Andrew McIlroy

Selective detection of Isomers with Photoionization mass spectrometry for studies of hydrocarbon flame chemistry

We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry (PIMS). Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressure laminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomeric composition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence of substantial concentrations of ethenol (vinyl alcohol) and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames.

Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source

A flame-sampling molecular-beam photoionization mass spectrometer, recently designed and constructed for use with a synchrotron-radiation light source, provides significant improvements over previous molecular-beam mass spectrometers that have employed either electron-impact ionization or vacuum ultraviolet laser photoionization. These include superior signal-to-noise ratio, soft ionization, and photon energies easily and precisely tunable [E∕ΔE(FWHM)≈250–400] over the 7.8–17-eV range required for quantitative measurements of the concentrations and isomeric compositions of flame species. Mass resolution of the time-of-flight mass spectrometer is m∕Δm=400 and sensitivity reaches ppm levels. The design of the instrument and its advantages for studies of flame chemistry are discussed.

A laser and molecular beam mass spectrometer study of low-pressure dimethyl ether flames

The oxidation of dimethyl ether (DME) is studied in low-pressure flames using new molecular beam mass spectrometer and laser diagnostics. Two 30.0-Torr, premixed DME/oxygen/argon flames are investigated with stoichiometries of 0.98 and 1.20. The height above burner profiles of nine stable species and two radicals are measured. These results are compared to the detailed chemical reaction mechanism of Curran and coworkers. Generally good agreement is found between the model and data. The largest discrepancies are found for the methyl radical profiles where the model predicts qualitatively different trends in the methyl concentration with stoichiometry than observed in the experiment.

REMPI temperature measurement in molecular beam sampled low-pressure flames

The cooling effect in molecular beam (MB) sampling from low-pressure flames and gas mixtures was investigated. Although the MB method is often used to study the flame structure of low-pressure flames, typically combined with mass spectrometric (MS) detection, it is poorly characterized. The temperature of the MB must be known, especially if species concentrations are to be measured with spectroscopic methods, like resonance-enhanced multiphoton ionization (REMPI) spectroscopy. In the present study, two independent MBMS instruments, which are very similar to those used previously by different groups, were investigated starting with pressures of 40 and 50 mbar in the burner chamber. The rotational temperatures of NO and benzene were determined using REMPI spectroscopy for different initial conditions: the ions were separated by time-of-flight mass spectrometers. Two REMPI excitation schemes were applied: for NO the first step was always the excitation of the A-X transition near 225 nm, while benzene was excited and ionized at wavelengths near 259 nm. Unexpectedly, molecular beams from cold-gas flows were cooled very slightly by 10%–25%. In the molecular beams derived from low-pressure flames, the cooling effect was stronger, with final rotational temperatures of 300–400 K, but the MB temperature was virtually independent of the initial temperature. A possible explanation of this finding would be that the cooling takes place to a large extent by wall collisions within the nozzle and to a lesser degree by intermolecular collisions.

Measurement of the sixth overtone band of nitric oxide, and its dipole moment function, using cavity-enhanced frequency modulation spectroscopy

We applied cavity-enhanced frequency modulation absorption spectroscopy (also known as noise-immune cavity-enhanced optical heterodyne molecular spectroscopy) to perform high-resolution spectroscopy of the sixth overtone band of nitric oxide near 797 nm. By using novel high-bandwidth balanced detectors, baseline variations caused by residual amplitude modulation were significantly reduced. The ultrahigh sensitivity (2 x 10(-10) cm(-1) minimum detectable absorption at 1 Hz detection bandwidth) of our spectrometer allowed accurate measurements of 15 individual line intensities of P- and R-branch transitions in the 2Pi(1/2)-2Pi(1/2) subband. A vibrational transition moment of 3.09(6) muD+/-13% and Herman-Wallis coefficients of a = -0.0078(26) and b = 0.001 25(45) were found by fitting the line intensities. Based on our measured transition moment, and those of other transitions from the literature, a new parametrization for the electric dipole moment function (EDMF) of nitric oxide, valid for 0.91 < or = r < or = 1.74 A, has been extracted. The residuals of this fit demonstrate that the derived EDMF is the most accurate representation to date of the dipole moment function. The new EDMF will be valuable for accurate intensity prediction of transitions that cannot be easily measured experimentally.

