Waruna D. Kulatilaka

Gas-phase single-shot thermometry at 1 kHz using fs-CARS spectroscopy

Single-laser-shot temperature measurements at a data rate of 1 kHz employing femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of N(2) are demonstrated. The measurements are performed using a chirped-probe pulse to map the time-dependent frequency-spread dephasing of the Raman coherence, which is created by approximately 80-fs pump and Stokes beams, into the spectrum of the coherent anti-Stokes Raman scattering signal pulse. Temperature is determined from the spectral shape of the fs-CARS signal for probe delays of approximately 2 ps with respect to the pump-Stokes excitation. The accuracy and precision of the measurements for the 300-2400 K range are found to be approximately 1%-6% and approximately 1.5%-3%, respectively.

Electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy of nitric oxide

A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.

Photolytic-interference-free, femtosecond two-photon fluorescence imaging of atomic hydrogen

We discuss photolytic-interference-free, high-repetition-rate imaging of reaction intermediates in flames and plasmas using femtosecond (fs) multiphoton excitation. The high peak power of fs pulses enables efficient nonlinear excitation, while the low energy nearly eliminates interfering single-photon photodissociation processes. We demonstrate proof-of-principle, interference-free, two-photon laser-induced fluorescence line imaging of atomic hydrogen in hydrocarbon flames and discuss the methods implications for certain other atomic and molecular species.

One-dimensional single-shot thermometry in flames using femtosecond-CARS line imaging

We report single-laser-shot one-dimensional thermometry in flames using femtosecond coherent anti-Stokes Raman scattering (fs-CARS) line imaging. Fs-CARS enables high-repetition-rate (1-10 kHz), nearly collision-free measurement of temperature and species concentration in reacting flows. Two high-power 800 nm beams are used as the pump and probe beams and a 983 nm beam is used as the Stokes beam for CARS signal generation from the N2Q-branch transitions at ∼2330 cm(-1). The probe beam is frequency-chirped for single-laser-shot imaging. All three laser beams are formed into sheets and crossed in a line which forms the probe region. The resulting 1D line-CARS signal at ∼675 nm is spatially and spectrally resolved and recorded as a two-dimensional (2D) image. Single-shot temperature measurements are demonstrated in flat-field flames up to temperatures exceeding 2000 K, demonstrating the potential of fs-CARS line imaging for high-repetition-rate thermometry in turbulent flames. Such measurements can provide valuable data to validate complex turbulent-combustion models as well as increase the understanding of the spatio-temporal instabilities in practical combustion devices such as modern gas-turbine combustors and augmentors.

Effects of quenching on electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide

We investigate the effects of gas-mixture composition on the electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) signals of nitric oxide (NO). From previous laser-induced fluorescence (LIF) studies, quenching rates are known to change drastically, by factors of 400–800, in mixtures of CO2∕O2∕N2. The observed ERE-CARS signal remains constant to within 30% whereas LIF signals from NO are predicted to decrease by more than two orders of magnitudes in the same environments. This is very significant for using NO ERE-CARS in high-pressure combustion environments where the electronic quenching rate can vary rapidly as a function of both space and time.

Pressure, Temperature and Velocity Measurements in Underexpanded Free Jets using Laser-Induced Fluorescence Imaging

We report measurements of pressure, temperature, and velocity in an underexpanded jet using planar laser-induced fluorescence of nitric oxide. Ultraviolet transitions near 44, 097.5 cm -1 in the A-X (0,0) system of nitric oxide were excited using narrow linewidth laser radiation generated from an optical parametric system, injection seeded using a distributed feedback diode laser. Planar laser-induced fluorescence images with excellent spatial resolution and signal-to-noise ratio were acquired by tuning the frequency of the laser radiation over the nitric oxide absorption line. The images were corrected on a shot-to-shot basis for fluctuations in the laser spatial profile. Line shapes constructed from the corrected planar laser-induced fluorescence images were used to determine pressure and temperature values along the centerline of the jet. Good agreement between laser-induced fluorescence and previously reported N 2 coherent anti-Stokes Raman scattering measurements was observed. The laser-induced fluorescence measurements also compared well with calculations of pressure and temperature using computational fluid dynamics codes. Velocity was measured in supersonic regions of the flowfield on the basis of the Doppler shift in the nitric oxide absorption lines. Planar laser-induced fluorescence images were acquired using laser sheets propagating at 90 and 45 degrees with respect to the flow direction; velocity was determined from the frequency shift of the absorption lines for these two shifts. The laser-induced fluorescence technique can potentially be applied to obtain instantaneous measurements of thermodynamic properties and multiple velocity components in high-speed turbulent flows.

