Experimental Spectroscopic Studies of Carbon Monoxide (CO) Fluorescence at High Temperatures and Pressures

100-kHz carbon monoxide TP-PLIF imaging using ultra-narrow-linewidth burst-mode OPO

A 100-kHz rate two-photon planar laser-induced fluorescence (TP-PLIF) imaging of carbon monoxide (CO) is successfully demonstrated utilizing a narrow-linewidth optical parametric oscillator (OPO) generating light at ∼230.1 nm. A specially designed injection-seeded burst-mode OPO was constructed and characterized for this purpose. This OPO efficiently converts the 355-nm output of a high-energy nanosecond burst-mode laser to ∼230.1 nm following parametric splitting and mixing processes. Generation of an ultra-narrow-linewidth 230.1 nm laser pulse is crucial for effectively exciting CO via a two-photon process from the ground X1Σ+ to the B1Σ+ electronic state-enabling PLIF imaging over a large area. The experimental setup is capable of tracking high-speed flow structures of a CO/N2 mixture, showcasing detection speeds 100 times greater than those achieved with previous femtosecond laser sources. This substantial increase in repetition rate will allow time-resolved CO-TP-PLIF measurements in highly dynamic hypersonic boundary layers and detonation-driven combustion processes for revealing chemical kinetics and turbulent aerodynamics.

Two-photon laser-induced fluorescence study of the CO B 1Σ+ (v' = 0) state in a 4850 K plasma plume: Modified molecular constants, evidence of predissociation, and J'-dependent photoionization

We report the two-photon absorption laser-induced fluorescence rotational spectrum of the CO B 1Σ+ ← X 1Σ+ Hopfield-Birge system (v' = 0, v″ = 0) Q-branch in an ∼4850 K, atmospheric pressure plasma torch plume at thermal equilibrium in both the quenching-dominated (low laser intensity) and photoionization-dominated (high laser intensity) regimes. We provide a detailed analysis of the photophysics in these two regimes using a rate equation approach and propose modeling considerations for them as well. In the experimental spectra, distinct rotational transitions up to J″ = 83 are observed, allowing analysis over a very large range of rotational states. Evidence of predissociation is observed for J' ≥ 64 and is likely due to the interaction with the D'1Σ+ electronic state, which has been proposed in the literature but never observed in the v' = 0 state. The line positions of higher rotational states show disagreement with line positions calculated from molecular constants in the available literature, suggesting the need for modifications to the constants, which are reported here. A shift in the B 1Σ+ ← X 1Σ+ absorption spectrum toward higher two-photon energy as a result of the second-order Stark shift was observed in the photoionization-dominated spectrum, and the second-order Stark shift cross section was estimated to be 7 ± 3 × 10-18 cm2. The mean photoionization cross section of the excited upper state was inferred by comparing the line broadening of the two spectra and was estimated to be 11 ± 7 × 10-18 cm2. In addition, weak J'-dependent variations of the photoionization cross section were observed and are reported here.

Modeling of Carbon Monoxide Two-Photon Laser-Induced Fluorescence (LIF) Spectra at High Temperature and Pressure

In this study, quantitative model of two-photon excitation and fluorescence spectra of carbon monoxide based on up-to-date spectroscopic constants collected during an extensive literature survey was developed. This semi-classical model takes into account Hönl-London factors, quenching effects (collisional broadening and shift), ionization and stark effect (broadening and shift), whereas predissociation is neglected. It was specifically developed to first reproduce with a high confidence level the behavior of our experimental spectra obtained from laser-induced fluorescence (LIF) measurements, and then to allow us to extrapolate the fluorescence signal amplitude in other conditions than those used in these experiments. Synthetic two-photon excitation and fluorescence spectra of CO were calculated to predict the fluorescence signal at high pressures and temperatures, which are representative of gas turbine operating conditions. Comparison between experimental and calculated spectra is presented. Influence of temperature on both excitation and fluorescence spectra shapes and amplitudes is well reproduced by the simulated ones. It is then possible to estimate flame temperature from the comparison between experimental and calculated shapes of numerical excitation spectra. Influence of pressure on both excitation and fluorescence spectra was also investigated. Results show that for temperature below 600 K and pressure above 0.1 MPa, the usual Voigt profile is not suitable to reproduce the shape of the excitation spectrum. We found that the Lindholm profile is well suited to reproduce the pressure-dependence of the spectrum in the range 0.1 to 0.5 MPa at 300 K, and 0.1 to 0.7 MPa at 860 K. Beyond 0.7 MPa, in this temperature range, it is shown that the Lindholm profile does no longer match the spectral profiles, in particularly the red wing. Further analyses taking into account the line mixing phenomenon at higher pressure are thus discussed.

