Jack A. Syage

Time-resolved imaging of flame kernels: Laser spark ignition of H2/O2/Ar mixtures

The shape and structure of developing flame kernels in laser-induced spark ignited hydrogen/air mixtures is investigated as a function of gas composition and time. Using planar laser-induced fluorescence (PLIF) to measure the spatial distribution of OH radicals produced inside the reaction zone, we have recorded the evolution of the nascent flame kernel in a series of images following the laser-induced spark. This series provides the rate of flame growth, the evolution of the flame shape, and the intensity of the PLIF signal as a function of time for both igniting flames and nonignition events. The reaction zones grow quickly at early times, but slowly decrease in propagation rate as the energy density within the flame kernel decreases. A distinct anisotropy is observed in the expanding spark and flame kernel. At short times (t < 100 μs), a toroidal shape is observed similar to that seen previously for electrode-spark ignitions and for laser ignitions in methane/air. There is also a tendency for the flame to grow back toward the ignition laser. Successful ignitions appear virtually identical to failed ignitions during the first 100 μs. Significant differences, notably in intensity, appear between 100 and 500 μs following the spark. These observations imply that early flame kernel growth is dominated by gas motion induced by the short-duration spark. The ultimate fate of an ignition lies with the chemistry of the reactions which determines whether the gas undergoes a transition from hot plasma to propagating flame.

Picosecond measurements of phenol excited‐state proton transfer in clusters. I. Solvent basicity and cluster size effects

Excited‐state proton transfer (ESPT) rates in molecular clusters were measured as a function of cluster size using picosecond spectroscopy in a molecular beam mass spectrometer. ESPT from the S1 state of phenol to base solvent clusters (NH3)n occurs for a critical solvent cluster size n≥5, with a rate constant of k=(60±10 ps)−1 for n=5–7. ESPT showing critical cluster‐size dependencies was also observed in the basic solvent N(CH3)3(n≂3). Proton transfer was not observed in the less‐basic solvent clusters (CH3OH)n and (H2O)n. Mixed‐solvent studies indicate that the addition of a dissimilar molecule to an otherwise neat solvent cluster impedes ESPT, presumably due to a disruption of the hydrogen bonding network. Evidence is also presented for the direct measure of solvent reorganization following ESPT. For (NH3)n solvation, the solvent reorganization appears as a long‐time‐scale component (0.3 ns) on the protonated solvent formation traces.

Dynamics of flame propagation using laser‐induced spark initiation: Ignition energy measurements

A method is described for measuring low‐energy ignition of combustable mixtures using nanosecond and picosecond laser‐induced breakdown. Ignition studies are reported for homogeneous H2/air mixtures. The extent of flame inhibition in H2/air was investigated as a function of CO2 concentration. Our electrodeless spark ignition results are compared with earlier work using electric discharge ignition. A simple model for flame ignition is advanced, based on the temporal and energy density characteristics of the source energy. Implications of laser‐induced spark ignition in homogeneous fuel mixtures to the molecular dynamics of flame propagation are discussed.

Picosecond mass‐selective measurements of phenol‐(NH3)n acid–base chemistry in clusters

The rate of proton transfer from the acidic S1 state of phenol to the basis solvent (NH3)n was measured as a function of solvent cluster size n. A distinct reaction threshold was observed for solvent size n=5 for 266 nm picosecond excitation. The proton transfer rate was measured to be ka=(60±10 ps)−1 for n=5–7. A competitive recombination rate of k−a =(350±100 ps)−1 occurs for n=5. Additional solvation stabilizes the product side causing the reaction enthalpy and consequently k−a to decrease. No evidence of proton transfer was observed when phenol was seeded in the less basic solvent clusters (CH3OH)n and (H2O)n.

Photofragment imaging by sections for measuring state‐resolved angle‐velocity differential cross sections

We describe a two‐dimensional (2D) imaging technique for recording state‐specific photofragment angle‐velocity (θ,v) distributions. In these experiments the photofragment images are recorded as 2D sections of the 3D angular distributions using state‐specific ionization in a time‐of‐flight mass spectrometer. We compare this method to previous methods that record 2D projections of the 3D distribution. The 2D sections represent cartesian flux‐velocity maps in the center of mass and are related to angle‐velocity differential cross sections by a simple geometric factor. Two studies are highlighted. In the first, new results are presented for the A state photodissociation of CH3I to CH3+I. (θ,v) images are presented for I atom in the 2P3/2 and 2P1/2 spin–orbit states following photodissociation at 266 and 304 nm. The principal result is detection of the weak perpendicular transitions to the 3Q1 state (at 304 nm) and the 1Q state (at 266 nm) that underlie the strong parallel transition to the 3Q0 state. We also ...

