Atomic Hydrogen In Flames Research Articles

Three-photon-excited laser-induced fluorescence detection of atomic hydrogen in flames.

In many recent studies, ultrashort femtosecond (fs) two-photon (2p) laser-induced fluorescence (LIF) of atomic hydrogen (H) has been demonstrated using a 205 nm excitation. However, 205-nm- deep ultraviolet (UV) pulses can be problematic in practical devices containing thick transmissive optics and can also be susceptible to photolytic production at high laser energies. In this Letter, we investigate the three-photon (3p) excitation scheme of H by using red-shifted 307.7 nm fs laser pulses. Efficient 3p excitation resulting from fs laser pulses enable the 3pLIF detection of H, which was previously unattainable in most flame conditions using ns or ps pulses. Measurements are reported in CH4/O2/N2 Bunsen jet flames and premixed CH4/air flames and compared to similar 2pLIF schemes with fs pulses. Saturation effects, photolytic interferences, and stimulated emissions effects are studied, as well as the benefits of 3pLIF in diagnostic hardware with thick optical windows.

Single-shot, spatially-resolved stand-off detection of atomic hydrogen via backward lasing in flames

We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration yielding 656-nm lasing in the backward direction upon 2-photon pumping with 205-nm femtosecond laser pulses. Spatial resolution is achieved by temporally-resolved detection of the backward lasing using a streak camera. The method is demonstrated in CH4/O2 flames; both in a setup consisting of two flames, with variable spacing between the flames, and in a single flame. Results from the two-flame experiment show that the backward lasing technique is able to determine changes in the separation between the flames as this distance was altered. By maximizing the temporal resolution of the streak camera, obtaining a highest spatial resolution of 1.65 mm, it is possible to resolve the hydrogen signal present in the two reaction zones in the single flame, where the separation between the reaction zones is ∼2 mm. The lasing signal is strong enough to allow single-shot measurements. Results obtained by backward lasing are compared with 2-photon planar laser-induced fluorescence (LIF) images recorded with detection perpendicular to the laser beam path and the results from the two methods qualitatively agree. Although further studies are needed in order to extract quantitative hydrogen concentrations, the present results indicate great potential for spatially resolved single-ended measurements, which would constitute a very valuable asset for combustion diagnostics in intractable geometries with limited optical access. It appears feasible to extend the technique to detection of any species for which resonant two-photon-excited lasing effect has been observed, such as O, N, C, CO and NH3.

Development of two-color laser system for high-resolution polarization spectroscopy measurements of atomic hydrogen

We have developed a high-spectral-resolution laser system for two-photon pump, polarization spectroscopy probe (TPP-PSP) measurements of atomic hydrogen in flames. In the TPP-PSP technique, a 243-nm laser beam excites the two-photon 1S-2S transition, and excited n=2 atoms are then detected by polarization spectroscopy of the n=2 to n=3 transition using 656-nm laser radiation. The single-frequency-mode 243 and 656-nm beams are produced using injection-seeded optical parametric generators coupled with pulsed dye amplifiers. The use of single-mode lasers allows accurate measurement of signal line shapes and intensities even with significant pulse-to-pulse fluctuations in pulse energies. Use of single-mode lasers and introduction of a scheme to select nearly constant laser energies enable repeatable extraction of important spectral features in atomic hydrogen transitions.

Analysis of 205-nm photolytic production of atomic hydrogen in methane flames

We investigate the 205-nm photolytic production of atomic hydrogen in methane flames. This process represents a significant interference in two-photon, laser induced-fluorescence (TP-LIF) detection of atomic hydrogen in flames. Relative TP-LIF profiles of the photolytically produced H atoms were measured using a pump-probe technique in atmospheric-pressure, premixed CH4/O2/N2 flames. A high-fluence, non-resonant, nanosecond pump laser created H atoms by photodissociating flame constituents, and a copropagating, non-perturbing picosecond laser probed the photolytically produced Hatoms via TP-LIF. Spatial profiles of photolytically produced H atoms indicate that both intermediate and product species contribute to the interference in all flames. Excellent agreement between simulated and measured interference signals is observed in the product region of the flames. Vibrationally excited H2O is the dominant source of interference in the product region, but an additional contribution is attributed to vibrationally excited OH radicals. In the flame-front region, CH3 is the dominant precursor, and photodissociation of C2H2 becomes increasingly important in rich flames. Mechanisms for sequential photodissociation of CH3 and C2H2 are presented, indicating that complete dissociation at 205 nm of both precursors is feasible.

