Double Imaging Photoelectron Photoion Coincidence Research Articles

The Unimolecular Decomposition Mechanism of Trimethyl Phosphate.

Trimethyl phosphate (TMP), an organophosphorus compound (OPC), is a promising fire-retardant candidate for lithium-ion battery (LIB) electrolytes to mitigate fire spread. This study aims to understand the mechanism of TMP unimolecular thermal decomposition to support the integration of a TMP chemical kinetic model into a LIB electrolyte surrogate model. Reactive intermediates and products of TMP thermal decomposition were experimentally detected using vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Phosphorus-containing intermediates such as PO, HPO and HPO2 were identified. Sampling effects could successfully be obviated thanks to photoion imaging, which also showed evidence for isomerization reactions upon wall collisions in the ionization chamber. Quantum chemical calculations performed for the unimolecular decomposition of TMP revealed for the first time that isomerization channels via hydrogen and methyl transfer (barrier heights of 65.9 and 72.6 kcal/mol, respectively) are the lowest-energy primary steps of TMP decomposition followed by CH3OH/CH3/CH2O or dimethyl ether (DME) production, respectively. We found an analogous DME production channel in the unimolecular decomposition of dimethyl methylphosphonate (DMMP), another important OPC fire-retardant additive with a similar molecular structure to TMP, which are not included in currently available chemical kinetic models.

Photoionization and Dissociative Photoionization of Acetaldehyde in the 10.0-13.7 eV Range by Synchrotron Photoelectron Photoion Coincidence Spectroscopy.

Photoionization and dissociative photoionization of acetaldehyde (CH3CHO) in the 10.0-13.7 eV energy range are studied by using synchrotron radiation double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO). The X2A' and A2A" electronic states of CH3CHO+ as well as the Franck-Condon gap region between these two states have been populated with several vibrational sequences and assigned in the high-resolution slow photoelectron spectrum (SPES). The adiabatic ionization energies (AIEs) of the X2A' and A2A" states are measured at 10.228±0.006 and 12.52±0.05 eV, respectively. The present results show that the X2A' state is a stable state while the A2A" state is fully dissociative to produce CH3CO+, CHO+ and CH4 + fragment ions. The 0 K appearance energies (AE0K) of CH3CO+ and CHO+ fragment ions are determined through the modeling of the breakdown diagram, i. e., AE0K(CH3CO+)=10.89±0.01 eV (including a reverse barrier of ~0.19 eV) and AE0K(CHO+)=11.54±0.05 eV. In addition, the dissociation mechanisms of CH3CHO+ including statistical dissociation, direct bond breaking and isomerization are discussed with the support of the calculated dissociation limits and transition state energies.

Conformer-Specific Photoelectron Spectroscopy of Carbonic Acid: H2CO3.

Carbonic acid (H2CO3) is a fundamental species in biological, ecological, and astronomical systems. However, its spectroscopic characterization is incomplete because of its reactive nature. The photoionization (PI) and the photoion mass-selected threshold photoelectron (ms-TPE) spectra of H2CO3 were obtained by utilizing vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence spectroscopy. Two carbonic acid conformers, namely, cis-cis and cis-trans, were identified. Experimental adiabatic ionization energies (AIEs) of cis-cis and cis-trans H2CO3 were determined to be 11.27 ± 0.02 and 11.18 ± 0.03 eV, and the cation enthalpies of formation could be derived as ΔfH°0K = 485 ± 2 and 482 ± 3 kJ mol-1, respectively. The cis-cis conformer shows intense peaks in the ms-TPES that are assigned to the C=O/C-OH stretching mode, while the cis-trans conformer exhibits a long progression to which two C=O/C-OH stretching modes contribute. The TPE spectra allow for the sensitive and conformer-selective detection of carbonic acid in terrestrial experiments to better understand astrochemical reactions.

Gas Phase Ionization Energy of Heptacene.

