Modeling and experimental verification of plasmas induced by high-power nanosecond laser-aluminum interactions in air.

It has been generally believed in literature that in nanosecond laser ablation, the condensed substrate phase contributes mass to the plasma plume through surface evaporation across the sharp interface between the condensed phase and the vapor or plasma phase. However, this will not be true when laser intensity is sufficiently high. In this case, the target temperature can be greater than the critical temperature, so that the sharp interface between the condensed and gaseous phases disappears and is smeared into a macroscopic transition layer. The substrate should contribute mass to the plasma region mainly through hydrodynamic expansion instead of surface evaporation. Based on this physical mechanism, a numerical model has been developed by solving the one-dimensional hydrodynamic equations over the entire physical domain supplemented by wide-range equations of state. It has been found that model predictions have good agreements with experimental measurement for plasma front location, temperature, and electron number density. This has provided further evidence (at least in the indirect sense), besides the above theoretical analysis, that for nanosecond laser metal ablation in air at sufficiently high intensity, the dominant physical mechanism for mass transfer from the condensed phase to the plasma plume is hydrodynamic expansion instead of surface evaporation. The developed and verified numerical model provides useful means for the investigation of nanosecond laser-induced plasma at high intensities.

Quantitative hydroxyl concentration time-series measurements in turbulent nonpremixed flames.

Quantitative hydroxyl concentration time-series measurements have been obtained by picosecond time-resolved laser-induced fluorescence in a series of methane-air and hydrogen-argon-air nonpremixed flames. The recovery of a quantitative time series is complicated by the need to account for fluctuations in the fluorescence lifetime. We have recently developed instrumentation that enables the simultaneous measurement of fluorescence signal and lifetime. The present research represents the first application of this technique to turbulent flames. The correction for hydroxyl lifetime fluctuations is shown to be significant for mean concentrations and thus probability density functions but negligible for power spectral densities (PSD's). The hydroxyl PSD's were found to vary slightly with radial and axial location in the flames and to vary significantly with Reynolds number. However, the PSD's in the H(2)-Ar-air flames are nearly identical to those in the CH(4)-air flames.

Photon-counting technique for rapid fluorescence-decay measurement.

We report on a novel laser-induced fluorescence triple-integration method (LIFTIME) that is capable of making rapid, continuous fluorescence lifetime measurements by a unique photon-counting technique. The LIFTIME has been convolved with picosecond time-resolved laser-induced fluorescence, which employs a high-repetition-rate mode-locked laser, permitting the eventual monitoring of instantaneous species concentrations in turbulent flames. We verify the technique by application of the LIFTIME to two known fluorescence media, diphenyloxazole (PPO) and quinine sulfate monohydrate (QSM). PPO has a fluorescence lifetime of 1.28 ns, whereas QSM has a fluorescence lifetime that can be varied from 1.0 to 3.0 ns. From these liquid samples we demonstrate that fluorescence lifetime can currently be monitored at a sampling rate of up to 500 Hz with less than 10% uncertainty (1 sigma) .

Comparisons of laser-saturated, laser-induced, and planar laser-induced fluorescence measurements of nitric oxide in a lean direct-injection spray flame.

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(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 ~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.

Effect of pulsed dye-laser wavelength stabilization on spectral overlap in atmospheric NO fluorescence studies.

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 +/-0.0001 nm in the dye fundamental and +/-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 ~5 to ~0.05%, which results in improved measurement accuracy.

Background corrections for laser-induced-fluorescence measurements of nitric oxide in lean, high-pressure, premixed methane flames.

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, CH(4)/O(2)/N(2) flames. Measurement interferences such as fluorescence and Raman scattering from secondary species become increasingly important for high-pressure LIF studies. O(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.

Time-series measurements of CH concentration in turbulent CH(4) / air flames by use of picosecond time-resolved laser-induced fluorescence.

We report a developing technique capable of making continuous time-series measurements of naturally occurring minor-species concentrations. The high repetition rate of the mode-locked laser used in this technique allows for the study of transient combustion events, such as turbulence, and their effect on minor-species concentrations. The technique is applied to make CH fluorescence time-series measurements and to calculate power spectral densities in a turbulent nonpremixed flame. To our knowledge, the reported time series represents the first such measurement for a naturally occurring minor species in a turbulent flame.

Measurements of hydroxyl concentrations and lifetimes in laminar flames using picosecond time-resolved laser-induced fluorescence.

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 CH(4)-O(2)-N(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 us, 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.

Atmospheric thermometry for metallic surfaces by laser-induced second-harmonic generation.

