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We demonstrate a two-color planar laser-induced fluorescence technique for obtaining two-dimensional temperature images in water. For this method, a pulsed Nd:YAG laser at 532 nm excites a solution of temperature-sensitive rhodamine 560 and temperature-insensitive sulforhodamine 640. The resulting emissions are optically separated through filters and detected via a charged-couple device (CCD) camera system. A ratio of the two images yields temperature images independent of incident irradiance. An uncertainty in temperature of [+ or -] 1.4 [degrees]C is established at the 95% confidence interval. OCIS codes: 120.0120, 300.2530.

Quantitative two-point hydroxyl time-series measurements have been performed in a turbulent nonpremixed flame by using two-point picosecond time-resolved laser-induced fluorescence. The current diagnostic system has been improved from its preliminary version to address optical aberrations and fluorescence lifetime fluctuations. In particular, with a newly designed collection system, the aberration-limited blur spot is reduced from 6 mm to 180 [micro]m. Additional photon-counting channels enable the recovery of absolute OH concentrations through a triple-bin integration method. The present research represents what we believe to be the first application of this two-point technique to turbulent flames. Two independent schemes have been applied to remove uncorrelated noise in the derived two-point statistics. We show that optical aberrations can have a significant effect on space-time correlations. However, the sampling rate and fluctuations in the fluorescence lifetime barely affect the spatial autocorrelation function and thus the integral length scale. Such length scales for hydroxyl are found to rise linearly with increasing axial distance at peak [OH] locations. Along the jet centerline, the integral length scale varies slightly below the flame tip but increases rapidly above the flame tip. In addition, the OH length scale demonstrates the same trend as the OH time scale along the jet centerline, but the opposite trend at peak [OH] locations. OCIS codes: 120.1740, 300.2530, 220.4830, 280.7060.

We report quantitative, spatially resolved measurements of methylidyne concentration ([CH]) in laminar, counterflow partially premixed and nonpremixed flames at atmospheric pressure by using both cavity ring-down spectroscopy (CRDS) and linear laser-induced fluorescence (LIF) in the A-X (0, 0) band. Three partially premixed ([[phi].sub.B] = 1.45, 1.6, 2.0) flames plus a single nonpremixed methane-air flame are investigated at a global strain rate of 20 [s.sup.-1]. These quantitative measurements are compared with predictions from an opposed-flow flame code when utilizing two GRI chemical kinetic mechanisms (versions 2.11 and 3.0). The LIF measurements of [CH] are corrected for variations in the electronic quenching rate coefficient by using predicted major species concentrations and temperatures along with quenching cross sections for CH that are available in the literature. The peak CH concentration obtained by CRDS is used to calibrate the quenching-corrected LIF measurements. Excellent agreement is obtained between CH concentration profiles measured by using the CRDS and LIF techniques. The spatial location of the CH layer is very well predicted by GRI 3.0; moreover, the measured and predicted CH concentrations are in good agreement for all the flames of this study. OCIS codes: 120.1740, 300.1030, 300.2530.

We report quantitative, spatially resolved laser-saturated fluorescence (LSF), linear laser-induced fluorescence (LIF), and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) concentration in a preheated, lean direct-injection spray flame at atmospheric pressure. The spray is produced by a hollow-cone, pressure-atomized nozzle supplied with liquid heptane, and the overall equivalence ratio is unity. NO is excited by means of the [Q.sub.2](26.5) transition of the [Gamma](0, 0) band. LSF and LIF detection are performed in a 2-nm region centered on the [Gamma](0, 1) band. PLIF detection is performed in a broad [approximately]70-nm region with a peak transmission at 270 nm. Quantitative radial NO profiles obtained by LSF are presented and analyzed so as to correct similar LIF and PLIF profiles. Excellent agreement is achieved among the three fluorescence methodologies.

We introduce an inexpensive application of a Fabry-Perot etalon to control long-term UV-laser line drift in atmospheric NO laser-induced fluorescence (LIF) measurements by monitoring the visible fundamental of a pulsed dye laser. A linear image sensor captures the interference pattern, and the dye grating can be adjusted to maintain a fixed wavelength through an interface with LABVIEW software. Results indicate that the laser wavelength can be fixed to an accuracy of [+ or -]0.0001 nm in the dye fundamental and [+ or -]0.00003 nm in the UV beam. Hence the average error in the LIF signal owing to fluctuations in spectral overlap between the laser and the NO absorption transition decreases from [approximately]5 to [approximately]0.05%, which results in improved measurement accuracy. Key words: Dye lasers, laser-induced fluorescence, Fabry-Perot etalon, spectral overlap fraction, wavelength stabilization.