The ionization energy is a fundamental property that is relevant to charge transport in organic semiconductors. We report adiabatic ionization energies (AIEs) of heptacene at 6.21 and 7.20 eV for the X̃+B2g and Ã+Au states, respectively, as the next larger member of the acene series using mass- and isomer-selective double imaging photoelectron photoion coincidence spectroscopy. The X̃+ state energy decreases monotonically with an increase in size within the homologous series of acenes and approaches an asymptotic limit [AIE(polyacene) = 5.94 ± 0.06 eV] based on a fit with an exponential decay function. As byproducts of heptacene formation from cycloreversion of diheptacenes, 5,18-, 7,16-, and 6,17-dihydroheptacene can be detected, and their AIE is similar to that of their largest acene subunit (anthracene and tetracene, respectively), in very good agreement with computational treatments.

Evidencing an elusive conical intersection in the dissociative photoionization of methyl iodide.

The valence-shell dissociative photoionization of methyl iodide (CH3I) is studied using double imaging photoelectron photoion coincidence (i2 PEPICO) spectroscopy in combination with highly-tunable synchrotron radiation from synchrotron SOLEIL. The experimental results are complemented by new high-level ab initio calculations of the potential energy curves of the relevant electronic states of the methyl iodide cation (CH3I+). An elusive conical intersection is found to mediate internal conversion from the initially populated first excited state, CH3I+(Ã2A1), into the ground cationic state, leading to the formation of methyl ions (CH3+). The reported threshold photoelectron spectrum for CH3+ reveals that the ν5 scissors vibrational mode promotes the access to this conical intersection and hence, the transfer of population. An intramolecular charge transfer takes place simultaneously, prior to dissociation. Upon photoionization into the second excited cationic state, CH3I+(B̃2E), a predissociative mechanism is shown to lead to the formation of atomic I+.

Combining mass spectrometry, i2PEPICO, and FTIR spectroscopy: Comprehensive speciation in DMM/NO oxidation

Oxymethylene ethers (OMEs) are potential alternative fuels that can be produced in a sustainable way based on CO2 and electricity. Additionally, they exhibit low tendencies to emit NOx or soot during practical combustion. Dimethoxy methane (DMM, CH3OCH2OCH3) has received increasing attention as the smallest OME with a O-CH2-O moiety. In the face of a possible application in exhaust gas recirculation (EGR) scenarios, understanding the influence of NOx on DMM oxidation is of importance. In this work, eleven mixtures of DMM/NO/O2/Ar were investigated in an atmospheric laminar flow reactor employing an extensive set of diagnostics. Molecular-beam mass spectrometry (MBMS), Fourier-transform infrared (FTIR) spectroscopy, NOx chemiluminescence detection, and double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy were combined to obtain a reliable speciation. All in all, species concentrations from different instruments showed a good agreement and one technique could counterbalance the physical limitations of another technique in many cases. In this manner, accurate mole fraction profiles of intermediates like e.g. CH4, C2H4, NO2, and methyl formate (C2H4O2) were gained. Based on i2PEPICO results, nitromethane (CH3NO2) and trans-HONO could additionally be identified as crucial intermediates in the NO-assisted oxidation of DMM. The present data set therefore provides an excellent basis to enhance future model development.

Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile.

