A study of CARS nitrogen thermometry at high pressure

The accuracy of CARS N₂ thermometry has been assessed using carefully measured experimental data at high pressures for the temperature range 295 to 870 K and pressure range 1–20 bar. Excellent agreement between calculated and experimental spectra was achieved using statistically based fitting laws to model rotational relaxation. Temperature errors of within ±60 K were achieved using the MEG fitting law, and within ±30 K using an extension to the model (XMEG). Results of separate ab initio IOS calculations with ECS corrections for rotational relaxation gave good overall agreement but did not model the structure of the motionally narrowed spectra as well as the empirical models. The various models were also compared with some complementary data measured by DLR, Stuttgart in their high pressure burner for pressures up to 40 bar. This study also investigated the sensitivity of CARS thermometry to pressure; in certain regions a significant reduction in sensitivity was found. Other key factors investigated include nonlinearities in the intensified diode array detector.Work supported by the CEC Project Turbulent Combustion and Diagnostics and by AEA Technology Underlying Programme     

H2 vibrational CARS thermometry

TheQ-branch spectra of molecular hydrogen have been used to determine rotational temperatures bycoherentanti-StokesRamanscattering (CARS). A supersonic jet expansion through a constant flow nozzle of adjustable temperature served as hydrogen source in the temperature range 300 to 2500 K. The analysis of the high-temperature data reveals details of the internal energy transfer in jet expansion.     

Is rotational CARS an alternative to vibrational CARS for thermometry?

Recent developments in rotational CARS thermometry and critical issues when comparing vibrational and rotational CARS thermometry are described. In particular, the development of dual broadband rotational CARS and the noise characteristics of this approach are emphasized. The difficulty with unambiguous temperature determination in vibrational CARS with unknown parameters, in particular the nonresonant background susceptibility, and the lower sensitivity of rotational CARS thermometry at flame temperatures are also discussed.     

Aspects of hydrogen CARS thermometry

The temperature of hydrogen in a graphite tube furnace has been determined using broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy. Significant laser-induced population changes were observed within the vibrational state manifold, whereas the rotational envelope in both fundamental and hot band remained unchanged.     

CARS thermometry in high temperature gradients

CARS is an effective non-intrusive technique for measuring gas temperature in combustion environments. In regions of high temperature gradient, however, the CARS signal is complicated by contributions from gas at different temperature. This paper examines theoretically the uncertainty associated with CARS thermometry in steep temperature gradients. In addition, the work compares the temperature predicted from CARS with the adiabatic mixed temperature of the gas resident in the measurement volume. This comparison helps indicate the maximum sample volume size allowed for accurate temperature measurements.     

CARS thermometry in a transversely heated graphite-tube atomizer used in atomic absorption spectrometry

An experimental setup for gas-temperature diagnostics using Coherent Anti-Stokes Raman Scattering (CARS) in very small sample volumes with great spatial and temporal temperature variations is presented. Studies have been started of a newly designed transversely heated graphite-tube atomizer for atomic absorption spectrometry (Perkin-Elmer, 4100 ZL). For the investigations, high spatial resolution and precise time synchronization of the measurements and also automatic control of the time-dependent intensity of the anti-Stokes signal were realized. The graphite-tube atomizer also offers the possibility of high-temperature-gas spectroscopy. A high-temperature CARS spectrum of CO₂, recorded in the graphite tube at 2300 K, is shown for the first time. A number of hot-band transitions in the spectral region of the Fermi doublet at a Raman shift from 1230 to 1450cm^–1 was observed and assigned, making possible the use of CO₂ as spectroscopic thermometer gas, similar to N₂.     

Parameter studies in practical nitrogen CARS thermometry using standard and advanced fitting codes

A systematic study of the influence of the collisional narrowing and the cross coherence effect on the temperature analysis of N₂-Q branch-CARS spectra at atmospheric pressure is presented. A comparison of calculated spectra over a temperature range 300–2000 K reveals that the standard theory neglecting these effects leads to temperature errors of +1.7% under flame conditions, when the nonresonant background is suppressed. This result is supported by the analysis of experimental CARS temperature measurements on a standard laminar diffusion flame. Furthermore, the temperature misreadings originating from erroneous slit function parameters and laser linewidth were investigated.     

Experimental study of vibrational and pure rotational coherent anti-stokes Raman scattering (CARS) in molecular hydrogen

Using a specially designed excimer-laser-pumped dye laser of adjustable bandwidth high-lying pure rotational transitions of both, ortho-and para-hydrogen have been identified by coherent anti-Stokes Raman scattering (CARS). As an interesting application H₂-based CARS-thermometry is discussed.     

Multiplex H_2 CARS thermometry in a microwave assisted diamond CVD plasma

Rapid temperature measurements in a low pressure, microwave assisted, diamond CVD plasma are reported. By using a “modeless” laser as the Stokes source for H CARS, accurate single-shot and averaged temperatures were obtained which agreed with values obtained from laser induced fluorescence measurements. The speed of data acquisition afforded by multiplex CARS allowed variations of temperature to be monitored with changing plasma conditions induced by variations of pressure, gas composition and microwave power. The application of the technique for “on-line” monitoring of plasma processes is briefly discussed. Received: 13 February 1996 / Accepted: 12 June 1996     

Study of processes of translational,rotational, and vibrational relaxation based on CARS spectra line shapes

This paper reviews the various physico-chemical processes responsible for actual linewidths encountered in high-resolution coherent anti-Stokes Raman spectroscopy (CARS). Most of the experimental data are based on linewidth measurements using a pulseamplified CARS spectrometer with an emission bandwidth (FWHM) of 2×10^–3 cm^–1. Detailed rotational and vibrational relaxation constants have been obtained from the analysis of theQ-branch profiles of C₂H₂, N₂, CH₄, and SiH₄.     

