Distributions Of Fragment Ions Research Articles

Inferring ejection distances and a surface energy profile in keV particle bombardment experiments

The disappearance cross-sections and kinetic energy distributions of fragment ions sputtered from polystyrene thin films under 12 keV 69Ga + ion bombardment are measured using a time-of-flight spectrometer. Even though the disappearance cross-sections are often used as an indicator of radial ion beam damage, the derived radii range from 2 to 10 Å depending on the particular hydrocarbon fragment. Therefore, the disappearance cross-sections cannot be directly related to a single global quantity of damage per incident particle. Likewise, the widths of the measured kinetic energy depend on the particular hydrocarbon fragment. Both the disappearance cross-sections and the widths of the kinetic energy distribution superficially relate to fragment mass but the correlation is not perfect. We develop a hypothesis that the disappearance cross-section and the width of the kinetic energy distribution actually correlate with the ejection radius of the particular fragment. Thus, the kinetic energy distributions provide an estimate of the radial extent of the energy density in the ejection region. Our interpretations are supported by molecular dynamics simulation results. For comparison with previously reported data, our results indicate that the deposited energy profile is 4–5 times narrower than for 72.3 MeV, electronic sputtering of PVDF [Phys. Rev. Lett. 77 (1996) 667].

The angular distributions of fragment ions from labelledand unlabelled N2O in intense laser fields

The mass spectra and angular distributions of fragment ionsarising from a Coulomb explosion of highly charged parent nitrousoxide ions, obtained in the femtosecond regime (~1016 W cm-2), are investigated. The N ion angulardistributions from 14N2O show maxima when the laserpolarization is parallel and orthogonal to the time-of-flightaxis. Measurements with labelled molecules(15N-14N = 16O) indicate that the maxima arisefrom the peripheral and central N atoms in the molecularstructure. The anisotropic distributions may be explained byassuming increased ionization and fragmentation when themolecular axis is parallel to the laser field. The bond angleprior to explosion is determined to be ~140°,irrespective of the charge state of the precursor, andcalculations of the kinetic energies imparted to the fragment ionssuggest that dissociation occurs at the equilibriuminternuclear distance of the neutral molecule.

Instrumentation of Kinetic Energy-Resolved Surface-Induced Dissociation in Fourier Transform Mass Spectrometry

A Bruker 7 T Fourier transform (FT) mass spectrometer fitted with electron impact (EI) and electrospray ionization (ESI) external ion sources has been modified for surface-induced dissociation (SID) studies utilizing a self-assembled monolayer (SAM) of CF3(CF2)9C2H4SH alkanethiolate chemisorbed on gold. Operational aspects of ion sources, intermediate hexapole ion storage, time-of-flight function of the ion optics and cell trapping mechanisms are discussed for both primary and secondary ions. Different methods of ion trapping in the FT ion cyclotron resonance cell were analyzed theoretically and dynamic voltage trapping (DVT) was chosen as the most suitable. Direct measurements of primary (short time scale) and secondary (delayed) SID fragmentation and kinetic energy distributions of the SID fragments illustrate the capabilities of the instrument as a research tool for investigating SID kinetics. SID of small molecules and model peptides was investigated over a moderate collision energy range and the ion population was sampled at various decomposition delay times. Kinetic energy distributions of SID fragment ions were measured and shown to be quite low over the range of energies investigated. Most of the primary ion kinetic energy is dissipated into surface excitation and internal energy of ions recoiled from the surface.

TOF‐SIMS Study of the Hydrocarbon‐generating Potential of Mineral‐Bituminous Groundmass

Abstract: The TOF‐SIMS fragment peak ascription of organic and inorganic ions of mineral‐bituminous groundmass of Jurassic source rocks in the Turpan‐Hami and Junggar basins was studied by using the high‐resolution Time of Flight Secondary Ion Mass Spectrometer (TOF‐SIMS). The characteristics of spectrum distribution and constitution of fragment ions of the mineral‐bituminous groundmass are discussed; then the methods of evaluating its hydrocarbon‐generating potential are developed. In addition, the typical parameters, XAL, Yox and ZAR, for indicating the hydrocarbon‐generating potential of mineral‐bituminous and other organic matter in source rocks are put forward to reflect the aliphatic, oxygenous, and aromatic structures. It is confirmed by Rock‐Eval that these parameters are significant in evaluating hydrocarbon generation. Moreover, the detection of the nitrogenous and oxygenous fragment ion, CH5NO3+, in the mudstone formed in semi‐deep lakes and in the carbargilite formed in the arms of lakes reflects the fact that microbes take an active part in biologic degradation.