The threshold photoionization and dissociative ionization of benzonitrile (C6H5CN) were studied using double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy at the Vacuum Ultraviolet (VUV) beamline of the Swiss Light Source (SLS). The threshold photoelectron spectrum was recorded from 9.6 to 12.7 eV and Franck-Condon simulations of ionization into the ionic ground state, X̃+, as well as the B̃+ and C̃+ states were performed to assign the observed vibronic structures. The adiabatic ionization energies of the X̃+, B̃+ and C̃+ states are determined to be (9.72 ± 0.02), (11.85 ± 0.03) and, tentatively, (12.07 ± 0.04) eV, respectively. Threshold ionization mass spectra were recorded from 13.75 to 19.75 eV and the breakdown diagram was constructed by plotting the fractional abundances of the parent ion and ionic dissociation products as a function of photon energy. The seven lowest energy dissociative photoionization channels of benzonitrile were found to yield CN˙ + c-C6H5+, HCN + C6H4˙+, C2H4 + HC5N˙+, HC3N + C4H4˙+, H2C3N˙ + C4H3+, CH2CHCN + C4H2˙+ and H2C4N˙ + c-C3H3+. HCN loss from the benzonitrile cation is the dominant dissociation channel from the dissociation onset of up to 18.1 eV and CH2CHCN loss becomes dominant from 18.1 eV and up. We present extensive potential energy surface calculations on the C6H5CN˙+ surface to rationalize the detected products. The breakdown diagram and time-of-flight mass spectra are fitted using a Rice-Ramsperger-Kassel-Marcus statistical model. Anchoring the fit to the CBS-QB3 result (3.42 eV) for the barrier to HCN loss, we obtained experimental dissociation barriers for the products of 4.30 eV (CN loss), 5.53 eV (C2H4 loss), 4.33 eV (HC3N loss), 5.15 eV (H2C3N loss), 4.93 eV (CH2CHCN loss) and 4.41 eV (H2C4N loss). We compare our work to studies of the electron-induced dissociative ionization of benzonitrile and isoelectronic phenylacetylene (C8H6), as well as the VUV-induced dissociation of protonated benzonitrile (C6H5CNH+). Also, we discuss the potential role of barrierless association reactions found for some of the identified fragments as a source of benzonitrile(˙+) in interstellar chemistry and in Titan's atmosphere.

Disentangling Multiphoton Ionization and Dissociation Channels in Molecular Oxygen Using Photoelectron-Photoion Coincidence Imaging.

Multiphoton excitation of molecular oxygen in the 392-408 nm region is studied using a tunable femtosecond laser coupled with a double velocity map imaging photoelectron-photoion coincidence spectrometer. The laser intensity is held at ≤∼1 TW/cm2 to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O2+ production is found to be resonance enhanced around 400 nm via three-photon excitation to the e'3Δu(v = 0) state, similar to results from REMPI studies using nanosecond dye lasers. O+ production reaches 7% of the total ion yield around 405 nm due to two processes: autoionization following five-photon excitation of O2, producing O2+(X(v)) in a wide range of vibrational states followed by two- or three-photon dissociation, or six-photon excitation to a superexcited O2** state followed by neutral dissociation and subsequent ionization of the electronically excited O atom. Coincidence detection is shown to be crucial in identifying these competing pathways.

Activation effect of ozone and DME on the partial oxidation of natural gas surrogates and validation of pressure-dependent ozone decomposition

To address current challenges related to climate change, novel concepts, such as engine-based polygeneration under fuel-rich conditions, have recently been developed. In this context, understanding and validation of the associated chemical kinetics are a key factor for prediction and optimizations. In this work, the combined effect of ozone and DME as additives on the partial oxidation of natural gas mixtures is investigated in a plug-flow reactor at a pressure of 4 bar and at temperatures ranging from 373 to 973 K. Double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy measurements are presented to study the conversion of the reactants and the formation of main products, and to identify important intermediates responsible for the low temperature chemistry phenomena and ignition characteristics of the natural gas/dimethyl ether/ozone mixture. Among the activation effect of ozone, the temperature-variant ozone dissociation at elevated pressures up to 20 bar is studied for the first time, revealing the start of ozone decomposition at 423 K in the investigated pressure range. In addition, the ozone decomposition is shown to be slightly pressure-dependent in the pressure range between 1 and 20 bar with lower ozone conversions at higher pressures. The experimental results are compared with model predictions using literature reaction mechanisms for validation and further analysis. The ozone decomposition initiates fuel conversion and the formation of oxygenates and hydroperoxides at very low temperatures, i.e., 423 K, resulting in an overall three-stage oxidation process at ∼450 K (1. stage), ∼550 K (2. stage), and ∼750 K (3. stage). Also, the overall fuel conversion is enhanced in the intermediate temperature range (between 550 K and 750 K) by up to 40%-points. The hydroperoxides, among other species, are clearly identified by mass-selected threshold photoelectron spectra from the literature. Excellent agreement between experiments and simulations is found, while deviations are observed for some oxygenates at very low temperatures showing the need for further model improvements.