H2 CARS thermometry in a fuel-rich,premixed, laminar CH4/air flame in the pressure range between 5 and 40 bar

To establish H₂ CARS thermometry at high pressure, accumulated H₂ Q-branch CARS spectra were recorded in the exhaust of a fuel-rich CH₄/air flame at pressures between 5 and 40 bar. Temperatures were deduced by fitting theoretical spectra to experimental data points. The Energy-Corrected Sudden (ECS) scaling law was employed to set up an empirical model for the calculation of H₂ linewidths in high-pressure hydrocarbon flames with H₂ as a minority species. Experimental H₂ CARS spectra could be simulated very accurately with this model. The evaluated temperatures agreed well with reference temperatures obtained by spontaneous rotational Raman scattering of N₂.     

CARS in combustion: Prospects and problems

The principles of CARS are reviewed. This technique has achieved important successes in combustion, but its use has been limited mostly to temperature measurements. We demonstrate the feasibility of sensitive single-shot concentration measurements (to 0.2%) with fair accuracy, in spite of the adverse effects of turbulence and saturation. Even better sensitivities will be achieved using electronic resonance enhancement.     

Multiplex rotational CARS of N2, O2, and CO with excimer pumped dye lasers: Species identification and thermometry in the intermediate temperature range with high temporal and spatial resolution

Pure rotational CARS spectra of N₂, O₂, air, and CO have been obtained using excimer laser pumped dye-lasers. The combination of the folded BOXCARS phase matching geometry with the broad-band laser multiplex method allowed high spatial and temporal resolution. Species and concentration analysis as well as thermometry up to 700 K is demonstrated, and possible applications are discussed.     

CARS spectroscopy of SF6 and UF6

CARS spectra of thev ₁-band of SF₆ and UF₆ performed under (static) cell conditions and under supersonic jet expansion have been recorded. The cooling behavior in supersonic jet expansions has been demonstrated for both gases.Work supported by BMFT-funds     

Determination of some optical constants of LiNbO3 by backward stimulated Raman Scattering

A study of the 2 lowest-frequency A₁ phonon modes is performed in various LiNbO₃ samples using backward stimulated Raman Scattering. The measured values of the damping constant are smaller than those previously reported in literature. Moreover, the nonlinear coefficient present in the Raman Scattering intensity expression is shown to be very sensitive to the crystal composition. This composition, and the homogeneity of the sample are characterized from second harmonic generation experiments.     

Relation of pump-beam quality and conversion efficiency in the Raman downward conversion

To investigate the full conversion of pump power to the Stokes waves in stimulated Raman scattering with a single-pass cell, unconverted pump energies have been analyzed in detail in relation to the first Stokes, second Stokes and first anti-Stokes energies. Adoption of a long focusing configuration with a high-pressure cell was found to be effective but insufficient to suppress the four-wave mixing effect, because a small amount of pump energy always remained undepleted. Comparison of the results using pump beams of various spectral and spatial mode characteristics from alexandrite and Nd:YAG lasers revealed that the spatial mode quality was the most critical parameter for full depletion of a pump pulse.     

Generation of 6.8–7.7 µm radiation in lithium iodate

Generation of infrared radiation in ts-infrared transmission window (6.8–7.7 µm) is reported for the first time by non-collinear difference-frequency mixing of the Nd: YAG second harmonic and the same-pumped dye (Rh-610) laser radiation in a lithium-iodate crystal. The spectrum of polythene sheets was run with the generated radiation.     

Analysis of Raman gain for focussed Gaussian pump beams

Several theoretical and numerical models have been published which describe the evolution of a Stokes beam in a Raman medium excited by a focussed pump beam. Generally, the published theoretical departures from the plane-wave theory of Raman scattering are based on assumptions about the power of the pump beam. In this paper we present a theoretical model which is shown to be in excellent agreement with an exact numerical treatment, and which is valid without restrictions on the pump power. Its predictions are used to indicate the range of validity of earlier theories.     

Simplified technique for subnanosecond pulse generation and injection mode-locking of a XeCl laser

A line narrowed XeCl oscillator/amplifier has been used to generate pulses of 100 ps duration by a combination of stimulated Brillouin scattering and optical breakdown. In addition, a Brillouin mirror has been used to control the amplified spontaneous emission in an injection mode-locked XeCl oscillator.     

Calculation of the signal time-profile in transient CARS experiments

The expressions for the resonant and non-resonant contributions to the CARS signal in a time-resolved CARS experiment are derived on the basis of a diagrammatic perturbative expansion of the polarization. The laser fields are assumed to be off-resonance with respect to any electronic level of the system. The probe pulse is supposed to be derived from one of the two pump pulses (₁). Under this assumption the effect of the noise content of ₁ laser pulses on the signal time-profile (coherence artefact) is correctly reproduced by modelling the noise as a stationary stochastic process. The choice of appropriate polarization geometries is shown to allow for direct measurement of the individual contributions to the CARS signal; the experiments performed on a KClO₄ single crystal fully confirm the results of the calculation based on our theoretical treatment.