Two-photon-excited fluorescence of CO: experiments and modeling

A model based on rate-equation analysis has been developed for simulation of two-photon-excited laser-induced fluorescence of carbon monoxide (CO) in the Hopfield-Birge band at 230 nm. The model has been compared with experimental fluorescence profiles measured along focused beams provided by lasers emitting nano-, pico-, and femtosecond pulses. Good quantitative agreement was obtained between simulations and experimental data obtained in premixed CH₄/C₂H₄-air flames. For excitation with femtosecond pulses, experimental and simulated fluorescence signals showed quadratic dependence on laser power under conditions of low laser irradiance, whereas different sublinear dependencies were obtained at higher irradiances due to photoionization. Simulations of CO signal versus femtosecond laser linewidth suggest the strongest signal for a transform-limited pulse, which is sufficiently broad spectrally to cover the CO Q-branch absorption spectrum. Altogether, the developed rate-equation model allows for analysis of two-photon excitation fluorescence to arrange suitable diagnostic configurations and retrieve quantitative data for CO as well as other species in combustion, such as atomic oxygen and hydrogen.

Dynamics of linear molecules in water: Translation-rotation coupling in jump motion driven diffusion

We study by computer simulations, and by theory, the coupled rotational and translational dynamics of three important linear diatomic molecules, namely, carbon monoxide (CO), nitric oxide (NO), and cyanide ion (CN^-) in water. Translational diffusion of these molecules is found to be strongly coupled to their own rotational dynamics which, in turn, are coupled to similar motions of the surrounding water. In particular, we find that coupled orientational jump motions play an important role in all three cases. While CO and NO show similar features, CN^- exhibits certain differences. Our results agree well with the known experimental values of the diffusion coefficient. We examined the validity of hydrodynamic predictions and found them to be inadequate, particularly for rotational diffusion. A mode coupling theory approach is developed and applied to understand the complexity of translation-rotation coupling.

Spectroscopic investigation of high-pressure femtosecond two-photon laser-induced fluorescence of carbon monoxide up to 20 bar

Extension of laser diagnostic methods to high pressure is critically important because most practical combustion systems operate at elevated pressures. However, increased collisional quenching, fluorescence trapping, and other spectroscopic complications make high-pressure laser diagnostics extremely challenging. As the pressure increases, collisional effects broaden and shift excitation spectral lines, reducing the excitation quantum efficiency of the laser-induced fluorescence (LIF) technique, in particular when using conventional narrowband, nanosecond (ns)-duration laser pulses. In this work, spectroscopic investigation of broadband, femtosecond two-photon LIF (fs-TPLIF) of carbon monoxide (CO) was performed in a high-pressure static gas cell, up to total pressures of 20 bar. The fluorescence emission spectrum broadened marginally at the highest pressure investigated and hence can be neglected in most cases. The subquadratic dependence of the CO fs-TPLIF signal on the laser fluence increased as the pressure is increased. Moreover, the CO fs-TPLIF signal decays more slowly with increasing pressure compared to the previously reported ns-TPLIF data. The signal drop when the total pressure is increased from 1 to 13 bar is approximately 30% in the fs excitation, as compared to approximately a 60% drop in the ns excitation in CO/N₂/O₂ mixtures. The pressure effects on the fluorescence signal were observed to be similar in the Ångström and third positive bands, suggesting that the third positive band could also be used for CO measurements with broadband fs laser pulses. Furthermore, experimentally investigated are the effect of pressure on the CO fluorescence signal in different quenching gases using fixed CO mole fractions, as well as number densities. Overall, the fs-TPLIF scheme is shown to be a promising diagnostics tool for CO detection in practical combustion systems at elevated pressures.

Comprehensive CO detection in flames using femtosecond two-photon laser-induced fluorescence

We demonstrate a femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique for sensitive CO detection, using a 230 nm pulse of 9 µJ and 45 fs. The advantages of fs-TPLIF in excitation of molecular species were analyzed. Spectra of CO fs-TPLIF were recorded in stable laminar flames spatially resolved across the flame front. A hot band (1, n) together with the conventional band (0, n) of the B→A transitions were observed in the burned zone and attributed to the broadband nature of the fs excitation. The CO fs-TPLIF signal recorded across the focal point of the excitation beam shows a relatively flat intensity distribution despite of the steep laser intensity variation, which is beneficial for CO imaging in contrast to nanosecond and picosecond TPLIF. This phenomenon can be explained by photoionization, which over the short pulse duration dominates the population depletion of the excited B state due to the high peak power, but only contributes in total a negligible X state depletion due to the low pulse energy. Single-shot CO fs-TPLIF images in methane/air flames were recorded by imaging the broadband fluorescence. The results indicate that fs-TPLIF is a promising tool for CO imaging in flames.