On the influence of NO addition to dimethyl ether oxidation in a flow reactor

Dimethyl ether (DME) is a widely recognized alternative fuel which can be sustainably produced from different feedstock. Its oxidation mechanism belongs to the most deeply understood within oxygenated fuels. The oxidation of DME in the presence of NO has gained renewed attention in recent experimental and modeling efforts because of a rather complex behavior in accelerating or inhibiting DME consumption, depending on the respective temperature and mixture conditions, similarly to what has been already observed for n- and iso-alkanes. The present investigation focuses on the interaction chemistry of DME–O2–NO mixtures in the low- to intermediate-temperature regime. Previously reported flow reactor data from mass spectrometric analysis with and without NO addition are extended by isomer-resolved detection of some key intermediates, including species of formula {HNO2} and {CH3NO2} by using double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Specifically trans-HONO is highlighted as the most abundant isomer. Starting from the recently published CRECK mechanism for DME oxidation and other models available in the literature, the relevant kinetics of DME/NOx interactions are included and analyzed. The model thus obtained is compared with new experimental data from this study and others from the literature and is used to interpret observed discrepancies. A systematic polynomial chaos expansion (PCE) analysis is also performed to assess the joint uncertainty of key influential reactions under the present conditions. Nevertheless, remaining differences of model and experiment can only be addressed jointly, demonstrating the value of a concurrent experimental–modeling approach.

Dissociation of State-Selected Ions Studied by Fixed-Photon-Energy Double-Imaging Photoelectron Photoion Coincidence: Cases of O2+ and CH3F+

This study presents the method of fixed-photon-energy double-imaging photoelectron photoion coincidence (i2PEPICO) utilized to investigate the dissociation of state-selected ions. Vacuum ultraviolet (VUV) synchrotron radiation at one fixed photon energy of hν = 21.2 eV, the He(I) atomic resonance energy, is employed as a light source to ionize molecules. Various dynamic information including time-of-flight (TOF) mass spectra, mass-selected photoelectron spectra (PES), and electron and ion kinetic energy correlation diagrams corresponding to each mass are obtained efficiently with the multiplexed capabilities of i2PEPICO, thereby revealing the detailed dissociation mechanisms of ions. As representative examples, dissociation of state-selected O2+ ions prepared in the b4∑g− and B2∑g− electronic states and CH3F+ ions in the X2E, A2A1, and B2E states were selected and investigated.

Investigation of the combustion chemistry in laminar, low-pressure oxymethylene ether flames (OME0–4)

Quantitative speciation data for alternative fuels is highly desired to assess their emission potential and to develop and validate chemical kinetic models. In terms of substitute choices for fossil diesel are oxymethylene ethers (OMEs) strongly discussed. Due to the absence of carbon-carbon bonds, soot emissions from combustion of OMEs are low, but significant emissions of unregulated pollutants such as aldehydes emerge.The combustion behavior of OME fuels with different chain lengths, OME0–4, was investigated in laminar premixed low-pressure flames using complementary molecular-beam mass spectrometry (MBMS) techniques. MBMS sampling provides an in-situ access directly into the reaction zone of the flame. Almost all chemical species involved in the oxidation process can be detected and quantified simultaneously. Neat OME0–3 flames were analyzed by electron ionization (EI) MBMS with high mass resolution (R ≈ 3900) providing exact elementary composition. To obtain isomer-specific information, an OME1-doped hydrogen flame and a stochiometric OME4 flame were studied by double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Both, EI-MBMS detection and i2PEPICO spectroscopy, enables a complete overview of all intermediates.The results show a dominance of oxygenated intermediates for all measured conditions. Mole fraction profiles for the most important species are presented (i.e. formaldehyde, methanol, methyl formate and formic acid) and compared to modeling results. Hydrocarbons with more than four carbon atoms were not detected under the investigated conditions. Isomers such as ethanol/dimethyl ether (m/z = 46) and ethenol/acetaldehyde (m/z = 44) could be separated using threshold photoelectron spectra for clear identification and photoionization efficiency curves for quantification. This investigation permits the discussion and analysis of systematic trends, including intermediate species, for the combustion of the studied series of oxymethylene ether fuels.