Interference-free two-photon LIF imaging of atomic hydrogen in flames using picosecond excitation

Interference-free, two-photon-excited planar laser-induced fluorescence (PLIF) imaging of atomic hydrogen is demonstrated in steady premixed methane flames. PLIF measurements of atomic hydrogen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the laser-induced fluorescence (LIF) signal on laser intensity. Previous two-photon LIF measurements of atomic hydrogen in hydrocarbon flames using nanosecond laser excitation have identified complications from photolytic production of hydrogen atoms by the excitation laser. Recent results from line-imaging studies in our laboratory indicate a significant advantage to using picosecond excitation for imaging atomic hydrogen with negligible photolytic interference. In the current study, we extend our capabilities for interference-free PLIF imaging of atomic hydrogen in steady premixed CH4/O2/N2 flames. Peak single-shot signal-to-noise ratios of approximately 6–8 are achieved, and the estimated single-shot detection limit is on the order of 1016cm-3. Avoidance of interference and stimulated emission and the effects of fluorescence quenching are discussed. Averaged composite PLIF images are generated by combining images from multiple axial locations in the flame. The images show enhanced number densities of atomic hydrogen near the flame tip, in accordance with numerical predictions of diffusional focusing of H-atoms resulting from the sharp curvature of the flame front.

Comparison of nanosecond and picosecond excitation for interference-free two-photon laser-induced fluorescence detection of atomic hydrogen in flames

Two-photon laser-induced fluorescence (TP-LIF) line imaging of atomic hydrogen was investigated in a series of premixed CH4/O2/N2, H2/O2, and H2/O2/N2 flames using excitation with either picosecond or nanosecond pulsed lasers operating at 205 nm. Radial TP-LIF profiles were measured for a range of pulse fluences to determine the maximum interference-free signal levels and the corresponding picosecond and nanosecond laser fluences in each of 12 flames. For an interference-free measurement, the shape of the TP-LIF profile is independent of laser fluence. For larger fluences, distortions in the profile are attributed to photodissociation of H2O, CH3, and/or other combustion intermediates, and stimulated emission. In comparison with the nanosecond laser, excitation with the picosecond laser can effectively reduce the photolytic interference and produces approximately an order of magnitude larger interference-free signal in CH4/O2/N2 flames with equivalence ratios in the range of 0.5< or =Phi< or =1.4, and in H2/O2 flames with 0.3< or =Phi< or =1.2. Although photolytic interference limits the nanosecond laser fluence in all flames, stimulated emission, occurring between the laser-excited level, H(n=3), and H(n=2), is the limiting factor for picosecond excitation in the flames with the highest H atom concentration. Nanosecond excitation is advantageous in the richest (Phi=1.64) CH4/O2/N2 flame and in H2/O2/N2 flames. The optimal excitation pulse width for interference-free H atom detection depends on the relative concentrations of hydrogen atoms and photolytic precursors, the flame temperature, and the laser path length within the flame.

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.

Two-photon-resonant four-wave-mixing spectroscopy of atomic hydrogen in flames

We report a new diagnostic technique based on resonant foru-wave-mixing spectroscopy of the two-photon 1s→2s transition in atomic hydrogen. Observations including Doppler-free spectral lineshapes and saturation behavior are related to models for the nonlinear optical processes. Two-photon wave-mixing measurements of atomic hydrogen concentration in laboratory flames at pressures of 30–760 Torr to be less susceptible to errors from collisional quenching than similar measurements made using laser-induced fluorescene.

An a b i n i t i o study of the reaction of atomic hydrogen with sulfur dioxide

The potential energy surface for H(1 2S)+SO2 has been investigated computationally in order to study the catalytic removal of atomic hydrogen in flames by sulfur dioxide. HF/3–21G(*) and MP2/3–21G(*) levels of theory were employed to locate stationary points, which were then characterized by calculation of the vibrational frequencies. Some geometries were also optimized with the 6–31G* basis set. Two adducts HOSO and HSO2, with H bonded to O or S, respectively, were studied. Energies were estimated at the optimized geometries using spin-projected MP4/6–31G* calculations, which show that planar cis HOSO is more stable than Cs HSO2. An H–OSO bond energy of 109 kJ mol−1 is predicted. By contrast HSO2 is predicted to be 25 kJ mol−1 endothermic with respect to H+SO2, and is insufficiently stable to be significant in combustion chemistry. Transition states were located and the information used to derive the kinetics of H+SO2+Ar⇄HOSO+Ar from 298 to 2000 K. An unusually large energy barrier to recombination, of about 95 kJ mol−1 relative to H+SO2, is proposed. The results are compared with available kinetic measurements. Other potential decomposition channels for HOSO, to SO+OH and isomerization to HSO2, were also analyzed.

Doppler-free two-photon-excited fluorescence spectroscopy of atomic hydrogen in flames

We describe Doppler-free 205-nm two-photon excitation studies of atomic hydrogen performed using a single-mode pulsed dye-laser system in low-pressure flames. Excitation spectra confirm calculated line-strength ratios for transitions to the 3S and 3D states. Collisional broadening rates are obtained in several flames and compared with quenching rates measured in the same flames, using time-resolved fluorescence measurements.

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).

