Search

The methanol-to-hydrocarbons (MTH) process involves the conversion of methanol, a C1 feedstock that can be produced from green sources, into hydrocarbons using shape-selective microporous acidic catalysts – zeolite and zeotypes. This reaction yields a complex mixture of species, some of which are highly reactive and/or present in several isomeric forms, posing significant challenges for effluent analysis. Conventional gas-phase chromatography (GC) is typically employed for the analysis of reaction products in laboratory flow reactors. However, GC is not suitable for the detection of highly reactive intermediates such as ketene or formaldehyde and is not suitable for kinetic studies under well defined low pressure conditions. Photoelectron–photoion coincidence (PEPICO) spectroscopy has emerged as a powerful analytical tool for unraveling complex compositions of catalytic effluents, but its availability is limited to a handful of facilities worldwide. Herein, PEPICO analysis of catalytic reactor effluents has been implemented at the FinEstBeAMS beamline of MAX IV Laboratory. The conversion of dimethyl ether (DME) on a zeolite catalyst (ZSM-5-MFI27) is used as a prototypical model reaction producing a wide distribution of hydrocarbon products. Since in zeolites methanol is quickly equilibrated with DME, this reaction can be used to probe vast sub-networks of the full MTH process, while eliminating or at least slowing down methanol-induced secondary reactions and catalyst deactivation. Quantitative discrimination of xylene isomers in the effluent stream is achieved by deconvoluting the coincidence photoelectron spectra.

The methanol‐to‐hydrocarbons (MTH) process involves the conversion of methanol, a C1 feedstock that can be produced from green sources, into hydrocarbons using shape‐selective microporous acidic catalysts – zeolite and zeotypes. This reaction yields a complex mixture of species, some of which are highly reactive and/or present in several isomeric forms, posing significant challenges for effluent analysis. Conventional gas‐phase chromatography (GC) is typically employed for the analysis of reaction products in laboratory flow reactors. However, GC is not suitable for the detection of highly reactive intermediates such as ketene or formaldehyde and is not suitable for kinetic studies under well defined low pressure conditions. Photoelectron–photoion coincidence (PEPICO) spectroscopy has emerged as a powerful analytical tool for unraveling complex compositions of catalytic effluents, but its availability is limited to a handful of facilities worldwide. Herein, PEPICO analysis of catalytic reactor effluents has been implemented at the FinEstBeAMS beamline of MAX IV Laboratory. The conversion of dimethyl ether (DME) on a zeolite catalyst (ZSM‐5‐MFI27) is used as a prototypical model reaction producing a wide distribution of hydrocarbon products. Since in zeolites methanol is quickly equilibrated with DME, this reaction can be used to probe vast sub‐networks of the full MTH process, while eliminating or at least slowing down methanol‐induced secondary reactions and catalyst deactivation. Quantitative discrimination of xylene isomers in the effluent stream is achieved by deconvoluting the coincidence photoelectron spectra. [ABSTRACT FROM AUTHOR]

C1 coupling reactions over zeolite catalysts are central to sustainable chemical production strategies. However, questions persist regarding the involvement of CO in ketene formation, and the impact of this elusive oxygenate intermediate on reactivity patterns. Using operando photoelectron photoion coincidence spectroscopy (PEPICO), we investigate the role of CO in methyl chloride conversion to hydrocarbons (MCTH), a prospective process for methane valorization with a reaction network akin to methanol to hydrocarbons (MTH) but without oxygenate intermediates. Our findings reveal the transformative role of CO in MCTH at the low pressures, inducing ketene formation in MCTH and boosting olefin production, confirming the Koch carbonylation step in the initial stages of C1 coupling. We uncover pressure‐dependent product distributions driven by CO‐induced ketene formation, and its subsequent desorption from the zeolite surface, which is enhanced at low pressure. Inspired by the above results, extension of the co‐feeding approach to CH3OH as another simple oxygenate showcases the additional potential for improved catalyst stability in MCTH at ambient pressure. [ABSTRACT FROM AUTHOR]

