Appearance Energies Of Fragments Research Articles

Absolute Photoionization Cross Section and Dissociative Ionization Pathways of Alpha-Pinene.

The absolute photoionization cross section of the monoterpenoid, alpha-pinene (AP), is presented together with the relative photoionization cross sections of its dissociative fragments for the first time. Experiments are performed via multiplexed vacuum ultraviolet (VUV) synchrotron photoionization (PI) mass spectrometry in the 8.0-11.0 eV energy range. Experimental work is conducted at the Advanced Light Source of the Lawrence Berkeley National Laboratory. Dissociative fragments were identified at m/z 121, 94, 93, 92, and 80. The photoionization cross section for the parent mass at 11.0 eV was determined to be 17±4 Mb with a total ionization cross section of 92±23 Mb at the same photon energy. Experimental appearance energies of dissociative ionization fragments and potential dissociative ionization pathways calculated at the G4 level of theory are presented as well.

Photoionization cross sections of Ethylene oxide

Abstract In this work a theoretical study for photoionization of Ethylene oxide is presented. The photoionization cross section (PICS) for each of the nine valence orbitals and also the summed cross sections are presented. Electronic structure calculations are done to obtain the required molecular properties and the Variational Schwinger method with Padé approximants is used to calculate the PICS. The results are shown in four different approaches: dipole-length/velocity form, static-exchange and static-exchange-polarization levels. The partial PICS for each orbital shows which symmetries of continuum states are responsible for resonance features and how the polarization effects affect the cross sections magnitudes near the threshold. PICS calculations indicate which parent ion state is generated according to the corresponding ionization energies. A correlation is proposed suggesting that the different ionic fragments can be preferentially produced by different electronic states of the parent ion, based on their ionic fragment appearance energies. The summed cross section of all valence orbitals of Ethylene oxide is compared with the results of Acetaldehyde and the analysis suggests that the PICSs, in a given energy range, can be used to identify these isomers. A comparison of our results for EtO and Acetaldehyde with available experimental results for Acetaldehyde and Ethenol demonstrated the the results of three isomers are similar in magnitude in the energy range from 9 to 12 eV. Furthermore, for acetaldehyde, we observed quantitative agreement with the experiment, indicating the reliability of our calculations.

Role of the Molecular Environment in Quenching the Irradiation-Driven Fragmentation of Fe(CO)5: A Reactive Molecular Dynamics Study

Irradiation-driven fragmentation and chemical transformations of molecular systems play a key role in nanofabrication processes where organometallic compounds break up due to the irradiation with focused particle beams. In this study, reactive molecular dynamics simulations have been performed to analyze the role of the molecular environment on the irradiation-induced fragmentation of molecular systems. As a case study, we consider the dissociative ionization of iron pentacarbonyl, Fe(CO)5, a widely used precursor molecule for focused electron beam-induced deposition. In connection to recent experiments, the irradiation-induced fragmentation dynamics of an isolated Fe(CO)5+ molecule is studied and compared with that of Fe(CO)5+ embedded into an argon cluster. The appearance energies of different fragments of isolated Fe(CO)5+ agree with the recent experimental data. For Fe(CO)5+ embedded into an argon cluster, the simulations reproduce the experimentally observed suppression of Fe(CO)5+ fragmentation and provide an atomistic-level understanding of this effect. Understanding irradiation-driven fragmentation patterns for molecular systems in environments facilitates the advancement of atomistic models of irradiation-induced chemistry processes involving complex molecular systems.

Mass spectra and appearance energies of cationic fragments for electron ionization of 2-butanol

Mass spectra and appearance energies of cationic fragments for electron ionization of 2-butanol

The impact of the third O2 addition reaction network on ignition delay times of neo-pentane

We studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O2 addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O2 addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O2 addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O2 addition reactions.

Ab initio study of energy characteristics of small polyatomic molecules in threshold electron-impact dissociative ionization processes

SynopsisA joint theoretical study of electron-impact dissociative ionization (DI) processes of small polyatomic molecules is presented. Different organic (CH4 and C2H6) and inorganic (SF6 and Sn, n = 2-8) target molecules were examined. The investigated dissociative ionization processes take place at threshold energies, so our main focus is to study the appearance energies (E ap) of different ion fragments as well as to analyse the corresponding DI reaction pathways.

