Paul E. Schrader

Distributed-feedback dye laser for picosecond ultraviolet and visible spectroscopy

We describe the design and operation of a tunable, picosecond laser system for use in time-resolved spectroscopic measurements in the visible and ultraviolet (UV) spectral region. The laser is designed for fine tuning and high wavelength stability. A Nd:YAG-pumped distributed-feedback dye laser (DFDL) generates pulses that are ∼100 ps in duration with a nearly transform-limited linewidth (∼5 GHz) at a 20 Hz repetition rate. The DFDL pulses are amplified in two bow-tie amplifiers, providing pulse energies of up to 3.0 mJ; the amplified pulses may be frequency doubled to the UV spectral region, providing up to 1.0 mJ. The DFDL wavelength is computer stabilized to within ±0.8 pm (±0.7 GHz, two standard deviations), allowing the wavelength to be stationed on a narrow atomic or molecular transition or permitting nearly continuous spectral scans. Application of the laser system to studies of OH energy transfer has been demonstrated; both laser-induced-fluorescence and degenerate-four-wave-mixing spectra have be...

High-peak-power (>1.2 MW) pulsed fiber amplifier.

We report results from Yb-doped fiber amplifiers seeded with two microchip lasers having 0.38-ns and 2.3-ns pulse durations. The shorter duration seed resulted in output pulses with a peak power of >1.2 MW and pulse energy of 0.67 mJ. Peak power was limited by nonlinear processes that caused breakup and broadening of the pulse envelope as the pump power increased. The 2.3-ns duration seed laser resulted in output pulses with a peak power of >300 kW and pulse energy of >1.1 mJ. Pulse energies were limited by the onset of stimulated Brillouin scattering and ultimately by internal optical damage (fluences in excess of 400 J/cm2 were generated). In both experiments, nearly diffraction-limited beam profiles were obtained, with M2 values of <1.2. Preliminary results of a pulse-amplification model are in excellent agreement with the experimental results of the amplifiers operating in the low-to-moderate gain-depletion regime.

Measurements of NO distributions and fluorescence lifetimes in a non-premixed counterflow CH4/air flame using picosecond time-resolved laser-induced fluorescence

We have applied picosecond laser-induced fluorescence (ps-LIF), excited in the linear regime, to obtainspatial profiles of NO concentrations and NO A 2 Σ + effective fluorescence lifetimes in a non-premixed, counterflow CH 4 /air flame at atmospheric pressure. We used an excitation pulse much shorter than the LIF lifetime and recorded the temporal evolution of the LIF decays. Analysis of the decays with corrections for instrumental broadening yielded NO concentrations nearly free of quenching uncertainties. This study is the first application, to our knowledge, of ps-LIF to measure naturally occurring NO concentrations in flames. Compared to saturated fluorescence measurements using nanosecond pulses (ns-LSF), linear ps-LIF was found to be less susceptible to interferences in fuel-rich regions of the flame and less affected by errors resulting from rotational energy transfer (RET) refilling. Analysis based on a simple four-level, density-matrix model for the ps-LIF excitation motivated the use of non-saturating excitation. Experimental NO concentrations compared favorably with flame model results within uncertainties of current prompt-NO reaction mechanisms. Experimental NO LIF lifetimes were substantially lower in fuel-rich regions than predictions based on quenching cross-section models and flame species calculations. Such overpredictions could explain discrepancies in fuel-rich regions between ns-LSF and linear, quenching-corrected ns-LIF measurements of NO.

Comparison of Gas Temperatures Measured by Coherent Anti-Stokes Raman Spectroscopy (CARS) of O(2) and N(2).