In this work, we investigate the relaxation processes of 2-thiouracil after UV photoexcitation to the S2 state through the use of ultrafast, single-colour, pump-probe UV/UV spectroscopy. We place focus on investigating the appearance and subsequent decay signals of ionized fragments. We complement this with VUV-induced dissociative photoionisation studies collected at a synchrotron, allowing us to better understand and assign the ionisation channels involved in the appearance of the fragments. We find that all fragments appear when single photons with energy > 11 eV are used in the VUV experiments and hence appear through 3+ photon-order processes when 266 nm light is used. We also observe three major decays for the fragment ions: a sub-autocorrelation decay (i.e., sub-370 fs), a secondary ultrafast decay on the order of 300–400 fs, and a long decay on the order of 220 to 400 ps (all fragment dependent). These decays agree well with the previously established S2 → S1 → Triplet → Ground decay process. Results from the VUV study also suggest that some of the fragments may be created by dynamics occurring in the excited cationic state. [ABSTRACT FROM AUTHOR]

In this work, we investigate the relaxation processes of 2-thiouracil after UV photoexcitation to the S2 state through the use of ultrafast, single-colour, pump-probe UV/UV spectroscopy. We place focus on investigating the appearance and subsequent decay signals of ionized fragments. We complement this with VUV-induced dissociative photoionisation studies collected at a synchrotron, allowing us to better understand and assign the ionisation channels involved in the appearance of the fragments. We find that all fragments appear when single photons with energy > 11 eV are used in the VUV experiments and hence appear through 3+ photon-order processes when 266 nm light is used. We also observe three major decays for the fragment ions: a sub-autocorrelation decay (i.e., sub-370 fs), a secondary ultrafast decay on the order of 300–400 fs, and a long decay on the order of 220 to 400 ps (all fragment dependent). These decays agree well with the previously established S2 → S1 → Triplet → Ground decay process. Results from the VUV study also suggest that some of the fragments may be created by dynamics occurring in the excited cationic state.

Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH 2 OMe) play an important role in the ignition of blended diesel fuels, but the underlying mechanisms are still debated. In these kinetic models, bimolecular hydrogen abstraction or unimolecular C–O bond fission are considered as the primary initial steps, while MeOCH 2 OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH 2 OMe combining imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation and CBS-QB3 theoretical calculations to unveil reaction paths and energetics. In the mass spectrum of MeOCH 2 OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH 3), 28 (CO), 29 (HCO), 30 (H 2 CO), 31 (CH 2 OH), 32 (CH 3 OH), 45 (CH 3 OCH 2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH 2 OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH 2 OMe pyrolysis products. The m/z 31 peak was assigned to the methoxy radical in the previous studies. However, our photoion mass-selected threshold photoelectron spectrum (ms-TPES) confirms that it originates from dissociative photoionization of the primary pyrolysis fragment methanol. Based on the experimental and computational results, a thermal decomposition mechanism of MeOCH 2 OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH 3 OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH 3 OCH and of dimethyl ether yields enhanced m/z 15, 28 and 29 signals at high temperature. Rate constants have been calculated to confirm the dominant role of MeOCH 2 OMe isomerization and to help improve predictive combustion models. [ABSTRACT FROM AUTHOR]

The dissociative photoionization of 1,3-dioxolane was studied by photoelectron photoion coincidence (PEPICO) spectroscopy in the photon energy range of 9.5–13.5 eV. Our statistical thermodynamics model shows that a total of six dissociation channels are involved in the formation of three fragment ions, namely, C3H5O2 + (m/z 73), C2H5O+ (m/z 45), and C2H4O+ (m/z 44), with two channels contributing to the formation of each. By comparing the results of ab initio quantum chemical calculations to the experimentally derived appearance energies of the fragment ions, the most likely mechanisms for these unimolecular dissociation reactions are proposed, including a description of the relevant parts of the potential energy surface. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Dissociation of Cr(CO) 6 + ions was studied by Photoelectron Photoion Coincidence. • Cr(CO) 6 + ions dissociate by losing all six CO ligands, yielding the Cr+ ion. • By modeling the breakdown diagram and fitting rate curves to measured dissociation rates, bond energies were obtained. • Repulsive doublet states lead to non-statistical dissociation at intermediate energies. • Bond energies disagree with literature, leading to revised Cr(CO) 6 thermochemistry. Abstract The fragmentation processes of internal energy selected chromium hexacarbonyl cations, Cr(CO) 6 +, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source. In the 9.3–21.5 eV photon energy range, Cr(CO) 6 + dissociates by six sequential carbonyl ligand losses. The fragment ion fractional abundances, plotted in the breakdown diagram, along with the time-of-flight mass spectra for the first three metastable CO-loss channels were modeled using a statistical approach. Between 12 and 16 eV, the statistical model overestimates the degree of fragmentation, which is explained by enhanced kinetic energy release in impulsive CO loss on repulsive electronic states of the parent ion Cr(CO) 6 +, as confirmed by TD-DFT calculations. This is the first reported example for an embedded non-statistical unimolecular dissociation regime, bracketed by statistical regimes at low and at high energies. The statistical model was employed to derive 0 K appearance energies for Cr(CO) n + (n = 0–5). The Cr(CO) 5 + appearance energy and the literature CO Cr(CO) 5 + bond dissociation energy yield an adiabatic Cr(CO) 6 + ionization energy of 8.195 ± 0.120 eV, independent of photoelectron spectroscopy measurements. The 0 K appearance energy of Cr+ is 61 kJ mol–1 higher than predicted based on literature enthalpies of formation, which we suggest is most likely due to an error in the enthalpy of formation of Cr(CO) 6 , and propose a revised Δ f H [Cr(CO) 6 , g] = –972.1 ± 4.1 and –968.9 ± 4.1 kJ mol–1 at 0 and 298 K, respectively, and Δ f H 298K [Cr(CO) 6 , c] = –1040.6 ± 4.2 kJ mol–1 for the crystalline state, based on the known enthalpy of sublimation. The measured Cr CO bond dissociation energies in [(CO) n Cr CO]+ (n = 0–5), and the enthalpies of formation of the chromium carbonyl ion series are also reported. [ABSTRACT FROM AUTHOR]

