The concept behind active thermochemical tables (ATcT) is presented. As opposed to traditional sequential thermochemistry, ATcT provides reliable, accurate, and internally consistent thermochemistry by utilizing the thermochemical network (TN) approach. This involves, inter alia, a statistical analysis of thermochemically relevant determinations that define the TN, made possible by redundancies in the TN, such as competing measurements and alternate network pathways that interrelate the various chemical species. The statistical analysis produces a self-consistent TN, from which the optimal thermochemical values are obtained by simultaneous solution in error-weighted space, thus allowing optimal use of all of the knowledge present in the TN. ATcT offers a number of additional features that are not present nor possible in the traditional approach. With ATcT, new knowledge can be painlessly propagated through all affected thermochemical values. ATcT also allows hypothesis testing and evaluation, as well as d...

Introduction to Active Thermochemical Tables: Several “Key” Enthalpies of Formation Revisited†

Recent contributions of flame-sampling molecular-beam mass spectrometry to a fundamental understanding of combustion chemistry

Photoionization cross sections for reaction intermediates in hydrocarbon combustion

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.

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

The combination of multiplexed mass spectrometry with photoionization by tunable-synchrotron radiation has proved to be a powerful tool to investigate elementary reaction kinetics and the chemistry of low-pressure flames. In both of these applications, multiple-mass detection and the ease of tunability of synchrotron radiation make it possible to acquire full sets of data as a function of mass, photon energy, and of the physical dimension of the system, e.g. distance from the burner or time after reaction initiation. The data are in essence an indirect image of the chemistry. The data can be quantitatively correlated and integrated along any of several dimensions to compare to traditional measurements such as time or distance profiles of individual chemical species, but it can also be directly interpreted in image form. This perspective offers an overview of flame chemistry and chemical kinetics measurements that combine tunable photoionization with multiple-mass detection, emphasizing the overall insight that can be gained from multidimensional data on these systems. The low-pressure flame apparatus is capable of providing isomer-resolved mass spectra of stable and radical species as a function of position in the flame. The overall chemical structure of the flames can be readily seen from images of the evolving mass spectrum as distance from the burner increases, with isomer-specific information given in images of the photoionization efficiency. Several flames are compared in this manner, with a focus on identification of global differences in fuel-decomposition and soot-formation pathways. Differences in the chemistry of flames of isomeric fuels can be discerned. The application of multiplexed synchrotron photoionization to elementary reaction kinetics permits identification of time-resolved isomeric composition in reacting systems. The power of this technique is illustrated by the separation of direct and dissociative ionization signals in the reaction of C2H5 with O2; by the resolution of isomeric products in reactions of the ethynyl (C2H) radical; and by preliminary observation of branching to methyl + propargyl products in the self-reaction of vinyl radicals. Finally, prospects for future research using multiplexed photoionization mass spectrometry are explored.

“Imaging” combustion chemistry via multiplexed synchrotron-photoionization mass spectrometry

A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104 suppression of the harmonics has been demonstrated with no measurable (<5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported.

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A differentially pumped harmonic filter on the Chemical Dynamics Beamline at the Advanced Light Source

We have demonstrated a resolution of 0.8 meV [full width at half-maximum (FWHM)] for threshold photoelectron measurements using a steradiancy-type zero kinetic energy photoelectron (ZEKE-PE) analyzer and the high resolution monochromatized vacuum ultraviolet (VUV) undulator synchrotron radiation of the chemical dynamics beamline at the advanced light source (ALS). Using this high resolution ZEKE-PE energy analyzer to filter prompt electrons and by employing a proper voltage pulsing scheme adapted to the timing structure of the ALS, we have achieved a resolution of 0.5 meV (FWHM) for pulsed field ionization photoelectron (PFI-PE) measurements with little contamination from prompt photoelectrons produced from direct photoionization and autoionizing processes. The experiment scheme presented here is generally applicable to PFI-PE studies using multi-bunch VUV synchrotron radiation at other synchrotron radiation facilities.

High resolution threshold and pulsed field ionization photoelectron spectroscopy using multi-bunch synchrotron radiation

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra for O2 in the energy range of 12.05–18.15 eV, covering ionization transitions O2+(X 2Π1/2,3/2g, v+=0–38,J+)←O2(X 3Σg−, v+=0,N″). While the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=3–5, 9, 11, 12, 22, and 25–38) reported here are the first rotational-resolved photoelectron measurements, the PFI-PE bands for O2+(X 2Π1/2,3/2g, v+=25–38) represent the first rotationally resolved spectroscopic data for these states. The simulation of spectra obtained at rotational temperatures of ≈20 and 220 K allows the unambiguous identification of O2+(X 2Π1/2,3/2g, v+⩾21) PFI-PE bands, the majority of which overlap with prominent PFI-PE bands for O2+(A 2Πu, v+=0–12) and O2+(a 4Πu, v+=0–18). Combined with spectroscopic data obtained in the previous emission study and the present PFI-PE experiment, we have obtained accurate Dunham-type expansion coefficients for ionization energies, vibrational constants, rotational constants, and spin–...

Rotationally resolved pulsed field ionization photoelectron bands of O2+(X 2Π1/2,3/2g, v+=0–38) in the energy range of 12.05–18.15 eV

The vacuum ultraviolet pulsed field ionization photoelectron (PFI-PE) band for OCS+(X 2Π) in the energy region of 11.09–11.87 eV has been measured using high resolution monochromatized synchrotron radiation. The ionization energies (IEs) for the formation of the (0,0,0) X 2Π3/2 and (0,0,0) 2Π1/2 states of OCS+ are determined to be 11.1831±0.0005 and 11.2286±0.0005 eV, respectively, yielding a value of 367±1.2 cm−1 for the spin–orbit splitting. Using the internally contracted multireference configuration interaction approach, three-dimensional potential energy functions (PEFs) for the OCS+(X 2Π) state have been generated and used in the variational Renner–Teller calculations of the vibronic states. The energies of all vibronic states (J=P) for J=1/2, 3/2, 5/2, and 7/2 have been computed in the energy range of ≈4000 cm−1 above the IE[OCS+(X 2Π3/2)] for the assignment of the experimental spectrum. By a minor modification of the ab initio PEFs, good correlations are found between the experimental and theoreti...

High resolution vacuum ultraviolet pulsed field ionization photoelectron band for OCS+(X 2Π): An experimental and theoretical study

Using the newly constructed photoelectron-photoion coincidence apparatus associated with the chemical dynamics beamline at the advanced light source, we have performed a high resolution energy-selected kinetic energy release measurement for the dissociative photoionization process SF6+hν→SF5++F+e−. After taking into account the center-of-mass kinetic energy release, the thermochemical threshold for this process is determined to be 14.11±0.08 eV. This value yields 18.5±1.9 and −202.9±2.2 kcal/mol for the heats of formation at 0 K for SF5+ and SF5, respectively.

Matthew D. Evans

Papers

A differentially pumped harmonic filter on the Chemical Dynamics Beamline at the Advanced Light Source

High resolution threshold and pulsed field ionization photoelectron spectroscopy using multi-bunch synchrotron radiation

Rotationally resolved pulsed field ionization photoelectron bands of O2+(X 2Π1/2,3/2g, v+=0–38) in the energy range of 12.05–18.15 eV

High resolution vacuum ultraviolet pulsed field ionization photoelectron band for OCS+(X 2Π): An experimental and theoretical study

A high resolution energy-selected kinetic energy release study of the process SF6+hν→SF5++F+e−: Heat of formation of SF5+