Laser-induced acoustic desorption coupled with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (LIAD/SVUVPI-MS) is employed to analyze aromatics prepared under different conditions from Lungu atmospheric residue (LGAR), i.e., the primary aromatics separated directly from LGAR, and the secondary aromatics after hydrogenation of LGAR and its resins. The mass spectra of the primary aromatics present a bimodal normal distribution in the range of 200–900 Da, in which the relative intensity of the two peaks changes significantly with the SVUV photon energies (9.0, 11.0, and 14.0 eV), indicating that at least two categories of compounds with different ionization energies (IEs) are included, i.e., polycyclic aromatics (IEs < 10.0 eV) in the mass range of 400–900 Da, and aliphatic and alicyclic compounds (IEs close to 11.0 eV) in 200–400 Da. Also detected in the aromatics are metalloporphyrins. Furthermore, the mass spectra of the secondary aromatics separated from LGAR and its resins at different hydrogenation temperatures (390, 400, 410, and 420 °C) are also recorded. The results indicate that the hydrogenation process, especially at higher temperatures, results in removal of alkyl-side and bridge chains in the aromatics, and the secondary aromatics from LGAR resins contain more alkyl side and bridge chains and metal compounds than those from LGAR.

Volatile species from pyrolysis of two kinds of bituminous coal, Huainan (HN) and Yima (YM), were investigated online with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Mass spectra of products at different photon energies and temperatures were measured during the pyrolysis process. Some isomeric products can be characterized by the SVUV-PIMS based on their different ionization energies. Aromatic compounds are dominant pyrolysis products of these coal samples, while a number of aliphatic products were also observed. In comparison of these two coals, the HN coal probably has more condensed aromatic structures than YM coal. The different structures of the macromolecular network between HN and YM coals probably lead to different pyrolytic products at the same temperature. This work demonstrates the good performance of SVUV-PIMS in online analysis of complex coal pyrolysis.

“Imaging” biomass conversion: pyrolysis is the first reaction involved in all thermal processes for biofuels and green chemicals production. Synchrotron light ionisation and mass spectrometry is used for the first time to investigate biomass pyrolysis. The soft and tunable ionisation source coupled with ab initio calculations reveals chemical mechanisms and new major intermediate species. This methodology could be extended to the thermal and catalytic conversion of all other materials. Primary volatile products are analysed online as a function of photon energy, biomass composition (cellulose, xylan, lignin), reactor temperature and time of conversion. Hydroxyacetaldehyde was detected at very minor yields for cellulose pyrolysis confirming that it is a secondary product. The effect of cellulose structure and ash content on primary tar formation was also studied. The mechanism of levoglucosan dissociative photoionisation is depicted. A new major intermediate product which could be a precursor of furanone-based species from cellulose is evidenced thanks to the soft ionisation and MSMS structural analysis of ions. Different lignin markers and evolutions upon time of conversion are shown for miscanthus and oak pyrolysis.

n-Butanol pyrolysis in a flow reactor was investigated at the pressures of 5, 30, 80, 200, and 760 Torr. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used for the identification of pyrolysis species and the measurement of their mole fractions. A detailed kinetic model consisting of 121 species and 658 reactions was developed to simulate the n-butanol pyrolysis. To enhance the accuracy of the model, the rate constants of unimolecular reactions of n-butanol and β-scission reactions of four n-butanol radicals (C4H8OH) were calculated with the variable reaction coordinate–transition-state theory (VRC–TST) and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory coupled with the master equation method. These rates are very sensitive to the mole fractions of pyrolysis species and have been well-validated by the pyrolysis experiment. The model was further validated by the low-pressure premixed flames at different equivalence ratios, oxidation data from the jet-stirred reactor, and ignition delay times. The comparison between predicted and measured results exhibited a good performance of this model. The reaction product analysis and sensitivity analysis were performed to elucidate the chemistry under different conditions.