Abstract Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH 3 bond in anisole (C 6 H 5 OCH 3) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C 6 H 5 O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C 5 H 6) and cyclopentadienyl radicals (C 5 H 5). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10−3–10−2) are CH 3 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , CH 2 O, C 5 H 5 (cyclopentadienyl radical), C 5 H 6 (cyclopentadiene), C 6 H 6 (benzene), C 6 H 5 OH (phenol), and C 6 H 5 CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C 6 H 5 CHO), (2) formation of phenol (C 6 H 5 OH), and (3) unimolecular decomposition of anisole to phenoxy (C 6 H 5 O) and CH 3 radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C 5 H 5 and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole. [ABSTRACT FROM AUTHOR]

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–O 2 –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 {HNO 2 } and {CH 3 NO 2 } 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/NO x 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. [ABSTRACT FROM AUTHOR]

The radicals produced by hydrogen abstraction in the initial fuel decomposition step are essential in combustion kinetics, but their experimental detection is very challenging. Imaging photoelectron photoion coincidence spectroscopy enables the detection and identification of even these isomeric radicals. Laminar low-pressure (40 mbar) hydrogen flames doped with different alkanes and alkenes are investigated systematically with the goal to identify the formation pathways and the fate of fuel radicals formed in hydrogen abstraction reactions. The abstraction reactions of primary, secondary, tertiary, and vinylic H atoms were never target of a systematic, direct semiquantitative investigation in a flame environment and this paper describes such a study for the first time. Performing the measurements at the vacuum ultraviolet beamline located at the Swiss Light Source enables isomer-selective detection of reactive radical species by imaging photoelectron photoion coincidence spectroscopy. For unambiguous identification of several isomeric radicals, threshold photoelectron spectra were compared with reference photoelectron spectra. H-abstraction ratios of isomeric radicals were determined and compared to literature reaction barriers and rate coefficients. In addition to the quantitative information, the peak positions of the profiles of radicals formed by hydrogen abstraction or addition to the fuel molecules as function of distance from the burner show faster H-abstraction for unbranched alkanes and alkenes than for branched fuels and faster H-addition than H-abstraction, respectively. [ABSTRACT FROM AUTHOR]

Double-imaging photoelectron/photoion coincidence (i²PEPICO) spectroscopy using a multiplexing, time-efficient, fixed-photon-energy approach offers important opportunities of gas-phase analysis. Building on successful applications in combustion systems that have demonstrated the discriminative power of this technique, we attempt here to push the limits of its application further to more chemically complex combustion examples. The present investigation is devoted to identifying and potentially quantifying compounds featuring five heavy atoms in laminar, premixed low-pressure flames of hydrocarbon and oxygenated fuels and their mixtures. In these combustion examples from flames of cyclopentene, iso-pentane, iso-pentane blended with dimethyl ether (DME), and diethyl ether (DEE), we focus on the unambiguous assignment and quantitative detection of species with the sum formulae C5H6, C5H7, C5H8, C5H10, and C4H8O in the respective isomer mixtures, attempting to provide answers to specific chemical questions for each of these examples. To analyze the obtained i2PEPICO results from these combustion situations, photoelectron spectra (PES) from pure reference compounds, including several examples previously unavailable in the literature, were recorded with the same experimental setup as used in the flame measurements. In addition, PES of two species where reference spectra have not been obtained, namely 2-methyl-1-butene (C5H10) and the 2-cyclopentenyl radical (C5H7), were calculated on the basis of high-level ab initio calculations and Franck-Condon (FC) simulations. These reference measurements and quantum chemical calculations support the early fuel decomposition scheme in the cyclopentene flame towards 2-cyclopentenyl as the dominant fuel radical as well as the prevalence of branched intermediates in the early fuel destruction reactions in the iso-pentane flame, with only minor influences from DME addition. Furthermore, the presence of ethyl vinyl ether (EVE) in DEE flames that was predicted by a recent DEE combustion mechanism could be confirmed unambiguously. While combustion measurements using i2PEPICO can be readily obtained in isomer-rich situations, we wish to highlight the crucial need for high-quality reference information to assign and evaluate the obtained spectra. [ABSTRACT FROM AUTHOR]