Reactive molecular dynamics simulations of organometallic compound W(CO)6 fragmentation,

Irradiation- and collision-induced fragmentation studies provide information about geometry, electronic properties and interactions between structural units of various molecular systems. Such knowledge brings insights into irradiation-driven chemistry of molecular systems which is exploited in different technological applications. An accurate atomistic-level simulation of irradiation-driven chemistry requires reliable models of molecular fragmentation which can be verified against mass spectrometry experiments. In this work fragmentation of a tungsten hexacarbonyl, W(CO)6, molecule is studied by means of reactive molecular dynamics simulations. The quantitatively correct fragmentation picture including different fragmentation channels is reproduced. We show that distribution of the deposited energy over all degrees of freedom of the parent molecule leads to thermal evaporation of CO groups and the formation of W(CO)n+ (n = 0 – 5) fragments. Another type of fragments, WC(CO)n+ (n = 0 – 4), is produced due to cleavage of a C–O bond as a result of localized energy deposition. Calculated fragment appearance energies are in good agreement with experimental data. These fragmentation mechanisms have a general physical nature and should take place in radiation-induced fragmentation of different molecular and biomolecular systems.

Electron-Impact-Induced Fragmentation of a Glutamine Molecule

The formation of ionized products upon single and dissociative ionization of a glutamine (C5H10N2O3) molecule by electron impact at low (70 eV) and high (11.5 MeV) energies has been studied by method of mass spectrometry. The mass spectra of glutamine molecules have been obtained upon electron irradiation to various doses (0, 5, 10, and 20 kGy) and for various threshold functions of ion fragment yield. The absolute values of ion fragment appearance energies are determined. Electron beams were generated by a three-electrode gun and electron accelerator (microtron). Comparative analysis of the measured mass spectra of unirradiated and irradiated glutamine molecules showed that high-energy irradiation produced irreversible changes in the molecular structure.

Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran.

Keto-hydroperoxides (KHPs) are reactive, partially oxidized intermediates that play a central role in chain-branching reactions during the gas-phase low-temperature oxidation of hydrocarbons and oxygenated species. Although multiple isomeric forms of the KHP intermediate are possible in complex oxidation environments when multiple reactant radicals exist that contain nonequivalent O2 addition sites, isomer-resolved data of KHPs have not been reported. In this work, we provide partially isomer-resolved detection and quantification of the KHPs that form during the low-temperature oxidation of tetrahydrofuran (THF, cycl.-O-CH2CH2CH2CH2-). We describe how these short-lived KHPs were detected, identified, and quantified using integrated experimental and theoretical approaches. The experimental approaches were based on direct molecular-beam sampling from a jet-stirred reactor operated at near-atmospheric pressure and at temperatures between 500 and 700 K, followed by mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation, and the identification of fragmentation patterns. The interpretation of the experiments was guided by theoretical calculations of ionization thresholds, fragment appearance energies, and photoionization cross sections. On the basis of the experimentally observed and theoretically calculated ionization and fragment appearance energies, KHP isomers could be distinguished as originating from H-abstraction reactions from either the α-C adjacent to the O atom or the β-C atoms. Temperature-dependent concentration profiles of the partially resolved isomeric KHP intermediates were determined in the range of 500-700 K, and the results indicate that the observed KHP isomers are formed overwhelmingly (∼99%) from the α-C THF radical. Comparisons of the partially isomer-resolved quantification of the KHPs to up-to-date kinetic modeling results reveal new opportunities for the development of a next-generation THF oxidation mechanism.

Фрагментация молекулы глютамина электронным ударом

Using the mass-spectrometric analysis technique, formation of the ionic products of the single and dissociative ionization of the glutamine molecule (C5H10N2O3) at its interaction with the low (70 eV) and high (11.5 MeV) energy electrons has been studied. The experimental mass-spectra of the above molecule measured at different irradiation doses (5, 10 and 20 kGy) and the near-threshold areas of the ionic fragment yield functions have been analyzed, the absolute ion fragment appearance energies have been found. The three-electrode electron gun and the electron accelerator (microtron) were used as the electron beam sources. The analysis of the behaviour of the measured glutamine molecule mass-spectra irradiated at the above doses demonstrates, as compared to the irradiated molecule mass-spectrum, that the high-energy irradiation of the initial molecule results in the irreversible changes of its structure.

Ion appearance energies at electron-impact dissociative ionization of sulfur hexafluoride molecule and its fragments

Theoretical analysis of appearance energies for SF + (k = 0−n) ion fragments of SF6 molecule as well as F+ and F 2 + ions at electron-impact dissociative ionization of SF n (n = 1−6) molecules is presented. Theoretical methods of GAMESS software package were used to calculate the total energies of neutral and charged molecular and atomic fragments. The dissociative ionization process is concluded to occur via repulsive highly-excited electronic states of the SF6 molecule and its fragments, due to which the observed appearance energies exceed the theoretical values. The electron binding energies on the molecular orbitals in the SF6 molecule are compared with the ion fragment appearance energies.

