CARS), using a narrowband, scanned colinear configuration, to measure temperatures, relative concentrations, and detect species in low pressure CVD of polycrystalline diamond. CARS measurements were obtained for methane, hydrogen, and acetylene in either or both a rf plasma reactor and a hot filament reactor. In the rf PACVD experiments a mixture of 1% CH4 in H2 was used at a total pressure of 5 torr. The rf power input to the plasma was 300 watts and the H2 and CH4 flow rates were 99 and 1 seem, respectively. As acetylene (C2H2) has been proposed as an intermediate in diamond growth, it was selected for the initial series of measurements. In the absence of rf power, a sensitivity of 5 mtorr was observed; in the plasma downstream of the rf coils, no observable signal attributable to C2H2 was evident. This places an upper limit to conversion of methane to acetylene at 20%, a figure representing the observed sensitivity to C2H2. In the hot filament reactor, the gas flow was 200 sccm of 1% CH4 in H2 at a total pressure of 150 torr. Under these conditions, C2H2 was detectable. Absolute concentrations were not calculated, but the observed spectra are within an order of magnitude of our sensitivity limit. This allows estimation of the C2H2 partial pressure near the substrate as 5–50 mtorr or from 0.66 to 6.6% conversion from methane. In view of this low conversion percentage, the absence of a signal in the rf experiments must be taken as inconclusive. CARS spectra of methane were also obtained in both reactors. In the rf reactor, under similar conditions to those described previously, the methane relative concentration decreased to 25% as the rf power was increased from zero to 400 watts. In the hot filament reactor, CH4 CARS signal profiles were obtained as a function of axial distance from the hot filament, and parametrically as a function of filament temperature. Comparison of these profiles, in which the observed signal decayed monotonically as the filament was approached and increased monotonically downstream of the filament, was made with theoretical calculations. This comparison showed that the fluctuations were attributable to temperature/pressure effects and not to chemistry. To determine if the observed depletion in the rf plasma was similarly attributable, the CARS signal of hydrogen was observed as a function of axial distance downstream of the rf coil centerline and parametrically as a function of rf power. In contrast to expected behavior in the thermal hot filament reactor, little rotational excitation was observed in the plasma. Rotational temperatures were assigned to hydrogen based upon comparison with theoretically derived spectra. At 450 watts of rf power, rotational temperatures of 340 K were observed 4 to 6 cm downstream of the coil, the region where the 25% decrease in CH4 was observed. Little or no density fluctuations accrue due to these temperatures, indicating that the observed depletion in methane signal is attributable to decomposition or chemical reaction in the plasma. In summary, CARS is applicable to reactant species (CH4) axial profiling in both reactors, but can be limited by sensitivity in the detection of intermediate or product species (C2H2). In addition, CARS thermometry can be utilized to profile the rotational temperatures of selected species.

Nitrogen CARS thermometry for a study of temperature profiles through flame fronts

Using coherent anti-Stokes Raman spectroscopy (CARS), we have performed detailed axial and radial temperature surveys in a premixed methane-air flame at atmospheric pressure with a special emphasis on measurements of temperature profiles through thin flame fronts. The temperature is inferred from N 2 CARS spectra. The thickness of flame fronts for flames with a stoichiometric mixture is about 700 μm, while it broadens considerably in a fuel rich flame. The spatial resolution for measuring the temperature profile is about 100 μm.

Pressure-Broadened Linewidths for N2 Coherent Anti-Stokes Raman Spectroscopy Thermometry

Coherent anti-Stokes Raman Spectroscopy (CARS) is a nonlinear, optical mixing technique that is being successfully applied as a diagnostic tool in a number of different fields. CARS is particularly well suited for combustion diagnostics because its coherence and high conversion efficiency make it capable of probing the high interference environments characteristic of practical devices. Because N2 is the dominant constituent of air-fed combustion, it has received the most attention for CARS thermometry. The usefulness of CARS has been demonstrated by N2 temperature measurements in a highly sooting diffusion flame, an internal combustion engine, and in liquid-fueled combustors. Single pulse CARS spectra have also been obtained successfully in combustion environments.

CARS thermometry in practical combustors

The feasibility of coherent anti-Stokes Raman spectroscopy (CARS) for remote, spatially and temporally resolved, measurements in practical combustion systems is demonstrated. N 2 CARS temperature measurements are reported for two different liquid-fueled combustors, a swirl burner, and a JT-12 combustor can, operated at atmospheric pressure and situated in a 50-cm diameter tunnel. Crossed-beam phase-matched CARS was employed to ensure good spatial precision. Delicate instrumentation was housed in a control room adjacent to the burner test cell and the CARS signals were piped out employing 60-μ diameter fiber optic guides. Average temperature measurements and demonstrations of single-pulse thermometry were performed.

CARS thermometry in a sooting flame

Coherent anti-Stokes Raman spectroscopy (CARS) has been employed to map the temperature field with high spatial precision throughout a small, highly sooting, laminar propane diffusion flame. The temperature is ascertained by comparing the experimental CARS signatures from N 2 recorded on an optical multichannel analyzer with computer-synthesized N 2 spectra.