Secondary Ion Spectra Research Articles

ToF‐SIMS quantification of polystyrene spectra based on principal component analysis (PCA)†

Monodisperses polystyrenes (PS) with molecular weight (Mn) ranging from 850 to 20000 g mol-1 and with different butyl end-groups have been analysed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Only the fingerprint part of the spectra (m/z<200) has been considered. This part comes from the fragmentation of the macromolecules and this is the only contribution in the secondary ion spectra of bulk polymers. In a former study, it has been shown that Mn has a significant influence on the PS secondary molecular ion intensities. It was found that not only the end-group characteristic ion intensities are modified for Mn<50000 g mol-1, but also those related to main chain fragments. In this study, the possibility to use this effect to quantify the molecular weight of the macrochains at the surface is investigated. Because the quantification method should preferably include all the information contained in the spectra (over 150 peaks), statistical methods are needed. Principal component analysis is applied to these ToF-SIMS data. Principal component analysis builds the best linear combination of the variables, namely the principal components that describe the greatest amount of variation of the sample set. It is found that only one principal component is required to account for the molecular weight variations and a second one allows the samples to be discriminated depending on their butyl end-groups. Whatever the PS end-groups (linear, secondary or tertiary butyl), the correlation found between the first principal component and the sample molecular weight leads to the definition of a universal curve that allows the molecular weight determination for an unknown monodisperse PS sample. © 1997 John Wiley & Sons, Ltd.

ToF-SIMS quantification of polystyrene spectra based on principal component analysis (PCA)†

Monodisperses polystyrenes (PS) with molecular weight (M-n) ranging from 850 to 20 000 g mol(-1) and with different butyl end-groups have been analysed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Only the fingerprint part of the spectra (m/z < 200) has been considered. This part comes from the fragmentation of the macromolecules and this is the only contribution in the secondary ion spectra of bulk polymers. In a former study, it has been shown that M-n has a significant influence on the PS secondary molecular ion intensities, It was found that not only the end-group characteristic ion intensities are modified for M-n < 50 000 g mol(-1), but also those related to main chain fragments. In this study, the possibility to use this effect to quantify the molecular weight of the macrochains at the surface is investigated. Because the quantification method should preferably include all the information contained in the spectra (over 150 peaks), statistical methods are needed, Principal component analysis is applied to these ToF-SIMS data. Principal component analysis builds the best linear combination of the variables, namely the principal components that describe the greatest amount of variation of the sample set, It is found that only one principal component is required to account for the molecular weight variations and a second one allows the samples to be discriminated depending on their butyl end-groups. Whatever the PS end-groups (linear, secondary or tertiary butyl), the correlation found between the first principal component and the sample molecular weight leads to the definition of a universal curve that allows the molecular weight determination for an unknown monodisperse PS sample. (C) 1997 by John Wiley & Sons, Ltd.

Effect of hydrogen on structure of steel welds

The results described in this report permit us to introduce certain corrective amendments to the conventional notion about hydrogen as an undesired impurity in welds. It was found that trace additions of boron to welds result in an acicular ferrite microstructure and simultaneously the residual hydrogen concentration increases. Secondaryion microscopic and mass-spectrometric experiments were carried out to study this effect. It was observed that in welds, the boron distribution is uniform and in spectra of secondary ions, the presence of particles BH + was found. These facts have allowed to conclude that the hydrogen and boron atoms form stable complexes. As a result the diffusion mobility not only of hydrogen, but also boron should be reduced. At the formation of the final weld metal structure it can result in accumulation of energy in the austenite matrix and then to formation with shift mechanism, the acicular ferrite microstructure that imparts excellent properties to the weldment.

Effect of the local electronic temperature on secondary-ion spectra

The problem of secondary-ion formation is investigated theoretically and experimentally. The effect of local electronic temperature on the collisional cascade region in the secondary-ion energy spectrum is examined. It is shown that the shift of the maximum of the energy spectrum with a change of the emission angle is explained well under the assumption of local heating of the electronic subsystem followed by cooling down. {copyright} {ital 1996 The American Physical Society.}

The unimolecular decays of Al n± and Cu n± sputtered clusters: a comparison between competitive decay modes

The method of kinetic energy release distribution measurement has been applied to the investigation of competitive channels of the unimolecular fragmentation of sputtered Cu n and Al n cluster ions. It has been shown how these methods could be used to understand the regularities observed in the mass spectra of secondary cluster ions.

