Energies Of Fragment Ions Research Articles

Experimental and theoretical study on the photoionization of styrene.

This study employed a vacuum ultraviolet synchrotron radiation source and reflectron time-of-flight mass spectrometry (TOF-MS) to investigate the photoionization and dissociation of styrene. By analyzing the photoionization mass spectrum and efficiency curve alongside G3B3 theoretical calculations, we determined the ionization energy of the molecular ion, appearance energy of fragment ions, and relevant dissociation pathways. The major ion peaks observed in the photoionization mass spectra of styrene correspond to C8 H8 + , C8 H7 + and C6 H6 + . The ionization energy of styrene is measured as 8.46 ± 0.03 eV, whereas the appearance energies of C8 H7 + and C6 H6 + are found to be 12.42 ± 0.03 and 12.22 ± 0.03 eV, respectively, in agreement with theoretical values. The main channel for the photodissociation of styrene molecular ions is the formation of benzene ions, whereas the dissociation channel that loses hydrogen atoms is the secondary channel. Based on the experimental results and empirical formulas, the required dissociation energies (Ed ) of C8 H7 + , C8 H6 + and C6 H6 + are calculated to be (3.96 ± 0.06), (4.00 ± 0.06) and (3.76 ± 0.06) eV, respectively. Combined with related thermochemical parameters, the standard enthalpies of formations of C8 H8 + , C8 H7 + , C8 H6 + and C6 H6 + are determined to be 964.2, 1346.3, 1350.2 and 1327.0kJ/mol, respectively. Based on the theoretical study, the kinetic factors controlling the styrene dissociation reaction process are determined by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This provides a reference for further research on the atmospheric photooxidation reaction mechanism of styrene in atmospheric and interstellar environments.

Long-lived molecular anions of brominated diphenyl ethers.

Resonance electron attachment in a series of brominated diphenyl ethers, namely 4-bromodiphenyl ether (BDPE), 4-bromophenyl ether (BPE), and decabromodiphenyl ether (DBDE), was investigated in the gas phase by means of dissociative electron attachment spectroscopy. In addition to channels of dissociation into stable fragments, long-lived molecular negative ions with an average lifetime relative to autodetachment of the order of 60 µs were found for the last two molecules. In the case of BDPE and BPE, the most intense dissociation channel is the bromine anion, and for DBDE-the [C6Br5O]- anion. The [C6Br5O]- anion sequentially decomposes with the elimination of the bromide anion on a microsecond time scale, which is confirmed by the registration of metastable ions with an apparent mass of 12.8 a.m.u. The electron affinity of the studied molecules and the appearance energy of fragment ions were estimated with CAM-B3LYP/6-311+G(d,p).

Investigation of vacuum ultraviolet photoionization of methylcyclohexane in energy region of 9−15.5 eV

Vacuum ultraviolet (VUV) photoionization and photodissociation of methylcyclohexane have been studied utilizing a reflectron time-of-flight mass spectrometer (RTOF-MS) with synchrotron radiation source. Photoionization efficiency curves (PIEs) of molecule ion C7H14+ and fragment ions C7H13+, C6H11+, C6H10+, C5H10+, C5H9+, C4H8+, C4H7+, and C3H5+ were observed. The ionization energy of methylcyclohexane was measured to be (9.80±0.03) eV, and appearance energies of fragment ions were determined from the PIEs. Optimized structures of transitional states, intermediates and product ions were characterized at the B3LYP/6-31G(d) level and the energies were calculated using G3B3 method. Formation channels of dominating fragment ions were proposed. Intramolecular hydrogen migrations and carbon ring-opening were the foremost processes in fragmentation pathways of methylcyclohexane.

Examination of a molecular se beam by mass spectrometry with electron ionization

The methodology and results of mass-spectrometric studies of producing positive ions as a result of the dissociative ionization of a molecular selenium beam by electron impact are discussed. The appearance energies of fragment ions were determined from the ionization efficiency curves. The dynamics of production of molecular selenium ions in the temperature range of 420–500 K was also examined. The energy dependences of efficiency of production of singly charged Sen+ ions for n = 1–4 and the doubly charged selenium ion in the interval from the threshold to 36 eV were studied for the first time. The observed specific features of effective ionization cross sections were analyzed.

An electron-impact ionization study of molecular selenium beams

We describe the method and presents results of a mass-spectrometric investigation of the yield of positive ions formed as a result of the dissociative ionization of a molecular selenium beam by electron impact. Appearance energies of fragment ions have been determined from the ionization efficiency curves. The dynamics of formation of the molecular ions of selenium in a temperature interval of 420–495 K has been studied. The energy dependences of the efficiency of formation of singly charged Sen+ (n = 1–3) and doubly charged Se++ ions are analyzed.