Photodissociation Mass Spectrometry of Trinuclear Carbonyl Clusters M3(CO)12 (M = Fe, Ru, Os)

Photodissociation of trinuclear carbonyl cluster compounds of Fe, Ru and Os was studied by recording the mass spectra produced from laser ablation of the cluster compounds. Under the experimental conditions, dissociation of the cluster compounds is very extensive, but the dissociation pathway of the osmium cluster is different from those of the iron and ruthenium clusters. The iron and ruthenium clusters not only lost their carbonyl ligands, but their cluster cores were also fragmented. As the osmium cluster dissociated, it ejected three pairs of oxygen atoms, in sequence, before losing the carbonyl ligands, but the trinuclear osmium core did not fragment. This specific dissociation scheme of the osmium cluster reveals its special structural stability. Not only does it have stronger metal-metal bonds, but also a relatively stable coordination bond formed between osmium and carbonyl ligands. In addition, different distributions of positive and negative fragment ions were observed in the experiment. This difference is interpreted as the result of different stabilities of their electronic structures.

High-energy collision-induced dissociation of small polycyclic aromatic hydrocarbons.

High-energy collision-induced dissociation (CID) experiments on polycyclic aromatic hydrocarbons (PAHs) having 2-6 rings, naphthalene, anthracene, phenanthrene, fluoranthene, pyrene and coronene, were performed, and the relative abundances of their fragment ions were investigated as a function of collision energy. The results revealed that the PAHs except naphthalene showed a bimodal-type distribution of positive fragmentation ions, which is closely similar to the fragment-ion distribution reported for the CID of three-dimensional fullerene, C(60)(+) and C(70)(+). The three-ring isomers of anthracene and phenanthrene and the four-ring isomers of fluoranthene and pyrene can be distinguishable in their spectra under an electron ionization energy of 70 eV, but the high-energy CID spectra of the three- and four-ring isomers were almost identical. The fragmentation corresponding to fragment ions in the low-mass region of the bimodal CID spectra could be interpreted by the simple statistical model that fragment ions are formed by random evaporation from the molecular ions after a considerable structural rearrangement, 'phase transition', occurring at some high-energy state.

An investigation of the angular distributions of fragment ions arising from the linear CS2 and CO2 molecules

The nonlinear interaction of the triatomic molecules CS2 and CO2 with the intense field of a linearly polarized laser beam of femtosecond (fs) pulse duration, was used to study the ionization and dissociation of the parent molecule. The fragment ion angular distributions arising from the Coulomb explosion of the parent ions were also measured. For CS2, the angular distributions of CS2+, CS22+, CS23+, CS+, CS2+, Sn+ (n6) and Cm+ (m4) ions are presented for a laser intensity of 1 × 1016 W cm-2 at a wavelength of 790 nm and pulse duration of 50 fs. The angular distributions of the parent molecular ions are all isotropic. The Sn+ fragments are peaked along the time-of-flight (TOF) axis, whereas the Cm+ fragments explode perpendicularly to this. Similar results for CO2 are also presented for comparison. The S ion distributions do not narrow as their ionic charge increases, and it is argued that the angular distributions for CS2 are due mainly to the angular dependence of the ionization probability. On the other hand, the distributions from the lighter CO2 molecule are thought to be at least partly due to alignment via dipole moments induced by the laser, as in this case the On+ angular distributions are seen to narrow as their charge increases. The conclusion of these results is that the laser pulse may be too short for the CS2 molecule to align in the pulse. Angular distributions are also presented for varying laser pulse durations, in the range of 50 fs to 300 ps. The dynamics of the ionization/dissociation mechanism are discussed in the context of the TOF mass spectra and angular distributions recorded for CS2.