Oxidation of oxymethylene ether (OME0−5): An experimental systematic study by mass spectrometry and photoelectron photoion coincidence spectroscopy

This paper presents a systematic study of oxymethylene ethers (OMEs) oxidation in an atmospheric laminar flow reactor setup. Oxymethylene ethers with different number of oxymethylene ether groups (n = 0–5) have been investigated under lean and rich conditions (750–1250 K). The flow reactor is coupled to an electron ionization molecular-beam mass spectrometer (EI-MBMS) with high mass resolution to measure speciation data. Additional isomer-selective speciation analysis was performed using a novel atmospheric laminar flow reactor combined with double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy at the vacuum ultraviolet radiation (VUV) beamline of the Swiss Light Source. The results show a dominance of oxygenated intermediates during the combustion of all OMEs in the investigated temperature regime. The observed species pool is thereby nearly independent of the OME’s chain length. In particular the presence of significant fractions of ethanol is remarkable and indicates unknown or underestimated reaction pathways to form C–C bonds from OME structures. Formation of combustion intermediates during oxidation of longer OMEs occurs at lower temperatures and correlates with the ignition delay time. No hydrocarbons with more than four carbon atoms are detected. The combination of high mass resolution provided by EI-MBMS detection and isomer-selective analysis by i2PEPICO enables a complete overview of all intermediates. This allows for in-depth discussion and analysis of systematic trends for several intermediate species.

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging.

Trifluoromethylsulfur pentafluoride (CF3SF5) was valence threshold photoionized in a double imaging photoelectron photoion coincidence spectrometer using vacuum ultraviolet synchrotron radiation. In the 12.5-16.4 eV photon energy range, CF3+, SF5+, and SF3+ cations were observed in both room temperature (RT) and molecular beam (MB) experiments. Their fractional abundances exhibited differences beyond the sample temperature. Kinetic energy analysis of the fragment ions confirmed the difference in the dissociative photoionization mechanism. In the RT experiment, the CF3+ kinetic energies were extrapolated to a 11.84 ± 0.15 eV threshold, which was used in an ion cycle to determine the enthalpy of formation of CF3SF5 as ΔfH°298K(CF3SF5) = -1593 ± 16 kJ mol-1. We also updated the enthalpy of formation of the sulfur pentafluoride radical as ΔfH°298K(SF5) = -854 ± 7 kJ mol-1 and discuss the discrepancy between the CF3 ionization energy based on the Active Thermochemical Tables and the value anchored to the CF ionization energy. A computed reaction enthalpy network optimization resulted in ΔfH°298K(CF3SF5) = -1608 ± 20 kJ mol-1. Both values for ΔfH°298K(CF3SF5) agree with previous ab initio ones in contrast to the original, experimental determination. SF3+ is formed by F-transfer processes both in the RT and MB experiments. Although the same peaks were observed in both experiments, the lower SF3+ onset energy and the more slowly rising CF3+ kinetic energy release in the MB experiment revealed clustering and intracluster F-transfer reactions upon ionization. The monomer and dimer cation potential energy surfaces were explored to rationalize the observations. In the dimer cation, the observer CF3SF5 catalyzes fluorine transfer and promotes CF4 formation, which ultimately leads to the SF3+ fragment ion.

Dissociation of High-Lying Electronic States of NO2+ in the 15.5-20 eV Region.

We present a dissociative photoionization study of NO2 in the 15.5-20 eV energy range using synchrotron radiation-based double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. The high-lying electronic states of the NO2+ cation, c 3B1, C 1B1, d 3A1, e 3B2, and D 1B2, are prepared in well-resolved vibronic states in order to study their individual dissociation mechanisms. Up to eight dissociation limits of NO2+ are reached, and mass-selected threshold photoelectron spectra (TPES) show that the c 3B1, C 1B1, and d 3A1 states predominantly dissociate into the NO+ + O products, while the e 3B2 and D 1B2 states can undergo fragmentation into both the NO+ + O and the O+ + NO channels, as well as the O2+ + N channel with a small yield. Overall, these product yields are found to be quite sensitive to autoionization processes. Mass-selected high-resolution electron and ion kinetic energy correlation diagrams reveal dissociative mechanisms that possess strong state-specific character.