Direct measurement of 1CH2 in flames by cavity ringdown laser absorption spectroscopy

Abstract Singlet methylene is an important combustion intermediate, but has remained difficult to measure in flames. Cavity ringdown laser absorption spectroscopy (CRLAS) allows the sensitive and selective detection of 1 CH 2 with good spatial resolution. Singlet methylene has been detected by CRLAS in a series of rich, low-pressure methane flames with stoichiometries of 1.0, 1.2, 1.4 and 1.6. The measured relative concentration profiles are compared with laminar flame models utilizing detailed chemical reaction mechanisms. The data show good agreement with models at stoichiometries of 1.0 and 1.2, but increasingly poor agreement at richer stoichiometries.

The vinyl radical (Ã2A″←X̃2A′) spectrum between 530 and 415 nm measured by cavity ring-down spectroscopy

High resolution (0.008 nm) transient absorption spectra have been measured for the vinyl radical (C2H3) A 2A″←X 2A′ transition between 530 and 415 nm using cavity ring-down laser absorption spectroscopy. This is over 200 times better resolution than the only previous recorded spectrum of vinyl in this wavelength region by Hunziker, Kneppe, McLean, Siegbahn, and Wendt [Can J. Chem. 61, 993 (1983)]. With the improved resolution in the present study, new vibrational bands are detected, and some rotational structure of the bands is resolved. A calculation of the envelope of the origin band was carried out, using a model for a c-type transition of an asymmetric top. For a best fit to the data, the model requires a 1 cm−1 Lorentzian linewidth, which suggests a short-lived excited state.

Vinyl radical visible spectroscopy and excited state dynamics

The vinyl radical (C2H3)A 2A″←X 2A′ spectrum has been measured between 530 and 385 nm using cavity-ringdown spectroscopy. The active vibrational progressions involve C–C stretching and alpha H–C–C bending vibrations. Optimal rotational constants and linewidths were determined for the first four vibrational bands by modeling the spectrum as an asymmetric top. The best-fit rotational constants obtained for the excited electronic state are consistent with the molecular geometry predicted by ab initio calculations. The lifetime of the vibrationless level in the excited electronic state is estimated to be a few picoseconds, and increasing vibrational excitation leads to a decrease in the lifetime, based upon an increasing linewidth. Various possibilities for the predissociation mechanism are discussed. The most likely is judged to be a conical intersection or seam of intersections. A preliminary CASSCF calculation has found the point on the relevant potential energy surfaces at which the ground and electronic...

CH+H2 reaction kinetics: Temperature and pressure dependence and Rice–Ramsperger–Kassel–Marcus‐master‐equation calculation

We investigate the reaction of CH(X 2Π) with H2 as a function of temperature in the range 240–470 K at 8.2 and 750 Torr of helium pressure and as a function of helium pressure in the range 8–750 Torr at 294 K. Methylidyne forms upon excimer‐laser photolysis of CHBr3 or CHClBr2 in a slow‐flow reactor and we time‐resolve its concentration profile using cw laser‐induced fluorescence. The title reaction proceeds through the formation of an excited methyl radical with multiple open decay channels. We observe dramatically different temperature dependencies at high and low helium bath‐gas pressures. At high pressure, collisional stabilization of CH*3 to CH3 dominates the reaction mechanism and the CH‐loss rate constant exhibits a negative temperature dependence over the range studied. At low pressure, the predominant product channel switches from CH3 to CH2+H as the temperature increases. We employ a Rice–Ramsperger–Kassel–Marcus‐master‐equation calculation to model the experimental results.

IDENTIFYING COMBUSTION INTERMEDIATES VIA TUNABLE VACUUM ULTRAVIOLET PHOTOIONIZATION MASS SPECTROMETRY

ABSTRACT The analysis of complex, reactive mixtures in flames remains challenging, but the need to understand the formation mechanisms of pollutants such as polyaromatic hydrocarbons (PAH) and soot drives innovation in this area. We have used molecular beam sampling and single-photon, tunable vacuum ultraviolet (VUV) ionization mass spectrometry to spatially resolve and identify the isomers of PAH precursors in low-pressure, one-dimensional, laminar H2/1,3-butadiene/O2/Ar flames. We have measured the photoionization efficiency spectra of many species in these flames to obtain their ionization thresholds. By comparison with literature ionization energies, the isomers can be uniquely identified. The approach shows promise as a universal probe of PAHs and their precursors from combustion processes.