A dual ion source to produce Cs + or I − ions for secondary ion mass spectrometry

Negative primary ions have an advantage for SIMS measurements when positive secondary ions are being observed, because they gain energy from the secondary ion accelerating voltage. Having both Cs + and I − ions available from the same ion source enables maximum bombarding energy to be obtained for both positive and negative secondary ion spectra without interrupting the vacuum.

Line shifts in mass spectra of secondary ions ejected from solids by MeV-ion impact

Emission of hydrogen ions has been used to investigate the charging of the nuclear track being produced by fission fragments in organic solids. The question of interest is, if in insulating material the lifetime of ionisation along the nuclear track is sufficiently long to allow atomic motion driven by Coulomb repulsion of neighbouring ionized atoms. It is shown in this article that the hydrogen ions experience the potential produced by the ionized track core, but weaker than Auger electrons (see Schiwietz et al., Phys. Rev. Lett. 69 (1992) 628). In order to determine the mean initial energy of emission, we measured the time-of-flight distributions of secondary ions by means of a linear TOF instrument being equipped with 2 acceleration grids and converted them into axial energy distributions. Monte Carlo simulations were used to simulate the time distributions and to estimate the mean charge along the nuclear track causing the observed line shifts.

Mass spectrometry of secondary negative ions emitted from low-energy sputtered YBCO ceramics

Mass spectra of negative secondary ions emitted from the surface of a stoichiometrical superconducting YBa2Cu3O7-x ceramic bombarded by 250 eV positive argon and oxygen ions were measured. The results show that the reduction of the primary ion energy reduces the yield of heavy secondary ions. It was observed that the supply of oxygen to the sample bombarded by Ar+ ions enhances the yield of secondary negative ions, although this yield is further enhanced by switching to positive oxygen primary ions. Moreover, the bombardment of a YBCO ceramic sample by oxygen ions is characterized by a particular enhancement of the yield of barium and barium-containing molecular ions.

Secondary ions produced from gaseous and frozen H2O under energetic (MeV/amu) Ar ion impact

Mass/charge spectra of secondary ions produced under energetic (∼MeV/amu) Ar ion impact on gaseous and frozen H2O targets are measured using a magnet-type mass spectrometer. The ion spectra are found to be quite different between these target phases. Multiply charged Oi+ ions with charge states i=2–6, as well as fragmented H+, O+, and OH+ ions, are clearly observed in the gas target, whereas the production of multiply charged ions is strongly suppressed in the frozen target. The most intense ion species is the parent H2O+ ion in the gas target; the H3O+ ion in the frozen target. The most outstanding feature in the frozen target is that the production of cluster ions of the type (H2O)nH+ (n=1–31), and their intensities, decreasing as the cluster size n increases, show anomalies between n=4 and n=5 and also around n=20, 21, and 22. Negatively charged cluster ions with formula (H2O)nO− and (H2O)nOH− are also efficiently produced in the frozen target.

Damaging of C60 films by MeV heavy ions

Thin films of C60 (99.99% purity) have been irradiated with 55 MeV 127I10+ ions in a fluence range from 2.5 × 109 to 3 × 1012 ions/cm2. Two methods have been employed to assess the modifications induced by the MeV ion irradiation: in situ plasma desorption mass spectrometry analysis and off-line micro-Raman spectroscopy. The yields of secondary low mass carbon cluster ions and intact C60 ions have been determined at different fluences. Damage cross-sections for the C60 molecules have been extracted from the ion yield curves as a function of MeV ion fluence. The secondary ion spectra of the C60 targets also show peaks corresponding to larger fullerenes with masses at least up to 2500 u. The evolution of the ion yields of these higher mass carbon clusters gives an evidence that they are not originally present in the target. They may be formed by coalescence reactions of C60 molecules as a result of an individual MeV ion impact.