Threshold photoelectron spectroscopy of the HO2 radical.

We report a synchrotron radiation vacuum ultraviolet photoionization study of the hydroperoxyl radical (HO2), a key reaction intermediate in combustion and atmospheric chemistry as well as astrochemistry, using double imaging photoelectron photoion coincidence spectroscopy. The HO2 radical is formed in a microwave discharge flow tube reactor through a set of reactions initiated by F atoms in a CH4/O2/He gas mixture. The high-resolution threshold photoelectron spectrum of HO2 in the 11 eV-12 eV energy range is acquired without interferences from other species and assigned with the aid of theoretically calculated adiabatic ionization energies (AIEs) and Franck-Condon factors. The three vibrational modes of the radical cation HO2 +, the H-O stretch, the H-O-O bend, and the O-O stretch, have been identified, and their individual frequencies are measured. In addition, the AIEs of the X3A″ ground state and the a1A' first excited electronic state of HO2 + are experimentally determined at 11.359 ± 0.003 eV and 11.639 ± 0.005 eV, respectively, in agreement with high-level theoretically computed results. Furthermore, the former AIE value provides validation of thermochemical networks used to extract the enthalpy of formation of the HO2 radical.

A pressurized flow reactor combustion experiment interfaced with synchrotron double imaging photoelectron photoion coincidence spectroscopy

A new pressurized low-temperature combustion experiment has been commissioned at the Swiss Light Source, Paul Scherrer Institute. The experiment uses photoionization with tunable synchrotron radiation and double imaging photoelectron photoion coincidence (i2PEPICO) detection at the vacuum ultraviolet beamline. The experimental setup is described, including the high-pressure reactor experiment, sampling interface, and reactant delivery system. The CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence) endstation and VUV beamline are briefly elaborated. The novel aspects of the apparatus and the new components are elucidated in detail, such as the fluid supply system to the reactor and the reactor integration into the endstation. We also present a system overview of the experimental setup. The technical details are followed by a description of the experimental procedure used to operate the pressurized flow reactor setup. Finally, first experimental results demonstrating the capability of the setup are provided and analyzed. A major advantage of this new experiment is that the excellent isomer resolution capabilities of the i2PEPICO technique can be transferred to the investigation of reactions at elevated pressures of several bars. This enables the investigation of pressure effects on the reactivity of fuel mixtures and covers more realistic conditions found in technical combustors. The capability to obtain quantitative oxidation data is confirmed, and the main and certain intermediate species are quantified for a selected condition. The results show excellent agreement with a chemical kinetics model and previously published reference measurements performed with a gas chromatography setup.

Vacuum ultraviolet photodynamics of the methyl peroxy radical studied by double imaging photoelectron photoion coincidences.

The vacuum ultraviolet photoionization of the methyl peroxy radical, CH3O2, and unimolecular dissociation of internal energy selected CH3O2 + cations were investigated in the 9.7-12.0 eV energy range by synchrotron-based double imaging photoelectron photoion coincidence. A microwave discharge flow tube was employed to produce CH3O2 via the reaction of methyl radicals (CH3) with oxygen gas. After identifying and separating the different sources of CH3 + from photoionization of CH3 or dissociative photoionization of CH3O2, the high resolution slow photoelectron spectrum (SPES) of CH3O2 was obtained, exhibiting two broad bands superimposed with a complex vibrational structure. The first band of the SPES is attributed to the X3A″ and a1A' overlapped electronic states of CH3O2 + and the second is assigned to the b1A' electronic state with the help of theoretical calculations. The adiabatic ionization energy of CH3O2 is derived as 10.215 ± 0.015 eV, in good agreement with high-accuracy theoretical data from the literature. The vertical ionization energy of the b1A' electronic state is measured to be 11.5 eV and this state fully dissociates into CH3 + and O2 fragments. The 0 K adiabatic appearance energy (AE0K) of the CH3 + fragment ion is determined to be 11.15 ± 0.02 eV.