We investigate the accuracy of temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) of O(2) and use measurements taken with N(2) CARS and a thermocouple for comparison. Scanning vibrational CARS spectra of O(2) and N(2) were recorded over a broad range of temperatures: between 294 K and 1900 K in air that was heated in a tube furnace and at approximately 2450 K in a fuel-lean CH(4)-O(2)-N(2) flame. Temperatures were derived from least-squares fits of simulated and experimental spectra. Both the fundamental vibrational band and the first hot vibrational band were included in fitting. In the case of the tube furnace, the N(2) and the O(2) CARS temperature measurements agreed to within 3%, and results were similar with the thermocouple; in the flame the agreement was to within 1%. We conclude that, for cases in which O(2) is present in sufficient concentrations ( approximately 10% or greater), the accuracy of O(2) thermometry is comparable with that of N(2).

Four-wave mixing in nanosecond pulsed fiber amplifiers

We present an experimental and theoretical analysis of four-wave mixing in nanosecond pulsed amplifiers based on double-clad ytterbium-doped fibers. This process leads to saturation of the amplified pulse energy at 1064 nm and to distortion of the spectral and temporal profiles. These behaviours are well described by a simple model considering both Raman and four-wave-mixing contributions. The role of seed laser polarization in birefringent fibers is also presented. These results point out the critical parameters and possible tradeoffs for optimization.

Spontaneous emission from C2(d 3Πg) and C3(A 1Πu) during laser irradiation of soot particles

In order to investigate the contribution of non-thermal processes to laser-induced vaporization of soot, we have recorded temporally and spectrally resolved emission from C2(d 3Πg–a 3Πu; Swan system) and C3(A 1Πu–X ; Swings system) following laser irradiation of soot at 532 and 1064 nm over a wide range of laser fluences. We compared the measured spectra with simulated spectra from C2-Swan and C3-Swings emission to gain new insight into the formation mechanism of the excited species. This comparison shows that the vibrational and rotational energy distributions of the nascent C2(d 3Πg) and C3(A 1Πu) depend strongly on laser wavelength and are not in local thermodynamic equilibrium with the soot. These results suggest non-thermal ejection of highly excited C2(d 3Πg) and C3(A 1Πu) from the particle surface. Multi-photon laser-induced fluorescence from thermally sublimed C2(a 3Πu) and C3(X ) is unlikely to be the source of the observed emission. This work provides fluence thresholds at 532 and 1064 nm for the appearance of spontaneous emission from C2(d 3Πg) and C3(A 1Πu) with sufficient intensity to be observable in the presence of laser-induced incandescence from soot and emphasizes the importance of non-thermal vaporization processes during laser irradiation of soot.

Formation and emission of large furans and oxygenated hydrocarbons from flames

Significance Furans and related large oxygenated organic carbon species (OC) are highly toxic pollutants. Their integration into soot particles may greatly enhance soot’s hygroscopicity, leading to regional and global climate change. We show that furans are the primary oxygenated functional group on soot formed in hydrocarbon combustion and report a reaction scheme that elucidates the interplay between nonoxygenated and oxygenated hydrocarbons. We expect this reaction pathway to be important in many hydrocarbon oxidation systems spanning geosciences, astrophysics, and energy research. We discovered ∼100 oxygenated species previously unaccounted for in hydrocarbon models. This study advances the understanding of the oxidation chemistry of OC, which is critical to many processes, from controlling emissions of toxic combustion by-products to reducing anthropogenic climate change. Many oxygenated hydrocarbon species formed during combustion, such as furans, are highly toxic and detrimental to human health and the environment. These species may also increase the hygroscopicity of soot and strongly influence the effects of soot on regional and global climate. However, large furans and associated oxygenated species have not previously been observed in flames, and their formation mechanism and interplay with polycyclic aromatic hydrocarbons (PAHs) are poorly understood. We report on a synergistic computational and experimental effort that elucidates the formation of oxygen-embedded compounds, such as furans and other oxygenated hydrocarbons, during the combustion of hydrocarbon fuels. We used ab initio and probabilistic computational techniques to identify low-barrier reaction mechanisms for the formation of large furans and other oxygenated hydrocarbons. We used vacuum-UV photoionization aerosol mass spectrometry and X-ray photoelectron spectroscopy to confirm these predictions. We show that furans are produced in the high-temperature regions of hydrocarbon flames, where they remarkably survive and become the main functional group of oxygenates that incorporate into incipient soot. In controlled flame studies, we discovered ∼100 oxygenated species previously unaccounted for. We found that large alcohols and enols act as precursors to furans, leading to incorporation of oxygen into the carbon skeletons of PAHs. Our results depart dramatically from the crude chemistry of carbon- and oxygen-containing molecules previously considered in hydrocarbon formation and oxidation models and spearhead the emerging understanding of the oxidation chemistry that is critical, for example, to control emissions of toxic and carcinogenic combustion by-products, which also greatly affect global warming.