Ionization of glycerin molecule by electron impact

The methods and results of studying the yield of positive ions produced due to direct and dissociative electron impact ionization of the glycerin molecule are described. The experiment is carried out using two independent setups, namely, a setup with a monopole mass spectrometer employing the method of crossing electron and molecular beams and a setup with a hypocycloidal electron spectrometer with the gas-filled cell. The mass spectra of the glycerin molecule are studied in the range of mass numbers of 10–95 amu at various temperatures. The energy dependences of the effective cross sections of the glycerin molecular ions produced by a monoenergetic electron beam are obtained and analyzed; using these dependences, the appearance energies of fragment ions are determined. The dynamics of the glycerin molecule fragment ions formation is investigated in the temperature range of 300–340 K.

Ionization of the glycerin molecule by the electron impact

The paper describes experimental method for and presents the results of studying positive ion yields produced due to direct and dissociative electron-impact ionization of the glycerin molecule. The mass spectra of the glycerin molecule are studied in the range of mass numbers of 10–95 amu at different temperatures. The energy dependences of the ionization efficiency cross sections of the glycerin molecule ions produced by a monoenergetic electron beam are analyzed; using these dependences, the appearance energies of fragment ions are determined. The dynamics of fragment ion formation is investigated in the temperature range of 300–340 K.

Ionization and dissociative ionization of methane molecules

A technique for mass-spectrometric investigation of the yield of positive ions produced by direct and electron-impact dissociative ionization of methane molecules is described, and respective experimental data are presented. Doubly charged C2+, CH32+, and CH42+ ions, as well as singly charged D2+, CD3+, and CD4+ ions, are detected in the mass spectrum of a methane molecule at electron energy Ue = 90 eV for the first time. From ionization efficiency curves, the ionization energy of the parent molecule and the appearance energy of fragment ions are determined. The ionization energy of the CH4 molecule is found to be 12.62 ± 0.20 eV. Electron-molecular reactions that may take place when a low-energy electron beam interacts with a methane molecule are analyzed. The ionization process and the formation of methane molecule fragments are studied.

Fragmentation of sputtered vanadium and niobium cluster ions, kinetic release and dissociation energy

The generation and unimolecular fragmentation of Vn+ and Nbn+ clusters formed in sputtering vanadium and niobium surfaces by Xe+ ions has been studied. The method of measuring the kinetic energy of fragment ions (kinetic energy release distribution) has been used to determine the dissociation energy. Kinetic energy spectra have been measured in the field-free zone (corresponding to a time window of 10−5–10−4 sec after emission) of an ion microanalyzer with double focusing in reverse geometry. The results of spectra measurement were treated using the Rice-Ramsperge-Kassel theory of unimolecular reactions and the “evaporative ensemble”, which allowed us to calculate the dissociation energies of homonuclear Vn1 (n= 5–11) and Nbn1 (n = 3–8) clusters.

Metastable dissociative decay of fluorofullerene negative ions

In the course of the resonant electron capture mass-spectrometric study of the fluorinated fullerenes C 60F 18, C 60F 36 and C 60F 48, the processes of the metastable (slow, delayed) decay of negative ions by loss of fluorine atoms, effectively competing with electron autodetachment were detected. Theoretical analysis of the unimolecular decomposition, carried out on the basis of the statistical RRKM theory, showed that the excess of the appearance energy of fragment ions over the energy threshold results from a kinetic shift. It is shown that rapid processes of fragmentation of fluorofullerene molecular ions take place in addition to a metastable decay.

Dissociative Photoionization of Sulfur Chlorides and Oxochlorides: Thermochemistry and Bond Energies Based on Accurate Appearance Energies