An experimental technique is presented that both minimizes and accounts for the interference background when laser-induced-fluorescence (LIF) measurements are made of NO in lean, high-pressure, premixed, C[H.sub.4]/[O.sub.2]/[N.sub.2] flames. Measurement interferences such as fluorescence and Raman scattering from secondary species become increasingly important for high-pressure LIF studies. [O.sub.2] fluorescence interferences are particularly troublesome in lean premixed flames. An excitation-detection scheme that minimizes the effects of these interferences is identified. A procedure that corrects the resulting LIF signal so as to account for any remaining interference signal is then developed. This correction is found to vary from less than 10% of the overall NO signal at atmospheric pressure to over 40% of the overall signal at 14.6 atm for LIF measurements of NO in a series of worst-case flames ([Phi] = 0.6, dilution ratio 2.2). The correction technique is further demonstrated to be portable over a useful range of flame conditions at each pressure.

We experimentally investigate the influence of [O.sub.2] interferences on laser-induced fluorescence measurements of NO in lean methane-fueled flames at a range of pressures for both narrow-band and broadband fluorescence detection. We identify NO excitation schemes that minimize [O.sub.2] interferences. From detection scans we obtain interference spectra for the different NO excitation schemes. We then identify optimum excitation-detection schemes for narrow-band detection measurements of NO. To simulate broadband detection experiments, we numerically apply five different filter combinations to the experimentally obtained detection scans. We develop filter-assessment parameters to judge the effectiveness of the different filtering schemes, and we establish a methodology for evaluating broadband excitation-detection strategies. From this research we identify optimum excitation-detection schemes for broadband detection measurements of NO.

To the best of our knowledge we report the first demonstration of surface thermometry using laser-induced second-harmonic generation (SHG) on a realistic metallic surface at atmospheric pressure. The surface is probed with a pulsed infrared laser beam and the SHG signal is monitored in reflection. For metallic silver, the SHG signal is found to be temperature dependent in the 25-120 [degrees] C range. The current accuracy of the method is [+ or -]5 [degrees] C. Future work with platinum should permit the application of SHG thermometry to much higher surface temperatures. Key words: Nonlinear optics, second-harmonic generation, surface thermometry.

Picosecond time-resolved laser-induced fluorescence (PITLIF) can potentially be used to obtain measurements of minor species concentrations in rapidly fluctuating flames. Previous studies demonstrated this potential for atomic sodium by monitoring the temporal fluorescence signal with both an equivalent-time and a real-time sampling method. In this developmental study, PITLIF is used to determine hydroxyl concentrations in laminar C[H.sub.4]-[O.sub.2]-[N.sub.2] flames by the measurement of both the integrated fluorescence signal and the fluorescence lifetime. The quenching environment can be monitored with real-time sampling, and thus the necessary quenching rate coefficient is obtained in 348 [[micro]second], which is fast enough for use in many turbulent flows. Fluorescence lifetimes of OH are also measured at different equivalence ratios in laminar flames by the use of the equivalent-time sampling technique. These results compare favorably with predicted lifetimes based on relevant quenching cross sections and calculated species concentrations. Key words: Laser-induced fluorescence, lifetime measurements, hydroxyl.

We report the development of a pump-probe instrument that uses a high-repetition-rate (82-MHz) picosecond laser. To maximize laser power and to minimize jitter between the pump- and the probe-pulse trains, we choose the asynchronous optical sampling (ASOPS) configuration. Verification of the method is obtained through concentration measurements of atomic sodium in an atmospheric methane-air flame. For the first time to our knowledge, ASOPS measurements are made on a quantitative basis. This is accomplished by calibration of the sodium concentration with atomic absorption spectroscopy. ASOPS measurements are taken at a rate of 155.7 kHz with only 128 averages, resulting in a corresponding detection limit of 5 X [10.sup.9] [cm.sup.-3]. The quenching-rate coefficient is obtained in a single measurement with a variation of ASOPS, which we call dual-beam ASOPS. The value of this coefficient is in excellent agreement with literature values for the present flame conditions. Based on our quantitative results for detection of atomic sodium, a detection limit of 2 X [10.sup.17] [cm.sup.-3] is predicted for the [Q.sub.1] (9) line of A 2[[Sigma].sup.+] (v = 0)-[X.sup.2][Phi](v = 0) hydroxyl at 2000 K. Although this value is too large for practical flame studies, a number of improvements that should lower the ASOPS detection limit are suggested. Key words: Combustion, turbulence, sodium, picosecond phenomena, mode-locked lasers, absorption, spectroscopy.

Measurement of radical concentrations is important in understanding the chemical kinetics involved in combustion. Application of optical techniques allows for the nonintrusive determination of specific radical concentrations. One of the most challenging problems for investigators is to obtain flame data that are independent of the collisional environment. We seek to obviate this difficulty by the use of picosecond pump-probe absorption spectroscopy. A picosecond pump-probe absorption model is developed by rate-equation analysis. Implications are discussed for a laser-pulse width that is much smaller than the excited-state lifetime of the absorbing atom or molecule. The possibility of quantitative, quenching-independent concentration measurements is discussed, and detection limits for atomic sodium and the hydroxyl radical are estimated. For a three-level absorber-emitter, the model leads to a novel pump-probe strategy, called dual-beam asynchronous optical sampling, that can be used to obtain both the electronic quenching-rate coefficient and the doublet mixing-rate coefficient during a single measurement. We discuss the successful demonstration of the technique in a companion paper. Key words: Combustion, sodium, picosecond phenomena, absorption, spectroscopy.