Dynamic and geometric alignment ofCS2in intense laser fields of picosecond and femtosecond duration

CS$_2$ is identified as a molecule for which distinction can be made between dynamic and geometric alignment induced by intense laser fields. Measured anisotropic angular distributions of fragment ions arise from (i) dynamic alignment of the S-C-S axes along the laser polarization vector for 35-ps laser pulses and (ii) geometric alignment due to an angle-dependent ionization rate in the case of 100-fs pulses. Results of classical calculations of the alignment dynamics support our observations. By comparing mass spectra obtained with linearly- and circularly-polarized light it is not possible to distinguish between dynamic and geometric alignment. 33.80.Rv, 33.90.+h, 42.50.Vk

Bimodal Distribution in the Fragmentation Behavior of Small Antimony ClustersSbx+(x=3–12) Scattered from a Highly Oriented Pyrolitic Graphite Surface

The fragment ion-yield curves for the scattering of antimony cluster cations from highly oriented pyrolitic graphite show two clearly separated abundance maxima. On the low-energy side ( $\ensuremath{\le}150\ifmmode\pm\else\textpm\fi{}30\mathrm{eV}$) the interaction is characterized by the unimolecular fragmentation of the clusters and by neutralization as the competing process. The neutralization efficiency reaches a maximum at collision energies of $150\ifmmode\pm\else\textpm\fi{}30\mathrm{eV}$. Above this boundary a transition to two different processes is indicated by a change in the fragment ion distribution. These competing processes are here the shattering and the implantation of the clusters. The threshold for implantation depends significantly on cluster size.

Mass spectral fragmentation pathways of propranolol related β‐fluorinated amines studied by electrospray and electron impact ionization

Propranolol, its 1″-mono-, di-, and trifluorinated analogs, and other related compounds were analyzed under electrospray ionization ion trap collision-induced dissociation (ESI-CID) and electron impact (EI) conditions. Interesting trends were observed in the fragment ions formed in both cases. Under ESI conditions, the abundances of product ions easily explained by protonation on the amine nitrogen decreased relative to the abundances of those formed from the ether-protonated species as the number of fluorines increased from zero to three. Under EI conditions, the distribution of fragment ions was shifted away from those arising from a nitrogen-centered cation radical and toward those arising from an ether oxygen-centered cation radical. The changes observed in apparent molecular sites of protonation and of ion radical formation in the mass spectra are consistent with the electron-withdrawing effects of the sequentially added fluorines. These effects are correlated with changes in solution phase pK a's of the fluorinated amines. Copyright © 1999 John Wiley & Sons, Ltd.

Influence of the primary ion beam parameters (nature, energy, and angle) on the kinetic energy distribution of molecular fragments sputtered from poly(ethylene terephthalate) by kiloelectron volt ions

Thin films of poly(ethylene terephthalate) (PET) cast on silicon were bombarded by 2–22 keV In + ions and 12 keV Ga + ions and the positive secondary ions were mass and energy analysed by means of a time-of-flight secondary ion mass spectrometer. Since the secondary ion intensities are seen to be dependent on the primary beam parameters, the purpose of this work was to check if these parameters (mass, primary energy, and incidence angle) also influence the kinetic energy distributions of the sputtered molecular fragment ions. This is done in order to gain a better understanding of the molecular ion emission from polymer targets. In general, the shape of the kinetic energy distribution of PET fragment ions is found to be almost independent upon the primary ion nature and energy, as witnessed by the similar spectra observed with 12 keV Ga + and 7–22 keV In + primaries. This confirms previous studies which indicated that the main parameters governing secondary ion kinetic energies in this energy range were the secondary ion nature and the chemical structure of the organic and polymer targets. Nevertheless, the energy spectra of the fingerprint fragment ions obtained at 2 keV with an impact angle of 65° with respect to the surface normal are broader than those observed for higher energy and lower angle of incidence (∼40°). In comparison with the latter, they exhibit an additional contribution centered around 5–6 eV, i.e. in the high energy tail of the distributions. The integrated intensity of this contribution increases with the fragment size, up to 7% and 15% of the total ion intensity for C 7H 4O + and C 17H 12O 5 +, respectively. The results are discussed in terms of collision cascade propagation in the surface region of the target, by comparing the experimental observations to simulations conducted with the TRIM code. TRIM calculations suggest that the variation of the impact angle is the predominant factor influencing the atom displacements in the first 25 Å below the surface. In the {2 keV, 65°} configuration, the energy transferred to recoils in this region is similar, but the number of interacting atoms is more than two times greater than for the other tested combinations of energy and angle, suggesting an increased probability of collective processes for molecular ion emission.