The absolute photoionization cross-sections of 24 nitrogenous compounds were measured in the photon energy range from their respective ionization thresholds to 11.5 eV, including n-propylamine, iso-propylamine, n-butylamine, 2-butylamine, iso-butylamine, tert-butylamine, diethylamine, dibutylamine, triethylamine, aniline, benzylamine, n-methylaniline, cyclohexanamine, nitromethane, nitroethane, nitropropane, nitrobenzene, pyrrole, pyridine, pyrrolidine, morpholine, N,N-dimethylethanolamine, benzonitrile and benzeneacetonitrile. The experiments were performed by photoionization mass spectrometry with tunable synchrotron vacuum ultraviolet (VUV) light as an ionization source. Binary-liquid-mixtures of investigated and standard species were used in this measurement. As a good solvent, benzene was chosen as a calibration standard, since its photoionization cross-section had been well know. Moreover, photodissociative fragments were also presented.Graphical abstractAbsolute photoionization cross-sections of 24 nitrogenous compounds were measured near ionization thresholds to 11.5 eV.Research highlights► Nitrogenous compounds can be found in coal, heavy oils and solid fuels from biomass to waste. ► Quantification of combustion species of these compounds is of great importance. ► We determined absolute photoionization cross-sections of 24 nitrogenous compounds. ► The data are important for quantification analysis of combustion intermediates.

An experimental study of n-heptane pyrolysis (2.0% n-heptane in argon) has been performed at low pressure (400 Pa) within the temperature range from 780 to 1780 K. The pyrolysis products were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV−PIMS). Photoionization mass spectra and photoionization efficiency spectra were measured to identify pyrolysis products, especially radicals and isomers. Mole fraction profiles of pyrolysis products versus temperature were also measured, indicating that H2, CH4, C2H2, and C2−C6 alkenes are major pyrolysis products of n-heptane. Meanwhile, the thermal decomposition pathways of n-heptane have been investigated using theoretical calculation. The calculation results are in good agreement with the experimental measurement. On the basis of the experimental observation and theoretical calculation, the pyrolysis channels of unimolecular dissociation are proposed to understand the pyrolysis process of n-heptane.

Combustion is one of the earliest developed human technologies and remains our primary source of energy, yet it embodies a complex suite of physical and chemical processes that are inadequately understood. Combustion chemistry involves both chemical thermodynamics and chemical kinetics, and experimental advances mostly depend on the development of combustion diagnostics, which effectively serve as the foundation of theoretical progress. The major objective of combustion diagnostics is to provide comprehensive product identification and concentration information of a flame species, which can be used to develop kinetic models for the simulation of practical combustion. However, conventional combustion diagnostic methods face difficult challenges in distinguishing isomeric species, detecting reactive radicals, obtaining real-time measurements, and so forth. Therefore, for deeper insight into combustion chemistry, a diagnostic method with high detection sensitivity, isomeric selectivity, and radical detectability is required. In this Account, we report recent applications of synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV−PIMS) in various areas of combustion chemistry research. The wide tunability of synchrotron photon energy can facilitate the selective identification of isomeric intermediates and the near-threshold detection of radicals (thus avoiding fragmentation interference). Moreover, the convenient combination of SVUV−PIMS with various laboratory-based combustion approaches demonstrates its universality in combustion studies. Recent experimental achievements have demonstrated the successful applications of this technique in premixed flames, pyrolysis in flow reactors, coflow diffusion flames, catalytic oxidation, plasma diagnostics, and analysis of polycyclic aromatic hydrocarbons (PAHs) and soot. More applications of SVUV-PIMS are expected in the near future, not only in combustion studies, but also in other research topics of chemistry such as analytical chemistry, photochemistry, biochemistry, and the like. In all applications, combustion intermediates, including isomers and radicals, can be distinguished unambiguously, extending our knowledge of intermediate pools and providing more precise targets for quantum chemical calculations of significant reaction channels. The observed mass range covers both small and large combustion products, such as PAHs with two to five carbonic rings. Such analyses present clues toward understanding the molecular growth process from fuel to PAHs and, consequently, soot in fuel-rich hydrocarbon flames. Furthermore, quantitative analyses of chemical structure are available in most applications. For example, one can acquire concentration profiles of flame species versus position in premixed and diffusion flames or versus temperature in pyrolysis and catalytic oxidation. The objectives of validating current kinetic models and developing new kinetic models are thus well served with SVUV-PIMS as an analytical tool in combustion research.

The near-threshold absolute photoionization cross-sections of 12 oxygenated hydrocarbons, including 1-butanol, 2-butanol, iso-butanol, tert-butanol, 1-pentanol, tert-pentanol, iso-pentanol, methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), furan, 2-methylfuran, 2,5-dimethylfuran, were measured in the photon energy range from respective ionization thresholds to 11.5 eV. The experiments were performed with photoionization mass spectrometry and tunable synchrotron vacuum ultraviolet (VUV) light. Binary-liquid-mixtures of the investigated sample and benzene were used in the measurements where benzene acts as a calibration standard, due to its well known photoionization cross-section at the photon energies from its ionization energy (9.24 eV) to 11.5 eV. Photodissociative fragments from the molecules were also observed, and their photodissociation cross-sections are also presented.Absolute photoionization cross-sections of 12 oxygenated hydrocarbons, including butanols, pentanols, MTBE, ETBE, furan and its derivatives, were measured near ionization thresholds to 11.5 eV.