Fragment appearance energies in dissociative ionization of a sulfur hexafluoride molecule by electron impact

Theoretical analysis of the fragment appearance energies corresponding to possible channels of formation of SFk+ fragments in dissociative ionization of the SF6 molecule by an electron impact is carried out. The total energies of neutral and ion molecular and atomic fragments are calculated using the theoretical methods of the GAMESS program complex. It is concluded that apart from dissociative ionization via autoionizing repulsive electronic states of the SF6 molecule, the excitation channels for SFk+ fragments and F2 molecules play a significant role, which leads to higher values of the observed fragment appearance energy as compared to theoretical values. The dependence of the energy corresponding to the formation of SFk+ c fragments on the number k of fluorine atoms is considered.

Fragmentation of methylphenylsilane and trimethylphenylsilane: A combined theoretical and experimental study

The electron impact ionization (EII) of methylphenylsilane (MPS), dimethylphenylsilane (DMPS) and trimethylphenylsilane (TMPS) was investigated using the mass spectrometry technique. The composition of fragmentation products from MPS and TMPS measured in this work is interpreted with respect to the ionization energy, appearance energies of fragments and bond dissociation energies of selected bonds. The results are compared to the previously published experimental data for DMPS. Comparison with the theoretical bond dissociation energies calculated using the density functional theory (DFT) calculations is presented. Using the combined experimental and theoretical approaches, we have focused our recent studies on the common features as well as basic differences of the fragmentation schemes of all three molecules. The elimination of the H2 molecule, specific for MPS but rarely observed in the other two compounds, was also of high interest in our studies. It can run in two mechanisms: (i) loss of two hydrogens one-by-one and (ii) elimination of H2 in one step. We can predict which mechanism is more probable according to the DFT calculated energy profile of reaction. The calculated predictions were in correlation with the composition of fragmentation products determined experimentally from mass spectra.

Realization of in-source collision-induced dissociation in single-photon ionization time-of-flight mass spectrometry and its application for differentiation of isobaric compounds.

Single-photon ionization mass spectrometry (SPI-MS) is a versatile and powerful analytical technique for online and real-time analysis of organic species; however, it is confronted with an intrinsic drawback of lacking structural information on the investigated molecules, let alone differentiation of isobaric compounds. In this work, we describe a first attempt to integrate in-source collision-induced dissociation (CID) to the SPI ion source in a SPI-MS instrument. The in-source CID was accomplished by elevating the pressure in the ion source to medium vacuum pressure (MVP) and raising the extraction voltage. With the aid of in-source CID, both the SPI-induced molecular ion and CID-generated fragment ion mass spectra can be obtained to endue each analyte with its unique spectrometric "fingerprint". The capability for differentiation of isobaric compounds is demonstrated by analyzing two groups of isobaric compounds with molecular weights of 72 and 106 Da, respectively, and quantitative analysis of p-xylene and ethylbenzene in gas mixture. As a result, isobaric compounds with different characteristic fragment ions or appearance energies can be successfully distinguished. The work presents a feasible method for practical applications of SPI-MS to differentiate isobaric compounds conveniently and rapidly without MS/MS technique or coupling additional separation technologies.

Dissociative VUV photoionization of butanediol isomers

Dissociative ionization of 1,3-butanediol and 2,3-butanediol isomers is investigated using threshold photoelectron-photoion coincidence (TPEPICO) measurements scanned across a photon energy range of approximately 4eV from threshold. Breakdown diagrams prepared from these internal energy-selected data display significant isomer-specific behavior. The observed fragment appearance energies are compared with calculated thresholds to allow identification, where possible, of the fragmentation pathways. In turn, these are rationalized by comparison with calculated parent ion structures. The roles of intermolecular H-bonding and weak one-electron CC bonding in the parent cations are discussed.

Electron induced reactions in gas phase MeCpPtMe3 and Pd(hfac)2

We present an experimental crossed electron-/ molecular beam study of dissociative electron attachment to the molecules trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe3) and Palladium(II) hexa-fluoroacetylacetonate (Pd(hfac)2) in the gas phase from close to 0 to 14 eV at room temperature. Further, we present appearance energies of important positive fragments determined by electron impact ionization.