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 οC range. The current accuracy of the method is ∓ οC. Future work with platinum should permit the application of SHG thermometry to much higher surface temperatures.

Experimental assessment of O(2) interferences on laser-induced fluorescence measurements of NO in high-pressure, lean premixed flames by use of narrow-band and broadband detection.

We experimentally investigate the influence of O(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(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.

Real-time acquisition of laser-induced fluorescence decays.

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(2)-O(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.

Rate-equation model for quantitative concentration measurements in flames with picosecond pump-probe 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 [Appl. Opt. 34, XXX (1995)].

Quantitative concentration measurements of atomic sodium in an atmospheric hydrocarbon flame with asynchronous optical sampling.

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 × 10(9) cm(-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 × 10(17) cm(-3) is predicted for the Q(1) (9) line of A (2)Σ(+) (v = 0)-X(2)II (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.

Formulation of a dimensionless overlap fraction to account for spectrally distributed interactions in fluorescence studies.

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(-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.

Performance of Pyrromethene 580 and 597 in a commercial Nd:YAG-pumped dye-laser system.

We report the performance of two new laser dyes, Pyrromethene 580 and 597, in a 10-Hz, Nd:YAG-pumped dye-laser system. These dyes demonstrate broader and more efficient tuning curves compared with those of Rhodamine 590 and 610. Pyrromethene 580 and 597 provide peak conversion efficiencies of 38% at 552 nm and 32% at 571 nm, respectively. Moreover, these new dyes provide conversion efficiencies of 31% near 572.5 nm, a wavelength used for fluorescence studies of NO. This enhanced efficiency should improve the signal-to-noise ratio in linear fluorescence [NO] measurements.

Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure C(2)H(6)/O(2)/N(2) flames.

We demonstrate saturation of a transition of the OH molecule in high-pressure flames by obtaining saturation curves in C(2)H(6)/O(2)/N(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 +/-50%.

Measurements of atomic sodium in flames by asynchronous optical sampling: theory and experiment.

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 are 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(2)H(4)/O(2)/N(2) flame and detected with the ASOPS instrument. When excited-state lifetimes are fitted by using the ASOPS theory, a 3P((1/2),3/2) ? 3S((1/2)) quenching-rate coefficient of 1.72 x 10(9) s(-1) and a 3P(3/2) ? 3P((1/2)) doublet-mixing rate coefficient of 3.66 x 109 s(-1) are obtained, in excellent agreement with literature values. ASOPS signals obtained over a wide 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.

Single-laser two-step fluorescence detection of atomic hydrogen in flames.

We describe a new single-laser, two-step fluorecence technique for detecting atomic hydrogen and demonstrate its application to flame measurements. This method provides the advantages of a previously demonstrated two-step method (two-photon 243-nm n = 1 ? n = 2 excitation and subsequent single-photon 656-nm n = 2 ? n = 3 excitation, by using two beams produced with two dye lasers) but with a much simpler experimental implementation (two-photon 243-nm n = 1 ? n = 2 excitation and subsequent single-photon 486-nm n = 2 ? 4 excitation, by using the fundamental and frequency-doubled beams from a single 486-nm dye laser).

Asynchronous optical sampling: a new combustion diagnostic for potential use in turbulent, high-pressure flames.

Asynchronous optical sampling (ASOPS) is a pump-probe method that has strong potential for use in turbulent, high-pressure flames. We show that rapid measurement of species number density can be achieved by maintaining a constant beat frequency between the mode-locking frequencies of the pump and probe lasers. We also describe the instrumental timing parameters for ASOPS and consider the optimization of these parameters. Measurement of the nanosecond decay for electronically excited sodium in an atmospheric flame demonstrates the viability of the ASOPS technique in highly quenched flame environments.

Flame temperature measurements using the anomalous fluorescence of pyrene.

We have evaluated the anomalous fluorescence of pyrene as a method for determining flame temperature. Vapor-phase pyrene was injected into four premixed hydrogen/oxygen flames at equivalence ratios of 1.0 and 1.5 and pressures of 22 and 72 Torr. For each flame, the relative laser-induced (S(2) ? S(0))/(S(1) ? S(0)) emission was monitored as a function of height above the burner and compared to the temperature profile as measured by miniature thermocouples. The thermal behavior of pyrene is discussed in terms of a modified intermediate strong-coupling model. A theoretical calculation of the temperature dependence of the S(2)/S(1) emission ratio agrees well with the experimental flame-temperature data. Based on this agreement, conditions are delineated for the most suitable use of pyrene as a molecular temperature probe.