Appearance Energies Of Fragment Ions Research Articles

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.

Fragmentation processes of ionized 5-fluorouracil in the gas phase and within clusters

We have measured mass spectra for positive ions produced from neutral 5-fluorouracil by electron impact at energies from 0 to 100 eV. Fragment ion appearance energies of this (radio-)chemotherapy agent have been determined for the first time and we have identified several new fragment ions of low abundance. The main fragmentations are similar to uracil, involving HNCO loss and subsequent HCN loss, CO loss, or FCCO loss. The features adjacent to these prominent peaks in the mass spectra are attributed to tautomerization preceding the fragmentation and/or the loss of one or two additional hydrogen atoms. A few fragmentions are distinct for 5-fluorouracil compared to uracil, most notably the production of the reactive moiety CF+. Finally, multiphoton ionization mass spectra are compared for 5-fluorouracil from a laser thermal desorption source and from a supersonic expansion source. The detection of a new fragment ion at 114 u in the supersonic expansion experiments provides the first evidence for a clustering effect on the radiation response of 5-fluorouracil. By analogy with previous experiments and calculations on protonated uracil, this is assigned to NH3 loss from protonated 5-fluorouracil.

Single photon ionization of methyl isocyanide and the subsequent unimolecular decomposition of its cation: experiment and theory.

Methyl isocyanide, CH3NC, is a key compound in astrochemistry and astrobiology. A combined theoretical and experimental investigation of the single photon ionization of gas phase methyl isocyanide and its fragmentation pathways is presented. Vacuum ultraviolet (VUV) synchrotron radiation based experiments are used to measure the threshold photoelectron photoion coincidence (TPEPICO) spectra between 10.6 and 15.5 eV. This allowed us to experimentally determine the adiabatic ionization energy (AIE) and fragment ion appearance energies (AE) of gas-phase methyl isocyanide. Its AIE has been measured with a precision never achieved before. It is found to be AIEexp = 11.263 ± 0.005 eV. We observe a vibrational progression upon ionization corresponding to the population of vibrational levels of the ground state of the methyl isocyanide cation. In addition, four fragment ion appearance energies (AEs) were measured to be AE (m/z 40) = 12.80 ± 0.05 eV, AE (m/z 39) = 13.70 ± 0.05, AE (m/z 15) = 13.90 ± 0.05 eV, AE (m/z 14) 13.85 ± 0.05 eV, respectively. In order to interpret the experimental data, we performed state-of-the-art computations using the explicitly correlated coupled cluster approach. We also considered the zero-point vibrational energy (ZPVE), core-valence (CV) and scalar relativistic (SR) effects. The results of theoretical calculations of the AIE and AEs are in excellent agreement with the experimental findings allowing for assignment of the fragmentations to the loss of neutral H, H2, CN and HCN upon ionization of CH3NC. The computations show that in addition to the obvious bond breakings, some of the corresponding ionic fragments result from rearrangements - upon photon absorption - either before or after electron ejection.

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.

Ligand Stabilization and Charge Transfer in Dissociative Ionization of Fe(CO)5 Aggregates

We probe dissociative ionization of iron pentacarbonyl, Fe(CO)5, in its elementary stage of aggregation: in clusters. Two types of targets are studied: (i) small pure Fe(CO)5 aggregates and (ii) small Fe(CO)5 aggregates deposited on free argon nanoparticles with the mean size of a few hundred atoms. In both cases, the presence of environment leads to suppressed ligand decay upon dissociative ionization. The appearance energies of fragment ions point to very different ionization mechanisms for the two target systems: while, in pure clusters, direct dissociative ionization dominates, on argon nanosupport, the ionization proceeds via a hole transfer from the support. The observed ligand stabilization and ionization mechanism shed new light on previous surface science studies of precursors used in focused electron beam induced deposition (FEBID).

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.

Size selective spectroscopy of Se microclusters

The electronic structure and photofragmentation in outer and inner valence regions of Se(n) (n ≤ 8) clusters produced by direct vacuum evaporation have been studied with size-selective photoelectron-photoion coincidence technique by using vacuum-ultraviolet synchrotron radiation. The experimental ionization potentials of these clusters were extracted from the partial ion yield measurements. The calculations for the possible geometrical structures of the Se(n) microclusters have been executed. The ionization energies of the clusters have been calculated and compared with the experimental results. In addition, theoretical fragment ion appearance energies were estimated. The dissociation energies of Se(n) clusters were derived from the recurrent relation between the gas phase enthalpies of the formation of corresponding cationic clusters and experimental ionization energies.

Photoionization of epichlorohydrin enantiomers and clusters studied with circularly polarized vacuum ultraviolet radiation

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.

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.