Capacitive radio frequency (RF) discharge of c-C4F8 (octafluorocyclobutane) has been studied with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry (PIMS) at 4 Torr and 33.33 kHz. Various free radicals and reactive intermediates have been identified through measurement of photoionization mass spectra and photoionization efficiency (PIE) spectra. CF2=CF2 is main product in the plasma, indicating that the dissociation of c-C4F8 into CF2=CF2 is one of prominent reactions in the present experimental conditions. The observation of large species including C5F8, C5F10 and C6F10 is presented in our work. Besides, the dependences of the signals of neutral species in the discharge of c-C4F8 on RF power are presented in this paper.

Vacuum ultraviolet (VUV) photon-induced ionization and fragmentation of N-methyl glycine (sarcosine) were investigated with infrared laser desorption/tunable synchrotron VUV photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. Fragment-controllable mass spectra of sarcosine were measured at various photon energies. By tuning the photon energy, the fragments at m/z 44, 45, 43, 42, 30, and 60 were gradually detected. The ionization energy of the precursor was obtained by measuring the photoionization efficiency spectrum. Possible formation pathways of the fragment ions at m/z 44 (CH3NHCH2+), 45 (CH3NH2CH2+), 43 (CH2NHCH2+), 42 (CH2NCH2+), 30 (CH2NH2+), and 60 (CH2COOH2+) were discussed in detail with the help of calculations at the G3B3 and B3LYP/6-31++G(d,p) levels.

Photoionization and dissociative photoionization of α-alanine have been studied with infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. By scanning photoionization efficiency (PIE) spectra, appearance energies (AEs) of the fragments CH3ĊHNH3+ (m/z = 45, doublet state) and CH3CH = NH2+ (m/z = 44, singlet state) are measured to be 9.53 and 9.21 ± 0.05 eV, respectively, which are consistent with the theoretical values of 9.61 and 9.37 eV with ab initio G3B3 calculations. Formation of the CH3ĊHNH3+ ion produced via intramolecular hydrogen transfer and subsequent decarboxylation processes is competitive to that of the CH3CHNH2+ ion derived from a direct C–C(O) bond cleavage. The photoionization mass spectrum obtained at the photon energy of 10.0 eV shows that the intensity of CH3CHNH2+ is much stronger than that of the CH3ĊHNH3+ fragment. The formation of the CH3ĊHNH3+ and CH3CHNH2+ ions is thought to be thermodynamically and kinetically favorable, respectively. Detailed formation pathways for the two cations have been proposed by using ab initio calculations. The calculated results provide a clear picture of the photoionization and dissociative photoionization processes of the α-alanine cation.

A low-pressure premixed toluene/O2/Ar flame with the equivalence ratio of 1.90 was investigated using tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Combustion intermediates up to C19H12 were identified by the measurements of the photoionization mass spectrum and photoionization efficiency spectrum. Mole fraction profiles of flame species were evaluated from the scan of burner position at photon energies near ionization thresholds. Furthermore, flame temperature was recorded by a Pt/Pt-13%Rh thermocouple. The comprehensive experimental data concerning the flame structure facilitate the discussion about the flame chemistry of toluene and other monocyclic aromatic fuels. Benzyl and benzene were found to be major primary intermediates of toluene degradation; and benzene is suggested to originate mainly from fuel degradation instead of radical recombination channels in fuel-rich monocyclic aromatic hydrocarbon flames. On the basis of the intermediate identification, comparison is made among the current mechanisms relevant to the formation of polycyclic aromatic hydrocarbons (PAHs). It is concluded that the molecular growth process in this flame is consistent with the synergy of the hydrogen-abstraction-carbon-addition (HACA) mechanism and the resonantly stabilized radical addition mechanism. In particular, the HACA mechanism can connect a great deal of aromatic intermediates observed in the present work and consequently explain the regular ring enlargement by consecutive addition of 2 or 4 carbon atoms, while the resonantly stabilized radical addition mechanism may have marked and sometimes predominant influences on the formation of many typical PAHs.