High resolution vibronic state-specific dissociation of NO2+ in the 10.0-15.5 eV energy range by synchrotron double imaging photoelectron photoion coincidence.

Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of NO2 in the 10.0-15.5 eV energy range have been investigated in detail by using high-resolution double imaging photoelectron photoion coincidence (i2PEPICO) at synchrotron SOLEIL. Five low-lying electronic states of the NO2+ cation, X1Σg+, a3B2, b3A2, A1A2 and B1B2, are prepared with well-resolved vibronic structures and their state-specific dissociation mechanisms are unraveled and discussed. The present experimental results clarify that except the X1Σg+ ground electronic state and the first three vibrational levels of the a3B2 electronic state, the other cationic states of NO2+ within the present energy range are totally dissociative towards the NO+(X1Σ+) + O(3P) and/or NO+(X1Σ+) + O(1D) dissociation limits. An energy barrier exists along the direct dissociation route of the a3B2 state, and the b3A2 electronic state is a quasi-bound state with a very shallow well, both of which adiabatically correlate to the NO+(X1Σ+) + O(3P) dissociation limit. The a3B2 state mainly with bending vibration excitations undergoes a non-adiabatic transition to the 23A''(3B1) repulsive state along its bending potential energy curve and then predissociates into the NO+(X1Σ+) + O(3P) products. Our experimental results firstly demonstrate that the NO+(X1Σ+, v) fragment ions produced from individual vibronic levels of the dissociative NO2+(a3B2, b3A2) states are produced at the v = 0 ground vibrational level with a high rotational population due to the excitation of the vibrational bending mode of NO2+ and the associated imparted torque upon dissociation. The slower predissociations of the A1A2 and B1B2 electronic states via their spin-orbit couplings with the repulsive 23A''(3B1) state produce the NO+(X1Σ+) and O(3P) fragments with a long vibrational progression. In addition, the B1B2 state can also undergo a radiationless transition such as internal conversion into the hot X1Σg+ state and then dissociate into the second dissociation channel correlated to the NO+(X1Σ+) and O(1D) products.

Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane.

Through the use of tunable vacuum ultraviolet light generated by the DESIRS VUV synchrotron beamline, a jet-stirred reactor was coupled for the first time to an advanced photoionization mass spectrometer based upon a double imaging PhotoElectron PhotoIon COincidence (i2PEPICO) scheme. This new coupling was used to investigate the low-temperature oxidation of n-pentane, a prototype molecule for gasoline or diesel fuels. Experiments were performed under quasi-atmospheric pressure (1.1 bar) with a residence time of 3 s for two equivalence ratios (1/3 and 0.5) with a fuel initial mole fraction of 0.01. The measured time-of-flight mass spectra are in good agreement with those previously obtained with other photoionization mass spectrometers and, like those previous ones, display several m/z peaks for which the related species assignation is ambiguous. This paper shows how the analysis of the coincident mass-tagged Threshold PhotoElectron Spectra (TPES) together with first principle computations, consisting of the determination of the adiabatic ionization energies and the spectra of some products, may assist products' identification. The results mostly confirm those previously obtained by photoionization mass spectrometry and gas chromatography, but also allow a more accurate estimation of the 1-pentene/2-pentene mole fraction ratio. Our data also indicate a higher formation of acetone and methyl ethyl ketone than what is predicted by current models, as well as the presence of products that were not previously taken into account, such as methoxyacetylene, methyl vinyl ketone or furanone. The formation of three, four and five membered ring cyclic ethers is confirmed along with linear ketones: 2- and 3-pentanone. A significant general trend in indicating higher amounts of ketones than are indicated by gas chromatography is noted. Finally, TPES of alkenylhydroperoxides are also provided for the first time and constrains on the isomers identification are provided.