Mass spectral fragmentation pathways of propranolol related beta-fluorinated amines studied by electrospray and electron impact ionization.

Propranolol, its 1"-mono-, di-, and trifluorinated analogs, and other related compounds were analyzed under electrospray ionization ion trap collision-induced dissociation (ESI-CID) and electron impact (EI) conditions. Interesting trends were observed in the fragment ions formed in both cases. Under ESI conditions, the abundances of product ions easily explained by protonation on the amine nitrogen decreased relative to the abundances of those formed from the ether-protonated species as the number of fluorines increased from zero to three. Under EI conditions, the distribution of fragment ions was shifted away from those arising from a nitrogen-centered cation radical and toward those arising from an ether oxygen-centered cation radical. The changes observed in apparent molecular sites of protonation and of ion radical formation in the mass spectra are consistent with the electron-withdrawing effects of the sequentially added fluorines. These effects are correlated with changes in solution phase pK( a)'s of the fluorinated amines.

Variable angle reflectron time-of-flight mass analyzer (VARTMAN) for studying synchrotron radiation photochemistry of polyatomic molecules

Abstract A new experimental apparatus, variable angle reflectron time-of-flight mass analyzer (VARTMAN), for studying synchrotron radiation photochemistry of the inner core excited organic molecules has been designed and constructed. A new type of reflectron time-of-flight (RTOF) mass analyzer, having a mesh-less reflector and tandem ion lens system, was designed to obtain moderate mass-resolution, even for fragment ions with kinetic energies of several eV while preserving kinetic energy information of the ions. In order to observe the angular distribution of fragment ions, the detection angle of the RTOF axis is variable with respect to the linear polarization axis of the synchrotron radiation. The new type of TOF can be operated in; (1) all solid angle (4π) collection mode for ions with kinetic energy up to 10 eV, (2) energy and angle resolve mode with a limited solid angle and (3) linear TOF mode. The mass resolution of the RTOF was measured to be 80 for fragment ions with kinetic energy up to 5 eV.

Dissociative ionization of benzene in intense laser fields of picosecond duration

Mass spectrometry experiments have been performed to probe the morphology of dissociative ionization of a ring molecule benzene when it is immersed in a laser field of 35-ps duration whose intensity is in the range ${10}^{13} {\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ The field-induced fragmentation pattern is grossly different from that obtained by electron impact. The intense laser field clearly gives rise to much more fragmentation. Covariance mapping has been applied to study field-induced formation of highly charged molecular ions. Results indicate that the overall fragmentation dynamics is complex and is likely to involve both fast ladder switching as well as multiple ionization of fragment molecules at some intermediate steps in the ladder. Angular distributions of fragment ions have been measured. Anisotropic distributions indicate that some fragments are produced from precursors that are strongly aligned along the direction of the laser polarization vector while at least one fragment is formed from a precursor whose angular distribution is markedly isotropic. The nature of the precursors remains to be discovered.

Coulomb explosion dynamics of N 2 in intense laser field by mass-resolved momentum imaging

The Coulomb explosion reactions of N 2, N 2 ( p+ q)+ → N p+ +N q+ , caused by the irradiation of intense laser light (0.73–6.9 PW/cm 2) were investigated by high-resolution time-of-flight (TOF) mass spectroscopy and by the mass-resolved two-dimensional momentum imaging (MRMI) method. Six explosion pathways ( p, q)=(1, 1), (1, 2), (1, 3), (2, 2), (2, 3) and (3, 3) in addition to (0, 1), (0, 2) dissociation pathways were identified, whose assignments were securely performed both from the momentum matching between the fragment pair and their correlation with the laser-field intensity dependence. The MRMI method, which visualizes the radial and angular momentum distributions of mass-selected fragment ions on a two-dimensional momentum plane, was used to correlate ion fragments produced after the Coulomb explosion. By examining theoretically synthesized MRMI maps, it was found that the momentum distributions of the fragment ions in a wide momentum range can be extracted by the MRMI technique. The relative yields of the explosion pathways at six different laser-field intensities were evaluated from the three-dimensional integration of the MRMI maps, and the intensity dependence of the formation of singly and multiply charged parent N 2 ( p+ q)+ ions prior to the fragmentation was derived.