The pyrolysis of pyrrole (6.46% pyrrole in argon) has been performed with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) technique. The experiment was carried out over the temperature range of 1260−1710 K at a pressure of 267 Pa. About 30 intermediates have been identified by near-threshold measurements of photoionization mass spectra, and the corresponding mole fractions versus temperatures have been obtained. Moreover, the isomers of some pyrolysis products have been identified by the measurement of photoionization efficiency spectrum. The major products are H2, C2H2, HCN, C3H4 (propyne), and C2H3N (acetonitrile). Meanwhile, some new intermediates, such as C4H4N (cyanoallyl radical) and C2H2N (cyanomethyl radical), have been determined. The major pyrolysis channels have been provided with the high-level ab initio G3B3 calculation and are well consistent with the experimental observation. The formation pathway of HCN via the cyclic carbene tautomer has been proved to be the lowest formation pathway, which is in accordance with previous theoretical work. The potential pathways of the early formed C4H4N species together with their subsequent consumption to C2H2N and C2H2 have been discussed in detail. Also, the formation pathways of the major products of C2H3N and C2H2 have been investigated as well.

We report a photoionization and dissociative photoionization study of β-alanine using IR laser desorption combined with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Fragments at m/z = 45, 44, 43, and 30 yielded from photoionization are assigned to NH3CH2CH2+, NH2CHCH3+, NH2CHCH2+, and NH2CH2+, respectively. Some new conformation-specific dissociation channels and corresponding dissociation energies for the observed fragments are established and determined with the help of ab initio G3B3 calculations and measurements of photoionization efficiency (PIE) spectra. The theoretical values are in fair agreement with the experimental results. Three low-lying conformers of the β-alanine cation, including two gauche conformers G1+, G2+ and one anti conformer A+ are investigated by G3B3 calculations. The conformer G1+ (intramolecular hydrogen bonding N−H···O═C) is found to be another precursor in forming the NH3CH2CH2+ ion, which is complementary to the previously reported formation pathway that only occurs with the conformer G2+ (intramolecular hydrogen bonding O−H···N). Species NH2CHCH2+ may come from the contributions of G1+, G2+, and A+ via different dissociation pathways. The most abundant fragment ion, NH2CH2+, is formed from a direct C−C bond cleavage. Intramolecular hydrogen transfer processes dominate most of the fragmentation pathways of the β-alanine cation.

Combustion intermediates of two low-pressure premixed pyridine/oxygen flames with respective equivalence ratios of 0.56 (C/O/N = 1:4.83:0.20) and 2.10 (C/O/N = 1:1.29:0.20) have been identified with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry techniques. About 80 intermediates in the rich flame and 60 intermediates in the lean flame, including nitrogenous, oxygenated, and hydrocarbon intermediates, have been identified by measurements of photoionization mass spectra and photoionization efficiency spectra. Some radicals and new nitrogenous intermediates are identified in the present work. The experimental results are useful for studying the conversion of volatile nitrogen compounds and understanding the formation mechanism of NOx in flames of nitrogenous fuels.

The photoionization and dissociative photoionization mechanism of 1,8-dihydroxyanthraquinone (1,8-DHAQ) have been investigated by infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (IR LD/VUV PIMS) technique and theoretical calculations. Consecutive losses of two carbon monoxides and elimination of hydroxyl group are found to be the major fragmentation channels in low photon energy range. Photoionization efficiency (PIE) spectrum of 1,8-DHAQ was measured in the photon energy range of 8.2−15.0 eV. Adiabatic ionization energy (IE) of 1,8-DAHQ (M) and appearance energies (AEs) of the major fragments (M−CO)+, (M−C2O2)+, and (M−OH)+ are determined to be 8.54 ± 0.05, 10.8 ± 0.1, 11.0 ± 0.1, and 13.1 ± 0.1 eV, respectively, which are in fair agreement with calculated results. The B3LYP method with the 6-31+G(d) basis set was used to study fragmentation of 1,8-DHAQ. Theoretical calculations indicate that five lowest-energy isomers of 1,8-DHAQ cations can coexist by virtue of bond rotation and intramolecular proton transfer. A number of decarbonylation and dehydroxylation processes of 1,8-DHAQ cations are well established.