In situ ToF‐SIMS/XPS investigation of nitrogen plasma‐modified polystyrene surfaces

AbstractPlasma modification of polymer surfaces is a well‐established technique for many technological applications. The control of the surface composition after plasma treatment requires in situ characterization by adequate surface analytical techniques. As an example, we investigated polystyrene modified by a microwave (2.45 GHz) nitrogen plasma by using an on‐line combination of time‐of‐flight secondary ion mass spectrometry and monochromatized photoelectron spectroscopy.Plasma treatment without subsequent air contact leads to the incorporation of nitrogen, the formation of amine, imine and nitrile bonds as evidenced by the C 1s and N 1s photoelectron lines, and the appearance of predominant CxNy clusters in the negative secondary ion spectra. These changes seem to be stable against organic solvents. A particular feature is the formation of a graphite‐like C 1s state after prolonged plasma treatment. The polystyrene surface after plasma treatment is very reactive, particularly the graphite‐like carbon state. Exposure to the atmosphere leads to the disappearance of this state, to considerable uptake of oxygen and to the formation of carbon‐nitrogen‐oxygen bonds, as evidenced by the CNO− secondary ion cluster and the amide binding state in the C 1s line.

Surface structure investigations using plasma desorption mass spectrometry and coincidence counting

Coincidence counting techniques were used to study the plasma desorption of ions from a polycrystalline sample of αZirconium bis(monohydrogen orthophosphate)monohydrate (αZrP). Using coincidence counting in conjunction with time-of-flight (TOF) mass analysis, it was possible to obtain secondary ion spectra with a signal to background ratio a factor of ten better conventional TOF alone. It also was possible to produce a spectrum of SI that were desorbed in coincidence with PO − 3 (characteristic of the αZrP). Cluster ions observed in this spectrum are related to the αZrP crystal structure. This supports a direct emission process for cluster ion production and demonstrates the utility of particle induced desorption in the determination of surface and crystal structure.

Comparison of the secondary-ion and electron-impact mass spectra of alkaloids

An advantage of using secondary-ion spectra for determining the molecular masses of quaternary bases and bases the molecular ions of which are unstable to electron impact is shown. The LSIMS spectra of four groups of diterpene alkaloids include the 100% peaks of the (M + H)+ ions and the peaks of the main fragments present in the EI spectra. The B/E = const. spectra of the key ions obtained by the different methods are compared. The closeness of the values of A of analogous transitions calculated from the B/E and MD spectra is confirmed.

Ionization probabilities of particles sputtered from high- Tc superconductors

The yields and the energy spectra of secondary neutrals, positive and negative ions sputtered from YBaCuO and BiSrCaCuO superconductors were measured. A strong correlation between the experimental surface binding energies U b and the atom site in the unit cell was observed. The velocity dependences of ionization probabilities P + ad P − of sputtered particles provide evidence for the electron exchange mechanism in secondary ion formation. An exponential dependence of absolute values of P + on the ionization potential is observed for atoms of main components of HTSCs.

Analysis for rare earth elements in silicates by ion microprobe using doubly-charged ions

A technique for measurement of rare earth element (REE) concentrations in silicates using a Camecaims-4f ion microprobe and doubly-charged, odd-mass isotopes has been developed. The secondary ion spectra of the doubly-charged odd-mass REE are virtually free of interferences, allowing measurements to be carried out at low energies and without the need for spectral stripping. Calibration lines have been established for La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, and Yb using a suite of clinopyroxene standards. This technique offers a relatively fast, simple approach for the in-situ analysis of REE on spots of <20 [mu]m and detection limits of <15 ppb for most elements. 17 refs., 2 figs., 5 tabs.