Highly excited electronic states are challenging to explore experimentally and theoretically--due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron--ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-- in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born--Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-- Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly,making it possible to identify the ultrafast reaction pathways. [ABSTRACT FROM AUTHOR]

This paper employs Bayesian probability theory for analyzing data generated in femtosecond pump-probe photoelectron-photoion coincidence (PEPICO) experiments. These experiments allow investigating ultrafast dynamical processes in photoexcited molecules. Bayesian probability theory is consistently applied to data analysis problems occurring in these types of experiments such as background subtraction and false coincidences. We previously demonstrated that the Bayesian formalism has many advantages, amongst which are compensation of false coincidences, no overestimation of pump-only contributions, significantly increased signal-to-noise ratio, and applicability to any experimental situation and noise statistics. Most importantly, by accounting for false coincidences, our approach allows running experiments at higher ionization rates, resulting in an appreciable reduction of data acquisition times. In addition to our previous paper, we include fluctuating laser intensities, of which the straightforward implementation highlights yet another advantage of the Bayesian formalism. Our method is thoroughly scrutinized by challenging mock data, where we find a minor impact of laser fluctuations on false coincidences, yet a noteworthy influence on background subtraction. We apply our algorithm to data obtained in experiments and discuss the impact of laser fluctuations on the data analysis.

We present here a flash pyrolysis source coupled with a threshold photoelectron photoion coincidence (TPEPICO) spectrometer and vacuum ultraviolet synchrotron radiation to investigate the spectroscopy and photochemistry of free radicals. The radicals are produced from pyrolysis in a heated silicon carbide tube, and the TPEPICO scheme provides a strategy to obtain pure spectra of the radicals without contamination from other byproducts. As a representative example, the methyl radical was studied, and its threshold photoelectron spectrum shows a series of umbrella vibrational transitions. The adiabatic ionization energy of the methyl radical was determined to be 9.84 ± 0.01 eV. [ABSTRACT FROM AUTHOR]

Advances in laser and Synchrotron Radiation instrumentation are continuously boosting fundamental research on the electronic structure of matter. At Elettra the collaboration between several groups active in the field of atomic, molecular and cluster physics and the Instrumentation and Detector Laboratory has resulted in an experimental set-up that successfully tackles the challenges posed by the investigation of the electronic structure of isolated species in the gas phase. The use of Synchrotron Radiation (SR) and Free Electron Laser (FEL) light, allows to cover a wide spectrum of targets from energetic to dynamics. We developed a Velocity Map Imaging (VMI) spectrometer that allows to perform as well SR as FEL experiments, just by changing part of the detection system. In SR experiments, at the Gasphase beamline of Elettra, a cross delay line detector is used, coupled to a 4-channel time-to-digital converter that reconstructs the position of the electrons. Simultaneously, a Time-of-Flight (TOF) mass spectrometer is used to acquire photoion spectra. Such a system allows PhotoElectron-PhotoIon-Coincidence (PEPICO) spectroscopy of atoms, molecules and clusters. In FEL experiments (notably differing from SR experiments in the much higher rate of events produced and detected, which forces one to forfeit coincidence detection), at the Low Density Matter (LDM) beamline of FERMI, a Micro Channel Plate (MCP) a phosphor screen and a CCD camera are used instead, capable of shot-by-shot collection of practically all events, albeit without time resolution. [ABSTRACT FROM AUTHOR]