Electron Capture by a Hydrated Gaseous Peptide: Effects of Water on Fragmentation and Molecular Survival

The effects of water on electron capture dissociation products, molecular survival, and recombination energy are investigated for diprotonated Lys-Tyr-Lys solvated by between zero and 25 water molecules. For peptide ions with between 12 and 25 water molecules attached, electron capture results in a narrow distribution of product ions corresponding to primarily the loss of 10-12 water molecules from the reduced precursor. From these data, the recombination energy (RE) is determined to be equal to the energy that is lost by evaporating on average 10.7 water molecules, or 4.3 eV. Because water stabilizes ions, this value is a lower limit to the RE of the unsolvated ion, but it indicates that the majority of the available RE is deposited into internal modes of the peptide ion. Plotting the fragment ion abundances for ions formed from precursors with fewer than 11 water molecules as a function of hydration extent results in an energy resolved breakdown curve from which the appearance energies of the b 2 (+), y 2 (+), z 2 (+*), c 2 (+), and (KYK + H) (+) fragment ions formed from this peptide ion can be obtained; these values are 78, 88, 42, 11, and 9 kcal/mol, respectively. The propensity for H atom loss and ammonia loss from the precursor changes dramatically with the extent of hydration, and this change in reactivity can be directly attributed to a "caging" effect by the water molecules. These are the first experimental measurements of the RE and appearance energies of fragment ions due to electron capture dissociation of a multiply charged peptide. This novel ion nanocalorimetry technique can be applied more generally to other exothermic reactions that are not readily accessible to investigation by more conventional thermochemical methods.

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.

Time-of-flight mass spectrometry study of the fragmentation of valence shell ionised nitrobenzene

A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in nitrobenzene as a result of valence shell photoionisation. Synchrotron radiation has been used to record spectra in the photon energy range 11–31 eV, and appearance energies have been determined for 18 fragment ions. These have enabled, previously unknown, heats of formation to be estimated for C 5H 2 + and C 6H 3 +. The fragment ion appearance energies have been compared to similar data for benzene and toluene in order to highlight the influence of the substituent on the fragmentation patterns. The time-of-flight spectra show that the peak associated with the NO + fragment changes shape as a function of excitation energy, and at high photon energy the peak consists of two components, one of which is narrow and the other broad. The latter component is due to fragments possessing substantial initial kinetic energy. In contrast, the peak associated with the NO 2 + fragment always appears broad. This behaviour is discussed in relation to the initial formation of a doubly charged ion and a subsequent coulomb repulsion. In a separate experiment the absolute photoabsorption cross section of nitrobenzene has been measured between the ionisation threshold and 35 eV using a double ion chamber. Some of the broad features have been attributed, tentatively, to valence shell excitations into π∗ orbitals.

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.

Measured appearance energies of fragment ions produced by electron impact on

We have used mass spectrometric techniques to determine the appearance energies for the formation of small, singly charged fragment ions produced by controlled electron impact on under single collision conditions. The measured appearance energies range from 79 eV to 91 eV and show, within the experimental error bar of eV, little variation with the cluster size. This result is distinctly different from the behaviour of the previously measured appearance energies for the large fragment ions to and indicates that the small fragment ions are the result of a different fragmentation pathway.

Time-resolved appearance energies for fragment ions from C 60

Time-resolved appearance energies (AEs) were determined for C 58 + and C 56 + by electron ionization of C 60. Two methods were applied: (1) electron-space-charge trapping in the ion source of a tandem mass spectrometer and (2) Fourier transform mass spectrometry in an ion cyclotron resonance spectrometer. The AE for C 58 + obtained in the limit of long ion trapping time at an evaporation temperature of 620 K is 44 ± 2 eV. This corresponds to a minimum intrinsic kinetic shift, IS = 30.4 eV due to radiative decay and agrees with recent results that cooling of C 60 (and C 60 +) is dominated by black-body radiative decay below an internal temperature of 2300 K.

Appearance Energies of C60 Fragment Ions Revisited

Following up on a serious discrepancy (factor of 2) in reported appearance energies for fragment ions produced by electron impact ionization of C60, we have measured here the appearance energies as a function of electron current using a crossed beam apparatus dedicated to ionization cross section measurements. Thus, it was possible to conclude that the low-energy peaks in the electron ionization curves of Baba et al. (which have been used to deduce the appearance energies) are very likely caused by consecutive ionization and excitation processes due to the high electron currents employed in these measurements. From this it follows that the true appearance energies are the high appearance energies reported earlier by Märk and co-workers (and Anderson and co-workers) which were determined at low ionizing electron currents to prevent secondary processes in the ion source. Moreover, we discuss here the crucial role of the kinetic shift in deriving activation energies and other thermochemical properties from measured appearance 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.