Structures and energetics of SiGeH z0,+1, Ge 2H z0,+1, and Si 2H z0,+1: A systematic theoretical study

The structural and energetic information of Si x Ge y H z and ions is crucial in understanding the deposition processes in producing Si x Ge 1− x semiconductor materials. This work presents theoretical studies on the structures and energetics of the simplest SiGe-hydrides and cations, SiGeH z 0,+1, as well as Ge 2H z 0,+1 and Si 2H z 0,+1 for comparison. The structures are obtained at DFT-B3LYP and MP2 levels with 6-31+G(2df,p) basis set, and the electronic energies at Gaussian-4 (G4) level. The G4 energies are used to calculate the relative energies, bond dissociation energies, the adiabatic ionization energies (IE as) of neutral species, and the appearance energies (AEs) of cation fragments from SiGeH 6, Ge 2H 6, and Si 2H 6. The relative energies and IE as for Si 2H z and the total atomization energies of Si 2H z and Ge 2H z are compared and are in close agreement with previous theoretical and experimental studies, while the agreements on the AEs of Si 2H z + from Si 2H 6 are less pronounced. The calculations suggest that the kinetic shift effect and potential barriers should be taken into account when using AEs for thermodynamic information of Si 2H 2 +, Ge 2H 2 + and SiGeH 2 +.

Photoionization Study of L-Valine in the Gas Phase by Vacuum Ultraviolet Synchrotron Radiation

The photoionization and photodissociation of L-valine are studied by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry at the photon energy of 13 eV. The ionization energy of L-valine and the appearance energies of major fragments are measured by the photoionization efficiency spectrum in the photon energy range of 8-11 eV. Possible formation pathways of the major fragments, NH(2)CHC(OH)(2)(+) (m/z=75), NH(2)(CH(3))(2)(CH)(2)(+) (m/z=72) and NH(2)CHCO(+) (m/z=57), are discussed in detail with the theoretical calculations at the B3LYP/6-31++G (d, p) level. Hydrogen migration is considered as the key way for the formation of NH(2)CHC(OH)(2)(+) (m/z=75) and NH(2)CHCO(+) (m/z=57). Furthermore, other fragments, NH(2)CHCOOH(+) (m/z=74), (CH(3))(2)(CH)(2)(+) (m/z=56), C(4)H(7)(+) (m/z=55), NH(2)CHOH(+) (m/z=46), NH(2)CH(2)(+) (m/z=30) and m/z=18, species are also briefly described.

Vacuum-Ultraviolet Photoionization and Mass Spectrometric Characterization of Lignin Monomers Coniferyl and Sinapyl Alcohols

The fragmentation mechanisms of monolignols under various energetic processes are studied with jet-cooled thermal desorption molecular beam (TDMB) mass spectrometry (MS), 25 keV Bi(3)(+) secondary ion MS (SIMS), synchrotron vacuum-ultraviolet secondary neutral MS (VUV-SNMS) and theoretical methods. Experimental and calculated appearance energies of fragments observed in TDMB MS indicate that the coniferyl alcohol photoionization mass spectra contain the molecular parent and several dissociative photoionization products. Similar results obtained for sinapyl alcohol are also discussed briefly. Ionization energies of 7.60 eV ± 0.05 eV for coniferyl alcohol and <7.4 eV for both sinapyl and dihydrosinapyl alcohols are determined. The positive ion SIMS spectrum of coniferyl alcohol shares few characteristic peaks (m/z = 137 and 151) with the TDMB mass spectra, shows extensive fragmentation, and does not exhibit clear molecular parent signals. VUV-SNMS spectra, on the other hand, are dominated by the parent ion and main fragments also present in the TDMB spectra. Molecular fragmentation in VUV-SNMS spectra can be reduced by increasing the extraction delay time. Some features resembling the SIMS spectra are also observed in the desorbed neutral products. The monolignol VUV-SNMS peaks shared with the TDMB mass spectra suggest that dissociative photoionization of ion-sputtered neutral molecules predominate in the VUV-SNMS mass spectra, despite the extra internal energy imparted in the initial ion impact. The potential applications of these results to imaging MS of biomolecules are discussed.

Electron impact-induced ionization and dissociation of the freon-12 molecule

An experimental technique is described, and the relative cross sections of the single and dissociative electron-impact ionizations of the freon-12 molecule (CCl2F2) in the near-threshold energy range are obtained. The experiment is performed on a device that provides the mass separation and recording of ions with a monopole mass spectrometer. The mass spectrum of the freon-12 molecule is measured at various ionizing-electron energies, and the relative cross sections of dissociative ionization are measured for the most intense ion fragments, including isotope-containing fragments. The threshold dependences of these cross sections are used to determine the appearance potentials of the ion fragments. The isotope shift in the thresh-old appearance energies of ion fragments [C35ClF2]+ and [C37ClF2]+ is measured for the first time.