The nitrous acid combustion intermediate has recently been detected in several reaction conditions using hydrocarbon fuels with different analytical techniques. Three of the several isomers of nitrous acid are expected to be produced during combustion: trans -HONO, cis -HONO and HNO 2. It has recently been shown that cis -HONO dissociates upon ionization, rendering isomer selective quantification with methods requiring photoionization prior to detection impossible. This fact is of importance, since cis -HONO is produced at a ten times higher rate than trans -HONO according to recently published isomer branching ratios, possibly leading to sensitivity issues when a detection of the isomer mix is attempted with photoionization methods. We provide a quantitative glimpse at the trans -HONO isomer in a systematic set of measurements of NO doped methane oxidized in a plug-flow reactor covering three reaction conditions in the lean and rich regimes. Reactions take place at equivalence ratios of 0.7 and 1.2 with 1000 ppm NO and at an equivalence ratio of 2.1 doped with 1% NO. Double imaging photoelectron photo ion coincidence spectroscopy, i2PEPICO, was used to selectively and assuredly detect and assign trans -HONO. We touch on the difficulties encountered when attempting to detect cis-HONO. HNO 2 remained undetectable despite recently published reaction rates for HNO 2 decomposition suggesting modelled concentrations of this species two orders of magnitude larger than previously believed, yet 10 times lower than the reported isomer branching ratio. The recent reaction rates add a new path for HNO 2 decomposition leading to formation of OH and NO which in turn influences the remaining decomposition kinetics of HNO 2. A literature model is modified to include the recently published reaction rates for HONO and HNO 2 decomposition and isomerization and compared to the measurements. Despite the higher predicted concentration of HNO 2 , that should be sufficient for detection, no HNO 2 is detected in the experiment. Other nitrogen containing species, such as nitromethane and NO 2 , the precursor of both HONO and nitromethane, have also been detected. Interestingly, ammonia was also present in significant concentration, albeit exclusively in the fuel-rich conditions, despite the relatively low maximum temperature of 923 K at which the experiments have been performed. We conclude that, facing the unfavorable photoionization properties of cis -HONO as well as the decomposition and formation kinetics of HNO 2 , a measurement of isomer branching fractions by means of selective and sensitive photoionization methods may remain unattainable. [ABSTRACT FROM AUTHOR]

Deep core photoionization of the sulfur hexafluoride molecule have been studied with X-ray photons at energies around the S K edge (from 2472 eV to 2489 eV), using time-of-flight multicoincidence technique with synchrotron radiation. Total and partial photoion yields for the fragments have been recorded as a function of the incident photon energy. The atomic fluorine ion, F + is the most abundant species observed in the mass spectra at the energy range studied. The SF 6 molecule undergoes a complete break up with the formation of atomic doubly and triply charged S 2+ , S 3+ , and F 2+ fragments. The intensity of absorption for the dipole-forbidden but vibronically allowed transition S K (1a 1g → 6a 1g ) was calculated and it is shown that it play a relevant role in the fragmentation pattern. [ABSTRACT FROM AUTHOR]

Charge state spectra of krypton ions generated after ionization (by a single photon) of the L shell have been measured by using the PEPICO technique. Relative abundances of Kr q+ ions in charge state up to 8+ were obtained using monochromatized synchrotron radiation. A comparison with other experimental and theoretical data is presented. [ABSTRACT FROM AUTHOR]

We present the threshold photoelectron spectroscopy of the imidogen radical (NH) recorded in the photon energy region up to 1 eV above its first ionization threshold. The radical was produced by reaction of NH 3 and F in a microwave discharge flow-tube and photoionized using vacuum ultraviolet (VUV) synchrotron radiation. A double imaging coincidence spectrometer was used to record mass-selected spectra and avoid contributions from the byproducts present in the reactor and background gas. The energy region includes the ground X + 2 Π and first electronically excited a + 4 Σ − states of NH + . Strong adiabatic transitions and weak vibrational progressions up to v + = 2 are observed for both electronic states. The rotational profile seen in the origin band has been modeled using existing neutral and cationic spectroscopic constants leading to a precise determination of the adiabatic ionization energy at 13.480 ± 0.002 eV. [ABSTRACT FROM AUTHOR]

We measure threshold photoelectron spectra of the benzyl radical, which show transitions to at least three electronic states of the benzylium cation:1A1,3B2, and1B2, with possible contributions from transitions to3A1. The main features in the vibrationally resolved threshold photoelectron spectrum between 7.1 and 10.5 eV are assigned with the aid of Franck–Condon simulations to these four electronic states of benzylium. We measure the adiabatic ionisation energy of the benzyl radical to be 7.252(5) eV and observe a well-resolved vibrational progression in the lowest triplet state,3B2, from which we obtain a measured singlet–triplet splitting of 1.928(7) eV in benzylium. [ABSTRACT FROM AUTHOR]