Infrared multiphoton excitation, dissociation and ionization of C 60

We report the observation of mid-infrared multiphoton excitation, dissociation, and ionization of C 60 by shaped CO 2 laser pulses. The results are interpreted in the framework of the general statistical theory of infrared laser chemistry, adding some simple model assumptions. The observations on intensity- and fluence-dependent fragment ion distributions are consistent with a mechanism of vibrational preionization from energies exceeding the threshold for ionization by about a factor of 8.

An experimental and theoretical study of the dissociative ionization of and by an intense laser field

The dissociative ionization of polyatomic molecules (tetrachloromethane), (trichloromethane) and (dichloromethane) by intense, 532 nm, 35 ps, linearly polarized, laser fields is explored. The laser-induced fragmentation pattern is found to be substantially different from that obtained using 70 eV electrons. Attempts are made to rationalize these differences in terms of distortions of the molecular electron density distributions caused by the intense laser field. Such distorted charge-cloud distributions are computed in an approximate fashion by ab initio quantum-chemical techniques. Angular distributions of various fragment ions have also been measured. A pronounced anisotropy is exhibited in many (but not all) cases, with the ion signal being detected mainly along the direction of the laser's polarization axis.

The fragment ion distribution of in close collision with fast carbon ions

We have measured the mass distribution of fragment ions of produced by collisions with 15.6 MeV carbon ions in different charge states. Close collisions were selectively measured using the coincidence method with the change of the projectile charge state. For the electron capture and loss by projectile ions, which are the K-electron processes, the multifragmentation was observed evidently. In L-electron loss channels of the projectile, the peaks of the multiply ionized ions and the multiply ionized fullerene-like fragment ions are more intense or as intense as the small fragment ions. We suppose that we can roughly classify the intermediate states of through the measurement of fragmentation in coincidence with the change of charge state for various projectiles.

Energy transfer and surface-induced dissociation for SiMe3+ scattering off clean and adsorbate covered metals

We scatter 10–70 eV SiMe3+ from clean Au(111), a hexanethiolate self-assembled monolayer on Au(111) (C6), and a NiO(111) layer grown on top of Ni(111). We examine both the scattered ion fragmentation patterns and the kinetic energy distribution spectra (KEDS) as a function of the incident ion energy E. Surface infrared and KEDS data indicate that we have prepared a saturated monolayer of hexanethiolate (C6) on Au(111) where the C6 carbon backbone is predominantly upright on the surface. C6 monolayers with a mixture of prone and upright C6 can also be prepared, but only the upright C6 monolayers are used for ion scattering experiments. The fragment ion distributions and the KEDS are then used to determine the channeling of the incident SiMe3+ion energy into the scattered ion internal energy Eint, and the scattered ion kinetic energy Escat. Overall, we find the order of Eint/E for SiMe3+ to be Au(111)≫NiO(111)>C6. From the Escat values, we find that MiSe3+ scattering off C6 is highly inelastic while scattering off Au is much more elastic. We use this information to calculate the energy (Esurf) which is channeled into the surface as a result of the collision for C6 and Au(111). By comparing the KEDS for the parent and fragment ions, we determine the importance of unimolecular dissociation off the various surfaces. In the accompanying paper, we present a series of classical dynamics simulations to explain and supplement these experimental results.

Deconvoluting Isotopic Distributions to Evaluate Parent/Fragment Ion Relationships

Traditionally, the broad isotopic distributions typical of most large molecules are seen as an impediment to the use of tandem mass spectrometry (MS/MS) for the sequencing of large biomolecules because they complicate fragment-ion spectra making them difficult to interpret. Clearly, if very large molecules are to be analyzed by MS/MS there is a need for methods of analysis that take into account the isotopic distributions of fragment ions. The present paper shows how to rigorously deconvolute the isotopic distribution of a leaving group whose chemical formula is postulated from a parent ion, whose isotopic distribution may be available from an experimental mass spectrum, giving, in the process, the isotopic distribution of a fragment ion. It is not necessary to know the chemical formula of either the parent ion or the fragment ion. This calculation is potentially useful because the deconvoluted spectrum can then be compared with the experimentally acquired fragment-ion mass spectrum in order to aid in the assignment of the spectrum. © 1997 John Wiley & Sons, Ltd.