An experimental study of methyl tert-butyl ether (MTBE) pyrolysis (3.72% MTBE in argon) has been performed at low pressure (267 Pa) within the temperature range from 700 to 1420 K. The pyrolysis process was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). About thirty intermediates are identified from near-threshold measurements of photoionization mass spectrum and photoionization efficiency spectrum. Among them, H2, CO, CH4, CH3OH and C4H8 are the major pyrolysis products. The radicals such as methyl, methoxy, propargyl, allyl, C4H5 and C4H7 are detected. The isomers of pyrolysis products are identified as well, i.e., propyne and allene, 1,2,3-butatriene and vinylacetylene, isobutene and 1-butene, propanal and acetone. Furthermore, the mole fractions of the pyrolysis products have been evaluated under various temperatures. Meanwhile, the initial formation temperatures of different pyrolysis products can be obtained. This work is anticipated to present a new experimental method for pyrolysis study and help understand the pyrolysis and combustion chemistry of MTBE and other oxygenated fuels.

The thermal decomposition pathways of MTBE have been investigated using the G3B3 method. On the basis of the experimental observation and theoretical calculation, the pyrolysis channels are provided, especially for primary pyrolysis reactions. The primary decomposition pathways include formation of methanol and isobutene, CH4 elimination, H2 elimination and C−H, C−C, C−O bond cleavage reactions. Among them, the formation channel of methanol and isobutene is the lowest energy pathway, which is in accordance with experimental observation. Furthermore, the secondary pyrolysis pathways have been calculated as well, including decomposition of tert-butyl radical, isobutene, methanol and acetone. The radicals play an important role in the formation of pyrolysis products, for example, tert-butyl radical and allyl radical are major precursors for the formation of allene and propyne. Although some isomers (isobutene and 1-butene, allene and propyne, acetone and propanal) are identified in our experiment, these isomerization reaction pathways occur merely at the high temperature due to their high activation energies. The theoretical calculation can explain the experimental results reported in part 1 and shed further light on the thermal decomposition pathways.

A procedure combining tunable vacuum ultraviolet (VUV) single-photon ionization with reflectron time-of-flight mass spectrometer has been developed for analysis of individual components in 90# unblended gasoline. Synchrotron radiation used as the ionization source can provide wide energy range and high resolution. All components can be identified by the molecular weights from photoionization mass spectrometry near threshold ionization and the ionization energies from photoionization efficiency spectra. About ninety components are detected in this study, including paraffins, olefins, and aromatics. Quantitative analysis can be obtained by the integrated ion intensity combining with data of photoionization cross-section. It appears that the tunable VUV photoionization mass spectrometry is a complementary method to previous existing ones for the analysis of individual components in gasoline. It may also be useful in other analytical applications.

Isomers of the C5H3 and C5H5 free radicals formed in benzene/oxygen flame are identified by analyzing the observed photoionization efficiency spectra with the aid of theoretical simulations based on calculated ionization energies (IEs) and Franck–Condon factors. For C5H3, the results reveal that two linear isomers (i-C5H3 and n-C5H3) are formed in the benzene flame, while, for C5H5, both linear and cyclic isomers (l-C5H5 and c-C5H5) are formed. Adiabatic IEs for i-C5H3, n-C5H3, l-C5H5 and c-C5H5 are measured to be 8.21, 8.32, 7.83 and 8.44 eV, respectively (all with an uncertainty of ±0.05 eV).

A comprehensive experimental study of the premixed benzene/oxygen/argon flame at 4.0 kPa with a fuel equivalence ratio (ϕ) of 1.78 has been performed with the tunable synchrotron photoionization and molecular-beam sampling mass spectrometry. Isomers of most observed species in the flame have been unambiguously identified by measurements of the photoionization efficiency spectra. Mole fraction profiles of species up to C16H10 have been measured at the selective photon energies near ionization thresholds, and the flame temperature profile is obtained using Pt/Pt–13%Rh thermocouple. Compared with previous studies on benzene flames by Bittner and Howard, and by Defoeux et al., a number of new species are observed in the present work. These new combustion intermediates should be included in the kinetic models of the growth of polycyclic aromatic hydrocarbons (PAHs) and benzene oxidation. Free radicals detected in the flame include CH3, C2H, C2H3, C2H5, C3H, C3H3, C3H5, C4H, C4H3, C4H5, C4H7, C5H3, C5H5, C5H7, C6H5, C6H5O, C7H7, and C9H7. More significantly, isomers of some PAHs have been identified, which should be of importance in understanding the mechanism of soot formation.