Coincidence spectroscopy is a powerful spectroscopic method addressing the recording of more than one particle involved in an ionization process simultaneously. In the current manuscript, an overview of the available flavors of coincidence experiments is given, and new results on the Auger decay of CH 3 F are discussed. An outlook of possible future experiments employing the approach is also briefly discussed. [ABSTRACT FROM AUTHOR]

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy has been used to determine 0 K appearance energies for the unimolecular dissociation reactions of several energy selected straight chain alkyl iodide cations 1-C n H 2 n +1 I + → C n H 2 n +1 + + I , ( n = 3–7). The 0 K appearance energy of iodine atom loss, yielding in fact the 2-alkyl radical cation up to n = 6, was determined to be 9.836 ± 0.010, 9.752 ± 0.010, 9.721 ± 0.010, 9.684 ± 0.010 and 9.688 ± 0.015 eV in 1-C 3 H 7 I, 1-C 4 H 9 I, 1-C 5 H 11 I, 1-C 6 H 13 I, and 1-C 7 H 15 I, respectively. In 1-iodohexane and the smaller molecules, these correspond to the transition state along the 1-iodoalkane cation → 2-iodoalkane cation reaction path, and can be used in conjunction with isodesmic reaction energies to determine the reverse barriers to dissociative photoionization. The small kinetic shift is indicative of little H tunneling during isomerization. Directly computed reverse barriers show that run-of-the-mill computational approaches are of limited use when applied to open shell systems containing period 5 elements. Hindered rotors were found to play a minor role in the internal energy distribution and the dissociation rate constants. [ABSTRACT FROM AUTHOR]

Quantitative species data for the development and critical examination of combustion mechanisms are in high demand regarding the need for predictive combustion models that may assess the emission potential of current and emerging fuels. Mass spectrometric investigation is one of the often-used techniques to provide mole fractions of stable and reactive intermediates including radicals from specifically designed laboratory experiments. Molecular-beam mass spectrometry (MBMS) has been coupled with electron ionization (EI) and photoionization (PI) to determine the species compositions, and combinations of these techniques have been successful in the investigation of the combustion pathways in flames of numerous hydrocarbon, oxygenated and nitrogenated fuels. Photoelectron/photoion coincidence spectroscopy (PEPICO) has recently emerged as a novel diagnostics to be combined with flame-sampling mass spectrometry, and its potential as a complement of existing techniques is just about being explored. In a multi-laboratory investigation, the present study has thus combined four different MBMS spectrometers (in Bielefeld, Germany, the Advanced Light Source in Berkeley, USA, the Swiss Light Source in Villigen, Switzerland, and the SOLEIL synchrotron in St. Aubin, France) to study a rich premixed argon-diluted low-pressure (40 mbar) ethylene–oxygen flame under comparable conditions. This was done with the aim of illustrating the respective properties and capabilities of the methods under these conditions, with an emphasis on the power offered by the synchrotron-based techniques, including PEPICO, for combustion chemistry studies. Examples include comparisons of selected species quantification as well as PEPICO spectra measured at different instruments. [ABSTRACT FROM AUTHOR]

Photoelectron photoion coincidence measurements have been performed for the thiazole (C3H3NS) molecule in gas phase, using time-of-flight mass spectrometry in the electron-ion coincidence mode and vacuum ultraviolet synchrotron radiation. photoelectron photoion coincidence spectra have been recorded as a function of the photon energy covering the valence range from 10 to 21 eV. The resulting photoionization products as well as the dissociation pathways leading to the ionic species were proposed and discussed. We have also performed density functional theory and ab initio calculations for the neutral molecule, its cation and the ion fragments produced in order to determine their electronic and structural parameters. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The valence and C 1 s core photoelectron spectra of three carboxylic acids (butanoic, hexanoic and octanoic) as well as the near-edge absorption fine structure spectra and photo-electron photo-ion coincidence spectra have been measured. Changes in the valence spectra are related to the lengthening of the aliphatic chains, and the core spectra have been assigned. Small shifts of the carboxylic carbon 1 s binding energy are related to screening of the core hole by the aliphatic group. The C K edge near-edge absorption fine structure spectra probe the unoccupied density of states and the main changes in the spectra are due to increasing length of the aliphatic chain and related Rydberg states. The coincidence spectra show that soft (near threshold) ionization of all compounds leads to parent ion formation for butanoic and octanoic acids. For hexanoic acid, both parent ions and fragments are produced for near threshold ionization. Fragmentation occurs at the C2 C3 bond for all carboxylic acids at the lowest energies, and other channels open for higher internal energy. • Electronic structure and ionic fragmentation patterns of fatty acids are reported. • The C K edge NEXAFS spectra display clear trends related to the aliphatic chain length in fatty acids. • The parent ion of butanoic and octanoic acids are the only products for threshold ionization. • Hexanoic acid fragments for threshold ionization. [ABSTRACT FROM AUTHOR]