The dissociative photoionization of four compounds, SCl(2), S(2)Cl(2), SOCl(2), and SO(2)Cl(2), were measured with threshold and imaging photoelectron photoion coincidence spectrometry (TPEPICO and iPEPICO). In all systems, the molecular ion loses a chlorine atom in a fast dissociation. The 0 K appearance energies of the first chlorine-loss fragment ions were determined to be 12.252 +/- 0.012 eV, 11.205 +/- 0.003 eV, 11.709 +/- 0.003 eV, and 12.505 +/- 0.003 eV, respectively. SCl(2) was measured on the laboratory-based TPEPICO instrument, in which the second Cl-loss dissociation could not be observed within the available photon energy. For S(2)Cl(2)(+) and SOCl(2)(+), the appearance energy of the fragment ion after two chlorine-loss dissociations were determined to be 13.32 +/- 0.02 eV and 14.88 +/- 0.02 eV, respectively. On the basis of the analysis of the breakdown curves, it was concluded that assuming three-dimensional translational degrees of freedom yields a more reliable statistical model of the product energy distributions. The literature heat of formation of the neutral precursor molecule thionyl chloride, SOCl(2) does not agree with our results based on the SO(+) cation and is revised by more than 10 kJ mol(-1) to -198.2 +/- 2.4 kJ mol(-1). A particularly broad Franck-Condon gap with vanishingly small threshold electron signal in the photon energy range for the second Cl-loss reaction in SO(2)Cl(2)(+) is discussed with regard to the mechanism of threshold ionization.

Direct determination of the ionization energy of histidine with VUV synchrotron radiation

Intact molecules of histidine are generated in the gas phase using impaction of nanoparticles on a heater. The direct determination of the ionization energy (IE) of histidine (155 m/ z) is performed using single photon ionization with tunable vacuum ultraviolet (VUV) synchrotron radiation and is reported to be 8.2(±0.1) eV. Ab initio calculations are performed at the B3LYP/6-311G (UB3LYP/6-311G) level of theory generating vertical and adiabatic ionization energies and possible fragmentation mechanisms. The theoretical adiabatic ionization energies derived for two lying conformers of histidine agree very well with the experimental result. Thermal dissociation at the heater gives rise to fragments, which are subsequently photoionized generating m/ z 82 and 111, while m/ z 110 is formed by dissociative photoionization of the histidine cation. Appearance energies of fragment ions at m/ z 82, 110 and 111 are 8.5(±0.1), 8.5(±0.1) and 8.4(±0.1) eV, respectively, at a heater temperature of 373 K. The H-atom dissociation energy of protonated histidine is derived to be 4.8(±0.1) eV using thermodynamic arguments.

Fragmentation of multiply ionized fullerene molecules

The dissociation of C60 fullerene molecules has been studied by means of an analysis of the kinetic energies of charged fragments formed upon the capture of several electrons from C60 by multiply charged Ar6+ ions. The kinetic energies of fragment ions of a multiply ionized C60 molecule are distributed in accordance with the Coulomb explosion mechanism of dissociation.

VUV peaks in absorption spectra and photoion yield curves of polycyclic aromatic hydrocarbons and related compounds

Photoion mass spectrometry (PIMS) was used to obtain the parent and fragment photoion yield curves in the 8–35 eV range of 25 aromatic hydrocarbons, mainly polycyclic aromatic hydrocarbons (PAHs) and related compounds. An intense peak, also observed in the absorption spectrum, occurs in the 16–18 eV region in the parent monocation yield curves and at similar or higher energies for fragment ions. Its origin, shape and width are discussed in terms of the electronic structure and photophysics of these carbonaceous species. The general behaviour of the ion yield curves as a function of photon energy is modelled for the prototype PAH benzene using data on the partial cross-sections for photoionization of this species. The appearance energies of fragment ions and the corresponding VUV peak profiles provide information about fragmentation mechanisms. In particular, the double peaks observed in the (M - 3) + ion yield curves for several species are interpreted as resulting from sequential mechanisms in the production of these ions. This constitutes a new diagnostic use of ion yield curves.

Relative dissociation energies of singly and doubly charged fullerene ions, C nz+ , for n = 52 to 70

Relative activation energies for dissociation of fullerene ions C n z+ into C n−2 z+ + C 2 have been determined for z = 1 and 2, covering the size range 52 ≤ n ≤ 70. These data are derived from measured appearance energies of fragment ions, obtained by electron impact ionization of C 70, with the help of the finite heat bath theory. The present results are compared with experimental and theoretical values for C 2 loss reported in the literature.

The ORELA positron-scattering program: Progress in the first energy-analysis experiments

Implantation and fragmentation by positron impact have been extensively studied by the positron group at Oak Ridge National Laboratory, but the group has never attempted an energy analysis of the ions produced. In this contribution, we report on the status of the first energy-analysis experiments planned with the ORELA (Oak Ridge Electron Linear Accelerator) positron beam. These first experiments will be similar to the original ORELA ionization–fragmentation experiments in that the target molecules will be in a stagnant gas, and the positrons will be confined in a Penning trap. We plan (i) to measure binding energies of several positronium compounds, (ii) to measure the energies of fragment ions produced by positrons with energies below fragmentation thresholds, and (iii) to detect secondary electrons from processes following the formation of ions, and to measure their energies.