In order to understand the interactions between 2-butanol and hydrocarbon fuels in combustion chemistry, experimental and kinetic modeling investigations were performed to study laminar coflow diffusion methane flames doped with two inlet mole fractions of 2-butanol (1.95% and 3.90%) in this work. Mole fractions of flame species along the flame centerline, particularly unsaturated C2–C5 hydrocarbons, C6–C16 aromatics and some free radicals, were measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed to simulate the fuel decomposition and the formation of benzene and PAHs in the investigated flames. The simulated results can reproduce the observed effects of 2-butanol addition. The reaction pathway analysis reveals that resonantly stabilized radicals, such as propargyl, cyclopentadienyl, phenyl, and benzyl radicals, are major precursors of indene and naphthalene. With the increasing inlet mole fraction of 2-butanol, the formation of these resonantly stabilized radicals increases significantly. PAHs with more carbon atoms, including acenaphthylene, phenanthrene, pyrene, and fluoranthene, are dominantly derived from indenyl and naphthyl radicals.

Combustion is directly related to energy conversion and the environment. Gas-phase chemical reactions such as thermal decomposition, oxidation and recombination play a critical role in combustion processes. Here we review six applications of synchrotron vacuum-ultraviolet (VUV) photoionization mass spectrometry (PIMS) in fundamental studies of combustion chemistry. These applications range from the use of flow reactors to probe elementary reaction kinetics, studies of pyrolysis in plug-flow reactors and oxidation in jet-stirred reactors, studies of spatial evolution of species concentrations in premixed and non-premixed flames, product distributions in pyrolysis of biomass, and analysis of polycyclic aromatic hydrocarbon (PAH) formation. These experiments provide valuable data for the development and validation of detailed chemical kinetic models. Furthermore, some additional potential applications are proposed.

Pyrrolidine serves as a model substance for a series of saturated nitrogenated heterocycles in plants, including certain amino acids such as proline and hydroxyproline. Thus the pyrolysis of this compound was investigated regarding the increasing interest on biofuels. The pyrolysis of pyrrolidine diluted with 95% argon was studied in a flow reactor at 40 mbar over the temperature range from 950 to 1450 K. Isomer-specific assignment and quantification was performed using molecular-beam mass spectrometry and ionization with tunable synchrotron vacuum ultraviolet radiation. The prominent decomposition pathways were analyzed based on the quantified mole fractions of pyrolysis species. Computations including an ab initio calculation and kinetic modeling for the primary fuel decomposition were used to refine the analysis and reveal the combustion chemistry of this saturated heterocyclic compound. Based on the theoretical calculation, a new pathway for the isomerization reaction pyrrolidine → CH2CHCH2CH2NH2 via a diradical intermediate was proposed. The rate constant calculations showed that this channel has a large contribution to pyrrolidine pyrolysis.

An experimental and kinetic modeling study is reported on three premixed nitroethane/oxygen/argon flames at low pressure (4.655 kPa) with the equivalence ratios (Φ) of 1.0, 1.5 and 2.0. Over 30 flame species were identified with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry, with their mole fractions quantified as the function of the height above burner. The flame temperature profiles were measured with a Pt–6%Rh/Pt–30%Rh thermocouple. A detailed kinetic mechanism with 115 species and 730 reactions was proposed and validated against experimental results. The computed predictions have shown satisfactory agreement with the experimental results. Basing on the rate-of-production analysis, the reaction pathways that feature the combustion of nitroethane were revealed, including the primary decomposition of C–N bond fission, the oxidation of C2 and C1 hydrocarbons and the formation of nitrogenous species. The presence of NO2 and NO has been proved to be important for these processes.

Experimental measurements were conducted for temperatures and mole fractions of C1–C16 combustion intermediates in laminar coflow non-premixed methane/air flames doped with 3.9% (in volume) 1-butanol, 2-butanol, iso-butanol and tert-butanol, respectively. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technique was utilized in the measurements of species mole fractions. The results show that the variant molecular structures of butyl alcohols have led to different efficiencies in the formation of polycyclic aromatic hydrocarbons (PAHs) that may cause the variations in sooting tendency. Detailed species information suggests that the presence of allene and propyne promotes benzene formation through the C3H3 + C3H4 reactions and consequently PAH formation through the additions of C2 and C3 species to benzyl or phenyl radicals. As a matter of fact, PAHs formed from the 1-butanol doped flame are the lowest among the four investigated flames, because 1-butanol mainly decomposes to ethylene and oxygenates rather than C3 hydrocarbon species. Meanwhile, the tert-butanol doped flame generates the largest quantities of allene and propyne among the four flames and therefore is the sootiest one.