The dissociative photoionization of energy-selected cis-1-bromopropene, trans-1-bromopropene, 2-bromopropene, 3-bromopropene (allyl bromide) and bromocyclopropane was investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. Br loss was the only dissociation channel observed in the measured energy range. Cis-1-bromopropene, trans-1-bromopropene and 2-bromopropene cations are metastable at the 0 K appearance energy of Br loss, whereas allyl bromide and bromocyclopropane cations dissociate quickly. The experimental onsets, supported by ab initio calculations, show that allyl bromide, cis- and trans-1-bromopropene, and bromocyclopropane dissociatively photoionize by Br loss to form the allyl cation, CH2CHCH2+. Due to the high barrier between the 2- and 3-bromopropene cations, the 2-bromopropene cation dissociates into the other, less stable C3H5+ isomer, the 2-propenyl cation, CH3C = CH2+. The 0 K appearance energy of the allyl cation from the cis- and trans-1-bromopropenes, allyl bromide and bromocyclopropane was determined to be 10.581 ± 0.014 eV, 10.561 ± 0.012 eV, 10.522 ± 0.005 eV and 10.211 ± 0.005 eV, respectively. Similarly, the 0 K appearance energy of the 2-propenyl cation from 2-bromopropene was determined to be 10.930 ± 0.015 eV. Using a mixed theoretical and experimental thermochemical network, the 0K heats of formation of cis-1-bromopropene, trans-2-bromopropene, allyl bromide, 2-bromopropene, and bromocyclopropane were determined as 64.7 ± 2.9, 66.4 ± 2.8, 70.3 ± 2.6, 58.5 ± 2.9, and 104.3 ± 4kJmol−1, respectively. [Copyright &y& Elsevier]

Abstract: Molecular LiCl and Li2Cl2 have been studied in the vapor phase with valence photoelectron and photoelectron–photoion coincidence spectroscopies. These two techniques determine the binding energies in fundamentally different ways. Binding energies obtained from photoelectron spectra are usually taken as the vertical ionization energies of the corresponding electronic states. In cases with several overlapping bands, corresponding to different electronic states, the coincidence measurement can separate the bands if the respective final states fragment differently. This applies well to the monomer case. To facilitate the determination of state-specific ionization energies in the dimeric molecule, a theoretical Franck–Condon analysis has been carried out. Moreover, ab initio coupled-cluster and density-functional-theory calculations have been used to analyze the fragmentation pattern based on asymptotic dissociation energies. The fragmentation pattern is largely common to all the accessible valence-ionized states of the dimer, consistent with rapid conversion to the ionic ground state before fragmentation. However, the highest-lying state of Li2Cl2 +, 2 A g , shows enhanced propensity for Li+ as dissociation product. [Copyright &y& Elsevier]

Abstract: Molecular fragmentation induced in organic molecules by absorption of VUV and soft X-ray radiation can cause damage on living organisms. In this work, the site-selective molecular fragmentation of pyrimidine and 2Br–pyrimidine has been studied and explained as a combined effect of the selective electron decay of a localised core vacancy and the selective fragmentation of specific valence shell electronic states. [Copyright &y& Elsevier]

Abstract: Ionization-site-dependent fragmentation of three cyclic biomolecules – uracil, 5-bromouracil and thymine – has been studied using electron-energy-resolved photoelectronphotoion-photoion coincidence spectroscopy. Previous studies concerning ionization site dependent fragmentation have mainly been carried out with linear molecules. The results reported here show that all studied molecules have dicationic fragmentation channels whose intensity depends on the initial core-ionization site, although these channels cover only a relatively small fraction of the total fragment yield. The present study shows on one hand, that it is often the surrounding bond(s) of the ionized atom that will break following the initial core ionization, and on the other hand, that some specific fragmentation channels can display strong site-dependency where there is no direct correlation between the ionization site and the bond breakage locations. [Copyright &y& Elsevier]