The energetics of resonant dissociative electron attachment to molecules of five-membered heterocyclic compounds

The thermochemistry of resonant dissociative electron attachment processes for furan, thiophene, selenophene, and pyrrole molecules has been studied. The structures of the dissociation products originating from negative molecular ions at energies ranging from 2 to 6 eV have been established using the measured appearance energies of fragment ions and the known thermodynamic functions of radical and molecular dissociation products. Heats of formation and electron affinities for some radicals and molecules have been assessed by calculations and estimated experimentally. It has been concluded that the majority of the fragment ions are formedvia rearrangement processes in molecular or fragment ions.

Absolute partial and total electron impact ionization cross sections for C3H8 from threshold up to 950 eV

Electron impact ionization of propane (C3H8) was studied using the ion beam deflection technique and a double focussing mass spectrometer in conjunction with a recently developed correction procedure that accounts for discrimination due to the initial kinetic energy of fragment ions. The relative corrected partial ionization cross sections for the production of C3H 8 + , C3H 7 + , C3H 6 + , C3H 5 + , C3H 4 + , C3H 3 + , C3H 2 + , C3H+, C 3 + ; C2H 5 + , C2H 4 + , C2H 3 + , C2H 2 + , C2H+, C 2 + ; CH 3 + , CH 2 + , CH+, C+; C3H 5 2+ , C3H 4 2+ , C3H 3 2+ and C3H 2 2+ were determined from threshold up to 950 eV. Absolute partial ionization cross sections were obtained by charge weighted summing of all the observed partial ionization cross sections and by normalizing to a recent accurate determination of the total ionization cross section at 100 eV by Djuric et al. 1991. The absolute total ionization cross section curve obtained is in excellent agreement in shape and magnitude with the low energy data (<200 eV) of Djuric et al. 1991 and the high energy data points of Schram et al. 1966.

Electron impact mass spectra of polycyclic aromatic compounds with several types of pyridino‐ and benzobenzanthrone skeletons

AbstractElectron impact mass spectra were measured for five isomers of pyridinobenzanthrones and three isomers of benzobenzanthrones. The fragmentation pattern and intensity of M2+, [M – H]+, [M – CO]i+, [M – CO – H(or 2H)]i+ and [M – CO – HCN]i+ (i = 1, 2) ions indicated remarkable differences and very interesting features according to the isomers with or without nitrogen and condensation positions of a pyridino or benzo ring to the benzanthrone skeleton. Further, the competition of decompositions through [M – H]+, [M – CO]+˙ or [M – HCN]+˙ ions was confirmed by the observation of metastable ions and the appearance energies of fragment ions. Interesting observations from these results were expulsion of an H atom in close proximity to the area around an OC group, a weak bonding interaction between sp2 CH and an OC group, inducing specific hydrogen rearrangement, and characteristic charge localization on heteroatoms.

Electrostatic repulsion between localized charges on hetero-atoms in a doubly charged ion and mechanism of ionization and fragmentation. Electron impact mass spectra of benzanthrone, 1-azabenzanthrone, 8-azabenzanthrone and their 3-bromo substituents

Electron impact mass spectra and ionization efficiency curves of fragment ions were taken for six benzanthrone homologues, i.e., benzanthrone, 1-azabenzanthrone, 8-azabenzanthrone, 3-bromobenzanthrone, 3-bromo-1-azabenzanthrone and 3-bromo-8-azabenzanthrone. The main fragmentation reactions were the elimination of CO and subsequently of HCN from the molecular ion M i+ ( i = 1,2) based on the observation of metastable ions and the value of appearance energies of fragment ions. The intensity of a doubly charged ion [MCOHCN (or C 2H 2 in benzanthrone)] 2+ compared with an ion (MCO) 2+ is strong for 8-azabenzanthrones, intermediate for 1-azabenzanthrones and weak for benzanthrones. This fact suggests that positive charges are localized on oxygen, nitrogen or other hetero-atoms immediately after the formation of a doubly charged molecular ion, that they exclude each other because of strong repulsion at short distances, and that this repulsive force promotes the above fragmentation. Approximate values of the first and second ionization potentials were interpreted from the easily achieved emission of non-bonding electrons from heteroatoms in the outer surface of molecules, which is correlated with the results of molecular orbital calculations.