Relative fluorescence-quenching measurements of atomic hydrogen in subatmospheric H 2/O 2/N 2 flames with photoionization controlled-loss spectroscopy

We present a novel techique, photoionization controlled-loss spectroscopy (PICLS), for obtaining relative changes in the fluorescence transition probability ( A + Q) under conditions where conventional measurements of the fluorescence lifetime are not feasible. Using PICLS, vertical profiles of relative changes in the transition probability have been measured in 20-torr premixed flat H 2/O 2/N 2 flames under stoichiometric and fuel-rich conditions. We compare these profiles with predictions from kinetic theory and discuss the implications of this comparison on quantitative fluorescence measurements of atomic hydrogen in flames.

Time-resolved two-photon-excited fluorescence detection of atomic hydrogen in flames

We describe time-resolved fluorescence decay measurements of atomic hydrogen in 20-Torr flames using twophoton, 205-nm excitation with 50-psec laser pulses. These studies provide measurements of the variations in collisional quenching rates of hydrogen atoms measured directly in combustion environments.

Two-photon-excited stimulated emission from atomic hydrogen in flames

The first observation to the author’s knowledge of optically excited stimulated emission in atomic hydrogen is described. The two-photon n = 1 → n = 3 transition of hydrogen atoms in low-pressure flames is excited with 205-nm radiation produced by using a beta barium borate crystal to frequency mix the fundamental and frequency-doubled radiation from a 615-nm pulsed dye laser. The resulting 656-nm n = 3 → n = 2 Balmer-α radiation is readily observable by eye as a coherent beam propagating in both the forward and reverse directions. We describe a variety of characteristics of the stimulated emission, comparing its behavior with simultaneous measurements of two-photon-excited fluorescence, and discuss the possibility that the stimulated-emission process may affect the quantum yield of fluorescence and ionization detection methods. Potential diagnostic applications of two-photon-excited stimulated-emission detection for measuring atomic hydrogen in flames are demonstrated, using simultaneous profile measurements of stimulated emission and fluorescence signals.

Photochemical effects in 243-nm two-photon excitation of atomic hydrogen in flames

This paper describes photochemical effects observed during two-photon 1S-2S excitation of atomic hydrogen in flames using 243-nm laser radiation. An I(4) intensity dependence is observed in regions of the flame where the natural atomic concentration is low, suggesting an I(2) photochemical production mechanism, which we believe is due to two-photon excitation of water molecules, which then predissociate to form H and OH fragments. In a measurement of OH created in the flame by the 243-nm beam, we observe the same I(2) intensity dependence with the laser detuned from the atomic hydrogen 1S-2S resonance, but an apparent I(3.4) dependence is observed when the laser is tuned to the resonance. We believe that a second photochemical mechanism contributes at the resonance, namely, two-photon excitation of H, followed by collisional energy transfer to water molecules, which then fall apart into H and OH fragments. We model this process and show that a combination of I(2) and I(4) dependences can lead to an intensity dependence that mimics a single I(3.4) dependence over a limited range of intensities.

Erratum: ac Stark splitting in multiphoton excitation of atomic hydrogen in flames: Abnormal peak asymmetry due to pressure broadening

Erratum: ac Stark splitting in multiphoton excitation of atomic hydrogen in flames: Abnormal peak asymmetry due to pressure broadening

Multiphoton Excitation Techniques in Combustion Diagnostics

AbstractMultiphoton excitation provides spectroscopic access to several species that are very important in flame studies, but that cannot be detected in combustion environments using conventional optical techniques. This paper describes applications of multiphoton excitation techniques for detecting such species in combustion environments, with an emphasis placed on measurements of atomic hydrogen in flames.

Photochemical effects in 205-nm, two-photon-excited fluorescence detection of atomic hydrogen in flames

This Letter describes photochemical effects observed by using 205-nm, two-photon-excited fluorescence spectroscopy to measure the atomic-hydrogen concentration in atmospheric-pressure hydrogen-oxygen flames. We found that the 205-nm radiation photolyzed water produced in the flame, forming atomic hydrogen in concentrations higher than those naturally present in lean flames and leading to erroneous concentration measurements. UV excitation of molecular oxygen transitions near 205 nm is also discussed; such excitation, followed by rapid predissociation to produce atomic oxygen, may also affect some flame measurements.

Ac Stark splitting in multiphoton excitation of atomic hydrogen in flames: Abnormal peak asymmetry due to pressure broadening.

The author develops a theory of ac Stark (or Autler-Townes) splitting occurring in double optical resonances, one of which is a two-photon transition. It includes both laser bandwidth and collisional broadening effects. Theoretical results are in excellent agreement with Goldsmith's observation of two-peaked structures in the multiphoton excitation spectra of atomic hydrogen in atmospheric-pressure flames. This is believed to be the first study demonstrating that the peak asymmetry in an ac Stark doublet can be reversed due to pressure broadening.

Two-step saturated fluorescence detection of atomic hydrogen in flames

We report a new variation on fluorescence detection, two-step saturated fluorescence spectroscopy, which can provide significant advantages over existing methods for detecting some atomic and molecular species. The method uses multiple-photon excitation of the selected atom or molecule, followed by saturated excitation of an optical transition from the initially excited state and subsequent detection of fluorescence emitted from the final state populated by the saturated excitation. The technique is demonstrated with the detection of atomic hydrogen in flames.