Picosecond time-resolved laser-induced fluorescence (PITLIF) has the potential to provide rapid measurements of minor-species concentrations by correction for local quenching conditions on the time scale of turbulence. Previous studies demonstrated that this technique could provide laser-induced fluorescence data and local quenching rates in flames but used equivalent-time sampling to obtain the required fluorescence decays. This precludes the use of PITLIF in turbulent systems. Fluorescence decays of sodium seeded into a laminar [H.sub.2]-[O.sub.2]-Ar diffusion flame are obtained from real-time data with an acquisition rate on the time scale of turbulence. The results obtained with this method are shown to be similar to those obtained from equivalent-time sampling. Key words: Laser-induced fluorescence, lifetime measurements.

Asynchronous optical sampling (ASOPS) is a pump-probe method for the measurement of species concentrations in turbulent high-pressure flames. We show that rapid measurement of species number density can be achieved in a highly quenched environment by maintaining a constant beat frequency between the mode-locking frequencies of the pump and the probe lasers. A model for the ASOPS method based on rate equation theory for three- and four-level atoms is presented. A number of improvements made to the basic ASOPS instrument, which result in a greatly enhanced signal-to-noise ratio. Atomic sodium is aspirated into an atmospheric pressure [C.sub.2.H.sub.4./O.sub.2./N.sub.2.] flame and dete instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a [3P.sub.1/2,3/2 (ar quenching-rate coefficient of 1. 72 x 10(9) [s.sup.-1] and a [3P.sub.3/2] [arrow right] [3P.sub.1/2] 10(9) s.sup.-1 are obtained, in excellent agreement with literature values. ASOPS signals obtained o range of pump and probe beam powers validate the rate equation theory. Improvements are suggested to improve the signal-to-noise ratio, since the present results are limited to laminar flows. key words: Combustion, quenching, spectroscopy, atomic sodium lifetime., A study using asynchronous optical sampling (ASOPS) to determine species concentration in flames is presented. In the experiment, an ASOPS device comprising two dye lasers pumped by mode-locked lasers was applied to an atmospheric pressure C2H4/O2/N2 flame containing atomic sodium. A rate equation theory for three- and four-level atoms was also developed for analyzing the data obtained from ASOPS measurements.

We demonstrate saturation of a transition of the OH molecule in high-pressure flames by obtaining saturation curves in [C.sub.2][H.sub.6]/[O.sub.2]/[N.sub.2] laminar flames at 1, 6.1, 9.2, and 12.3 atm. In addition we present quantitative fluorescence measurements of OH number density at pressures to 12.3 atm. To assess the efficacy of the balanced cross-rate model for high-pressure flames, we compare laser-saturated fluorescence measurements, which were calibrated in an atmospheric-pressure flame, with absorption measurements at 3.1 and 6.1 atm. At 3.1 atm the absorption and fluorescence measurements compare well. At 6.1 atm, however, the concentrations given by laser-saturated fluorescence are 25% lower than the absorption values, indicating some depletion of the laser-coupled levels beyond that at atmospheric pressure. By using a reasonable estimate for the finite sensitivity to quenching, we anticipate that fluorescence measurements that are calibrated at 1 atm can be applied to flames at 10 atm with absolute errors within [+ or -] 50%. Key words: High-pressure flames, laser-induced fluorescence of OH.

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 2Σ+←X 2Π (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. [ABSTRACT FROM AUTHOR]

A combination of planar laser imaging and theoretical modeling has been used to examine matrix-assisted pulsed-laser evaporation direct write (MAPLE-DW) in the bubble regime. MAPLE-DW is a method for patterning substrates via laser-initiated forward transfer of an organic fluid containing metallic particles and coated on a transparent support. For our conditions, best deposition of a silver-based, thick-film ink was found to occur when laser-initiated vaporization forces the ink outward as a bubble. Planar laser imaging was used to monitor bubble growth as a function of time for three different ink films with nominal thicknesses of 12, 25, and 50 μm and two laser beam diameters of 30 and 60 μm. From these measurements, correlations were developed for predicting the maximum height and velocity of bubbles via three known process variables: laser energy, ink thickness, and beam diameter. Further insight on the physics of the MAPLE-DW process was obtained by developing a theoretical model for bubble growth based on a simple force balance associating vapor-pocket pressure and viscous forces. Primary parameters specifying the subsequent differential equation were related to the above process variables. Numerical solutions to the differential equation were used to predict successfully bubble growth versus time for the conditions analyzed in the imaging experiments. [ABSTRACT FROM AUTHOR]

An overlap term is used in fluorescence theory to account for the spectrally distributed interaction between laser radiation and molecular transitions. We present a dimensionless overlap fraction formulation. Compared with the more common dimensional overlap term [in units of inverse wave number (1/[cm.sup.-1])], this form of expression of the interaction between a laser and an absorption transition has a much more practical interpretation and simplifies the equations that describe fluorescence measurements. Key words: Spectral overlap, laser-induced fluorescence, fluorescence modeling.