Pyrolysis is one of the most important methods to convert biomass into biofuel, which is a potential substitute for fossil fuel. The pyrolysis process of poplar biomass, a potential biofuel feedstock, has been studied with tunable synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry (PIMS). The mass spectra at different photon energies, temperatures, and time-evolved profiles of selected species during poplar pyrolysis process were measured. Our results reveal that poplar is typical of hardwood according to its relative contents of three lignin monomeric precursors. As temperature increases from 300 to 700 °C, the overall intensities of pyrolysis products decrease due to the gas-phase cracking. Observed intensities of syringyl and guaiacyl subunits of lignin in poplar at low temperature present different trends: the intensities of syringyl subunits of lignin undergo an increase firstly and then a decrease, whereas those of guaiacyl subunits of lignin show decrease continuously. Time-dependent data demonstrate that hemicellulose pyrolysis is faster than lignin in poplar. This work reports a new application of SVUV PIMS in biomass pyrolysis, which performs very well in products analysis.

Three premixed ethylbenzene/O2/Ar flames with equivalence ratio (φ) of 0.75, 1.00, 1.79 were studied at low pressure (4.0 kPa) to represent the lean, stoichiometric, and rich ethylbenzene flames. Flame species were identified using synchrotron vacuum ultraviolet photoionization mass spectrometry, and their mole fractions were evaluated. The maximum mole fractions of hydrocarbon intermediates were observed to increase with φ increasing. A kinetic model including 176 species and 804 reactions was developed with detailed submechanisms of ethylbenzene and toluene. The validation of the model was made by simulating the measured mole fractions of flame species, showing good agreement in reproducing the mole fractions of most observed species. Furthermore, rate of production analysis reveals the main formation and consumption channels of some key hydrocarbon intermediates involved in ethylbenzene decomposition and PAHs formation. The main reaction channels of these species in the rich flame have salient differences with those in the lean flame, indicating the different chemistry between the pyrolysis dominated and oxidation dominated circumstance, while the chemistry in the stoichiometric flame is more similar to that in the lean flame. Furthermore, reactions involving phenyl and benzyl are concluded to be critical for PAHs formation in the rich ethylbenzene flame.

An experimental and modeling study is reported on three premixed nitromethane/oxygen/argon flames at low pressure (4.655 kPa) with equivalence ratios (ϕ) of 1.0, 1.5 and 2.0. Flame species were identified with tunable synchrotron vacuum ultraviolet photoionization. The mole fraction profiles of more than 30 species including intermediates and radicals were obtained by near-threshold burner scan with selected photon energies. With a Pt–6%Rh/Pt–30%Rh thermocouple, the flame temperature profiles were measured. A detailed kinetic mechanism with 88 species and 701 reactions was proposed and validated against experimental results of all three flames. The computed predictions showed satisfactory agreement with the experimental results. Based on a rate-of-production analysis, reaction pathway diagrams were obtained to describe the hydrocarbon oxidation process and nitrogenous species chemistry in the nitromethane flame with ϕ = 1.5.

A laminar premixed pyrrolidine/oxygen/argon flame with volume ratio C/O/N of 1.0/2.4/0.25 at 40 mbar was investigated using molecular-beam mass spectrometry (MBMS) with tunable synchrotron vacuum ultraviolet (VUV) photoionization. About 50 species were identified from photoionization efficiency (PIE) spectra, including various air pollutants such as nitric oxide, ammonia, hydrogen cyanide, acetonitrile, formaldehyde, acetaldehyde, and propanal. Some radicals were detected including C4H8N, C4H7, C3H5, C3H3, C2H5, CHO, and CH3. Detected molecular intermediates include the isomers of C2H4O (ethenol and acetaldehyde), C3H4 (propyne and allene), C4H6 (1,3-butadiene and 1-butyne) and C4H8 (1-butene and 2-butene). Based on the experimental observations, the decomposition and ring-opening pathways of pyrrolidine were calculated at G3B3 level. Also, likely reaction channels from the products of H- and H2-elimination reactions were proposed. This combined experimental and theoretical investigation intends to shed light on the decomposition channels of pyrrolidine which may assist the development of a kinetic model for pyrrolidine pyrolysis and combustion.