Direct measurements of collisionally broadened Raman linewidths of CO2 S-branch transitions

We report direct measurements of S-branch Raman-coherence lifetimes of CO(2) resulting from CO(2)-CO(2) and CO(2)-N(2) collisions by employing time-resolved picosecond coherent anti-Stokes Raman scattering spectroscopy. The S-branch (ΔJ = +2) transitions of CO(2) with rotational quantum number J = 0-52 were simultaneously excited using a broadband (~5 nm) laser pulse with a full-width-at-half-maximum duration of ~115 ps. The coherence lifetimes of CO(2) for a pressure range of 0.05-1 atm were measured directly by probing the rotational coherence with a nearly transform-limited, 90-ps-long laser pulse. These directly measured Raman-coherence lifetimes, when converted to collisional linewidth broadening coefficients, differ from the previously reported broadening coefficients extracted from frequency-domain rotational Raman and infrared-absorption spectra and from theoretical calculations by 7%-25%.

Detection of atomic hydrogen in flames using picosecond two-color two-photon-resonant six-wave-mixing spectroscopy

We report an investigation of two-color six-wave-mixing spectroscopy techniques using picosecond lasers for the detection of atomic hydrogen in an atmospheric-pressure hydrogen-air flame. An ultraviolet laser at 243 nm was two-photon-resonant with the 2S(1/2) <-- <-- 1S(1/2) transition, and a visible probe laser at 656 nm was resonant with H(alpha) transitions (n=3 <-- n=2). The signal dependence on the polarization of the pump laser was investigated for a two- beam polarization-spectroscopy experimental configuration and for a four- beam grating configuration. A direct comparison of the absolute signal and background levels in the two experimental geometries demonstrated a significant advantage to using the four-beam grating geometry over the simpler two-beam configuration. Picosecond laser pulses provided sufficient time resolution to investigate hydrogen collisions in the atmospheric-pressure flame. Time-resolved two-color laser-induced fluorescence was used to measure an n=2 population lifetime of 110 ps, and time-resolved two-color six-wave-mixing spectroscopy was used to measure a coherence lifetime of 76 ps. Based on the collisional time scale, we expect that the six-wave-mixing signal dependence on collisions is significantly reduced with picosecond laser pulses when compared to laser pulse durations on the nanosecond time scale.

Perturbative theory and modeling of electronic-resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy of nitric oxide.

A theory is developed for three-laser electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) spectroscopy of nitric oxide (NO). A vibrational Q-branch Raman polarization is excited in the NO molecule by the frequency difference between visible Raman pump and Stokes beams. An ultraviolet probe beam is scattered from the induced Raman polarization to produce an ultraviolet ERE-CARS signal. The frequency of the ultraviolet probe beam is selected to be in electronic resonance with rotational transitions in the A (2)Sigma(+)<--X (2)Pi (1,0) band of NO. This choice results in a resonance between the frequency of the ERE-CARS signal and transitions in the (0,0) band. The theoretical model for ERE-CARS NO spectra has been developed in the perturbative limit. Comparisons to experimental spectra are presented where either the probe laser was scanned with fixed Stokes frequency or the Stokes laser was scanned with fixed probe frequency. At atmospheric pressure and an NO concentration of 100 ppm, good agreement is found between theoretical and experimental spectral peak locations and relative intensities for both types of spectra. Factors relating to saturation in the experiments are discussed, including implications for the theoretical predictions.

Temperature profile measurements in the near-substrate region of low-pressure diamond-forming flames

Coherent anti-Stokes Raman scattering (CARS) spectroscopy of diatomic hydrogen was used to measure gas-phase temperature profiles in low-pressure, stagnation-flow, diamond-forming flames. The objectives of the study were to measure detailed temperature profiles for comparison with a numerical flame model and to investigate the presence and magnitude of the temperature jump at the deposition substrate surface. At distances of several hundred microns or greater from the deposition substrate, the measured temperatures were in excellent agreement with the results of a numerical flame model incorporating elementary chemical kinetic and multi-species transport models. The estimated hydrogen CARS temperature accuracy was ±4% near the deposition substrate and was ±5% away from the substrate. The estimated spatial resolution of the measurements was 30 μm normal to the surface, and the gradient in temperature was resolved fully up to approximately 25 μm from the surface. The excellent spatial resolution for the near-surface measurements allowed the investigation of the presence and the magnitude of the discontinuity between the gas and surface temperature at the deposition substrate, a phenomenon known as the temperature jump. Temperature jumps of approximately 100 K were observed in these rich, premixed oxy-acetylene flames ranging from 30 Torr. to 125 Torr. The influence of gas flow rate and pressure on the temperature jump was investigated. The observed temperature jumps are consistent with an accommodation co-efficient in the range of 0.1 to 0.2 for the exchange of energy between the diamond-forming flame gases and the molybdenum surface or the diamond film on the deposition substrate. In these low-pressure flames, the ability to fully resolve the near-substrate temperature profiles will be extremely useful for the validation and improvement of surface chemistry models.