Yield enhancements of secondary negative molecular ions consisting of matrix elements under cesium primary ion bombardment of GaAs

A high secondary negative molecular ion yield that is high relative to the secondary negative monomer ion yield has been observed. The secondary negative ion spectrum of a GaAs chemical compound semiconductor sputtered by a cesium primary ion under the secondary ion mass spectroscopy dynamic mode was examined and it was found that the secondary negative GaAs dimer ion was as intense as the secondary negative arsenic monomer. The GaAs dimer ion was three times higher in its intensity than another dimer of the As2 ion. Moreover, the secondary matrix trimer ions containing at least one Ga atom have one order higher intensities than the arsenic trimer ions. At a stable state of the sputtering process, yield variations of secondary molecular ions made up of the matrix elements of GaAs have been examined as a function of the primary ion current density. The high yield of secondary molecular ion species consisting of the matrix elements of GaAs and containing at least one Ga atom is considered to be indirectly related to the arsenic depletion on the GaAs surface sputtered by cesium primary ions.

Secondary-ion and electron production from surfaces bombarded by large polyatomic ions.

Heavy molecular ions with energies in the range 10-20 keV and masses from 276 u to 132,000 u, produced by matrix-assisted laser desorption, were used as primary projectiles to produce secondary-ion spectra from a variety of surfaces in a tandem time-of-flight mass spectrometer. In the negative mode the ratio of electron emission to secondary-ion emission was found to decrease rapidly with increasing projectile mass. Ion emission was found to dominate for primary ions larger than approximately 10,000 u. Positive or negative molecular ions and cations were observed from several organic targets of masses up to 1140 u (gramicidin S) for incident projectiles up to mass 132,000 u, i.e., for projectile speeds down to approximately 7000 m/s. Other ions characteristic of the target were also observed for these projectiles. Thus, large polyatomic ions can cause secondary-ion desorption even at very low velocity. The background ions of both polarities are similar to those found in keV particle bombardment by monatomic projectiles. The same ions are observed for all the projectiles; most can be identified with hydrocarbon background. The relative intensities of the background positive ions are largely independent of projectile, and for both polarities the ratio of the ions characterizing the target to those forming the background is approximately constant for all the projectiles. These results strongly suggest that the background ions come from the usual layer of organic impurities attached to the target surface. No direct evidence for surface-induced dissociation was observed in this mass and energy range.

Secondary-emission mass spectrometry of hederagenin glycosides and medicagenic acid glycosides

The secondary emission (SE) spectra of six glycosides of medicagenic acid (MA) and of hederagenin have been studied. The (M + H)+ ions were observed in the secondary ion spectra (LSIMS) of mono- and biosides of MA and also of a triand a tetraoside of hederagenin in a glycerol matrix. The (M + Na)+ ions were recorded in the spectra of tri- and tetraosides of MA. In the majority of cases processes involving the successive splitting out of the carbohydrate units were revealed. Positive- and negative-ion spectra have been obtained on the ionization by bombardment with fast atoms — FAB “+” and FAB “−” — of a trioside and a tetraoside of MA. The FAB “−” spectra are the most characteristic: They show intense peaks of the (M - H)− ions and of the products of the sequential elimination of the sugar residues.

Sequencing an “unknown” peptide by time-of-flight secondary ion mass spectrometry

The procedure in sequencing the peptide dynorphin 1–17 (mass 2146.20 u) with a reflecting time-of-flight mass spectrometer is reported in some detail. Samples (1.5 μg) of the peptide were distributed as unknowns to participating laboratories in preparation for a workshop at the 1990 meeting of the American Society for Mass Spectrometry. At the time of the workshop about half the sequence had been determined correctly from the secondary ion spectrum with the aid of enzymatic digestions carried out on the target. Subsequent measurements have defined the remainder, apart from a couple of uncertainties.

The application of static secondary ion mass spectrometry to the study of epoxy adhesive constituents

The application of fast atom bombardment secondary ion mass spectrometry (FAB-SIMS) with quadrupole mass analysis to the study of some of the common constituents present in epoxy adhesive formulations is described and the potential of the technique evaluated. The compounds investigated were Epikote 828 (mainly diglycidyl ether of bisphenol-A) and the cross-linking agent dicyandiamide. The characteristic secondary ion spectra recorded for both materials exhibited a number of identifiable and uniquely diagnostic signals, which should be of use in the study of surface and interface adhesive chemistry.