Chemical structures of premixed toluene/O2/Ar flames with five equivalence ratios (0.75, 1.00, 1.25, 1.50, and 1.75) were studied at low pressure (4.0 kPa). Synchrotron vacuum ultraviolet photoionization mass spectrometry was used for the isomeric detection of flame species and the measurements of their mole fractions. The global trends of the flame temperature and mole fractions of detected species with varying equivalence ratio were observed and discussed, drawing complete pictures for the chemical structures of premixed toluene flames. Based on the experimental results, a kinetic model was developed from previous models and updated with many recently studied reactions related to toluene decomposition and polycyclic aromatic hydrocarbons (PAHs) formation. The model was validated by simulating the measured mole fractions of flame species, showing good agreement in reproducing the maximum mole fractions of most observed species and their global trends with varying equivalence ratio. The rate of production analysis reveals the major formation and consumption channels of some key intermediates involved in toluene decomposition in the ϕ = 0.75 and 1.75 flames, and the major formation channels of some large aromatics and typical PAHs in the ϕ = 1.75 flame, indicating the importance of benzyl in toluene flames.

The formation of hydroperoxides postulated in all the kinetic models for the low temperature oxidation of alkanes have been experimentally proved thanks to a new type of apparatus associating a quartz jet-stirred reactor through a molecular-beam sampling system to a reflectron time-of-flight mass spectrometer combined with tunable synchrotron vacuum ultraviolet photoionization. This apparatus has been used to investigate the low-temperature oxidation of n-butane and has allowed demonstrating the formation of different types of alkylhydroperoxides, namely methylhydroperoxide, ethylhydroperoxide and butylhydroperoxide, and of C4 alkylhydroperoxides including a carbonyl function (ketohydroperoxides). In addition, the formation of products deriving from these ketohydroperoxides, such as C4 molecules including either two carbonyl groups or one carbonyl and one alcohol functions, has been observed. Simulations using a detailed kinetic model have been performed to support some of the assumptions made in this work.

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N2 and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C3H4, C4H6, C4H8, C2H2O, C2H4O, CH3CN, H2CNHO, C3H3N and C3H7N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.

A slightly sooting premixed ethylbenzene flame with an equivalence ratio of 1.90 was investigated at low pressure (4.0 kPa) using molecular-beam mass spectrometry (MBMS) and tunable synchrotron vacuum ultraviolet (VUV) photoionization. Basing on the ionization threshold measurements of photoionization efficiency (PIE) spectra, combustion intermediates up to C19H12 were identified, including a number of radicals and isomeric species. Mole fraction profiles of observed flame species were evaluated from the measurements of burner scan at the photon energies near ionization thresholds. Besides, the flame temperature profile was measured by a Pt/Pt–13%Rh thermocouple. From the intermediate identification and mole fraction measurements, the degradation of ethylbenzene, as well as the formation of some interested polycyclic aromatic hydrocarbons (PAHs), was discussed in detail. It is suggested that the formation of most typical PAHs observed in this work can be related to the H-abstraction/C2H2-addition (HACA) mechanism. Furthermore, the high concentration levels of intermediates in this flame is ascribed to the weak C–C bonds in the sidechain of ethylbenzene, which provides a potential explanation of the high sooting tendencies of ethylbenzene and other monocyclic aromatic fuels with complex sidechain structure. This study is anticipated to be constructive for combustion investigations of aromatic fuels, and the discussion is hoped to be helpful for further modeling studies concerning PAHs formation in combustion process.

Photoionization and dissociative photoionization of α-alanine have been studied with infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. By scanning photoionization efficiency (PIE) spectra, appearance energies (AEs) of the fragments CH3ĊHNH3+ (m/z = 45, doublet state) and CH3CH = NH2+ (m/z = 44, singlet state) are measured to be 9.53 and 9.21 ± 0.05 eV, respectively, which are consistent with the theoretical values of 9.61 and 9.37 eV with ab initio G3B3 calculations. Formation of the CH3ĊHNH3+ ion produced via intramolecular hydrogen transfer and subsequent decarboxylation processes is competitive to that of the CH3CHNH2+ ion derived from a direct C–C(O) bond cleavage. The photoionization mass spectrum obtained at the photon energy of 10.0 eV shows that the intensity of CH3CHNH2+ is much stronger than that of the CH3ĊHNH3+ fragment. The formation of the CH3ĊHNH3+ and CH3CHNH2+ ions is thought to be thermodynamically and kinetically favorable, respectively. Detailed formation pathways for the two cations have been proposed by using ab initio calculations. The calculated results provide a clear picture of the photoionization and dissociative photoionization processes of the α-alanine cation.