Picosecond mass-selective measurements of molecular cluster reactions: (CH3I)n à state excitation

The picosecond reaction dynamics of (CH3I)n clusters, initiated by A state excitation, were studied by molecular beam mass spectroscopy. Time-resolved measurements indicate that the prominent I2 and I2(CH3I)n-2 fragments are formed within the 10 ps limit of our resolution. The cluster dynamics and geometries allow for sequential loss of CH3 radicals and substantial caging of fragment I atoms and recombined I2 molecules. Loss of up to five CH3 radicals for cluster sizes n⩾8 occurs within the 25 ps excitation pulse duration as indicated by the presence of ion signals such as I5(CH3I)3+. The evidence suggests that the extensive demethylation in the larger clusters is driven in part by the energy of intracluster recombination of two CH3 radicals and the elimination of ethane.

A time-resolved photoelectron study of the double excited-state proton-transfer reaction in 7-azaindole dimer

Abstract A study employing picosecond and subpicosecond excitation in a mass and photoelectron spectrometer is reported for the 7-azaindole (7-AI) dimers, reactive and unreactive. The 7-AI photoelectron spectrum is structured and has a sharp ionisation threshold at 8.17 eV above the neutral ground electronic state. The reactive dimer ionisation threshold was measured as 7.19 eV. The excited-state lifetime of the reactive dimer was measured by a technique that monitors the ionisation signal as a function of pulse duration. 1 + 1 resonance ionisation photoelectron spectra were recorded using 0.8 ps and 5 ps pulses. Our results indicate a lifetime substantially less than a picosecond, however, consistent with recent real time studies. Advantages of the method are discussed.

Ozone decomposition on alumina: Implications for solid rocket motor exhaust

Rates of ozone decomposition on aluminum oxide (alumina) particles were measured in a flow tube reactor equipped with molecular beam sampling mass spectrometry and ultraviolet absorption spectroscopy, and in a static reaction cell equipped with ultraviolet absorption spectroscopy. Reaction probabilities η are reported for ozone on α-alumina, γ-alumina, and Chromatographic alumina (hydroxylated alumina), respectively, over the temperature range −60 to 200°C. This work addresses the potential for stratospheric ozone depletion by launch vehicle solid rocket motor exhaust. Considering best estimates of plume particle size distributions and dispersion rates, we calculate ozone depletion profiles, for direct decomposition on alumina only. The calculated ozone holes are rather narrow. In the worst case, ozone levels are within 5 × 10−5 of ambient in the center of the plume. A simple analysis of the global impact of alumina particles on ozone decomposition indicates a potential steady-state daytime depletion of < 2.6 × 10−8 at present launch rates.

Measurement of cluster reorganization by time-resolved photoelectron spectroscopy

Abstract An excited-state proton transfer was initiated in the molecule phenol seeded in a solvent cluster of (NH 3 ) n , and the subsequent reorganization of the solvent about the proton measured by time-resolved photoelectron spectroscopy. A shift in the broad photo-electron band on a timescale of 300 ps is attributed to a changing Franck—Condon distribution for ionization arising from reorganization of the solvent geometry.

DETECTION OF THE PERPENDICULAR A STATE TRANSITIONS OF CH3I BY IMAGING OF PHOTOFRAGMENT ANGLE-VELOCITY DISTRIBUTIONS

Photofragment angle‐velocity distributions are measured in a molecular beam using a new two‐dimensional imaging technique. We report on the detection of weak perpendicular transitions for CH3I A state photodissociation at 266 and 304 nm. The ratio of cross sections and the 3Q0−1Q crossing probability are σ⊥/σ∥=0.00−0.10 and p=0.22−0.27 at 266 nm and σ⊥/σ∥=0.20−0.30 and p=0.43−0.48 at 304 nm.