Abstract: Charge state spectra of Ne ions generated by photoionization from 2 to 3.3keV X-ray with Ne atoms have been measured by the PEPICO technique. Relative abundances of Ne q+ ions in charge state up to 5+ were obtained. A comparison with other experimental and theoretical data is presented. In addition, by combining the present data with various theoretical and experimental data, we have shown that double ionization of Ne atoms by fast electron impact occurs mainly via outer-shell ionization. [Copyright &y& Elsevier]

Abstract: Dissociation of acetonitrile molecules following resonant core excitations of carbon and nitrogen core electrons to the LUMO orbital was investigated using the electron-energy-resolved photoelectron–photoion coincidence (PEPICO) technique. The fragment ion mass spectra were recorded in coincidence with the resonant Auger electrons, emitted in the decay process of the core-excited states. Deuterated and 13 C-substituted samples were used for fragment identification. The results showed the initial core hole localization to be of minor importance in determining the dissociation pattern of the molecular cation. The participator and spectator Auger transitions produce entirely different fragmentation patterns and the latter indicates that complex nuclear rearrangements take place. Using also the findings from the 13 C labelling, it is suggested that the bending motion, induced in the core-excited state by the Renner–Teller effect, plays an important role in determining the most intense dissociation pathways of the spectator Auger final electronic states. [Copyright &y& Elsevier]

Abstract: We report on the development of a new temperature controlled inlet system for the study of gas phase unimolecular reaction dynamics using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Temperatures in the range of 220–400K can be achieved, with a deviation of less than 5K over the course of a 48h experiment. Iodine loss from energy selected 1-butyl iodide (1-C4H9I) ions was studied at four temperatures, 220, 275, 298 and 400K. The fractional ion abundances, in the form of breakdown diagrams are presented. Particular attention is paid to the slopes of the molecular and daughter ion abundances in crossover region of the breakdown curve since they are governed solely by the internal energy distribution, P(E), of the neutral precursor. P(E) is a function of both the temperature and vibrational frequencies of the neutral precursor. A detailed discussion regarding the transposition of the neutral thermal energy distribution to the ionic manifold is presented. From the four experimental measurements, the E 0 for the production of the 2-C4H9 + ion was determined to be 9.738±0.015eV. [Copyright &y& Elsevier]

Abstract: Dissociation processes of the CCl4 molecule excited in the valence, Cl 2p, and C 1s core levels have been studied using the photoelectron–photoion (PEPICO) coincidence technique combined with synchrotron radiation. Doubly and triply charged fragments not previously reported have been observed. Kinetic energy releases for the fragments have been estimated using the peak width method. In the PEPICO spectra an increasing broadening in the peak widths has been observed as function of the incident photon energy, indicating the repulsive role played by the σ* orbital. [Copyright &y& Elsevier]

Abstract: Electron-ion coincidence technique (PEPICO) and synchrotron radiation have been employed in the study of the ionic fragmentation of epichlorohydrin (C3H5ClO) molecule. Total ion yield and mass spectra have been recorded for the first time for this molecule in the vicinity of the C 1s, O 1s and Cl 1s core excitation edges. We have found evidences of site selective fragmentation upon core excitation, as well as indication of molecular quasi-alignment in the photoexcitation and dissociation at the Cl 1s resonance. [Copyright &y& Elsevier]

Abstract: A new electron-energy-resolved electron-ion coincidence setup was developed based on the Scienta SES-100 hemispherical electron analyzer and a Wiley–McLaren type ion TOF spectrometer. With this setup, an electron-ion coincidence study of gas-phase acetonitrile () was performed using synchrotron radiation. The fragmentation patterns in coincidence with resonant Auger electrons were recorded, following N 1s core excitations to the molecular orbital of the character. Also the dissociation of doubly charged molecular ions produced after normal Auger decay of N core-hole states was investigated from PEPICO and PEPIPICO maps. Kinetic energy releases in some reactions were determined with the aid of Monte-Carlo modelling. [Copyright &y& Elsevier]

Abstract: Single and multiple photoionization branching ratios of Ar have been measured for photon energies around the K-edge region between 3198 and 3212eV using synchrotron radiation and time-of-flight mass spectrometry. The present data is normalized to the absolute values using the accurately measured data of Zheng et al. [L. Zheng, M. Cui, Y. Zhao, K. Chen, J. Electron. Spectrosc. Rel. Phenom. 152 (2006) 143–147]. A comparison with other experimental and theoretical data is presented and discrepancies are discussed. [Copyright &y& Elsevier]