High-power fiber amplifier with widely tunable repetition rate, fixed pulse duration, and multiple output wavelengths

We report a pulsed, fiber-amplified microchip laser providing widely tunable repetition rate (7.1 - 27 kHz) with constant pulse duration (1.0 ns), pulse energy up to 0.41 mJ, linear output polarization, diffraction-limited beam quality (M(2) < 1.2), and < 1% pulse-energy fluctuations. The pulse duration was shown to minimize nonlinear effects that cause temporal and spectral distortion of the amplified pulses. This source employs passive Q-switching, single-stage single-pass amplification, and cw pumping, thus offering high efficiency, simplicity, and compact, rugged packaging for use in practical applications. The high peak power and high beam quality make this system an ideal pump source for nonlinear frequency conversion, and we demonstrated efficient harmonic generation and optical parametric generation of wavelengths from 213 nm to 4.4 mum with Watt-level output powers.

Fiber-based laser with tunable repetition rate, fixed pulse duration, and multiple wavelength output

We report a pulsed, Nd:YAG (1064 nm) microchip laser amplified by a mode-filtered, Yb-doped fiber amplifier. The system provided a widely tunable repetition rate (7.1-27 kHz) with constant pulse duration (1.0 ns), pulse energy up to 0.41 mJ, linear output polarization, diffraction-limited beam quality, and <1% pulse-energy fluctuations. Detailed spectral and temporal characterization of the output pulses revealed the effects of four-wave mixing and stimulated Raman scattering, and we investigated the effects of fiber length and Yb-doping level on system performance. The amplifier output was efficiently converted to a variety of wavelengths between 213 and 4400 nm by harmonic generation and optical parametric generation, with Watt-level output powers. The laser system employs a simple architecture and is therefore suitable for use in practical applications.

Evolution of maturity levels of the particle surface and bulk during soot growth and oxidation in a flame

ABSTRACT We performed a study of the evolution of soot composition and fine structure, i.e., maturity level, in an atmospheric ethylene-air diffusion flame. We used laser-induced incandescence (LII) to provide information about maturity level of the bulk primary particle and X-ray photoelectron spectroscopy (XPS) to provide complementary information about particle-surface-maturity level. The results demonstrate that the bulk material and the particle surface evolve separately in the flame. Increased soot-maturity level is associated with increased long-range order of the particle fine structure. This increased order leads to an increase in the absorption cross-section in the visible and near-infrared and a shift of the absorption to longer wavelengths with increasing maturity level of the bulk particle. These trends result in a decrease in the dispersion exponent (?) and increase in the absorption cross-section scaling factor (?), as inferred from LII measurements. LII measurements demonstrate that bulk-maturity level increases with height-above-the-burner (HAB) until it reaches a plateau in the center of the flame at the maximum in the soot volume fraction. Bulk-maturity level only slightly decreases as soot is oxidized at larger HABs. Increased maturity level also leads to an increase in long-range sp2 hybridization. XPS measurements of the sp2/defect ratio demonstrate an increase in soot surface-maturity level with increasing HAB, but the surface-maturity level increases more gradually with HAB than the bulk-maturity level. Whereas the bulk-fine-structure order decreases slightly in the oxidation region, the surface order decreases dramatically, indicating that oxidation occurs preferentially at the surface under these conditions. Copyright