Characteristic Fragmentation of Polysiloxane Monolayer Films by Bombardment with Monatomic and Polyatomic Primary Ions in TOF-SIMS

This study reports the characteristic fragmentation patterns from two polysiloxane polymers that form ordered overlayer on silver substrates. Results are compared for the bombardment of various monatomic and polyatomic projectiles of Cs⁺, C₆₀⁺ (10 keV), Bi₁⁺, and Bi₃⁺ (25 keV) in the high mass range time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra. Results are reported from sub-monolayer (solution cast) coverages of poly(dimethylsiloxane)s with the number average molecular weights (M_n) of 2200 and 6140 Da, respectively, and Langmuir-Blodgett monolayers of poly(methylphenylsiloxane) with molecular weights (MW) from 600 and 1000 Da. For each film, Bi projectiles resulted in the emission of positive silver cluster ions from the substrate under the polymer overlayer and peaks corresponding to silver cluster ions with larger mass were observed by impact of polyatomic 25 keV Bi₃⁺ projectiles. In addition, depending on the change of energy of Bi₃⁺, a different pattern of fragments was observed. With Cs⁺ and C₆₀⁺ impact, however, the emission of silver cluster ions was not detected. In the case of C₆₀⁺ impact for PDMS-6140, peaks corresponding to silver-cationized intact oligomers were not observed. In this paper, these results are explained by the possible bombardment mechanism for each projectile, based on its mass, energy, and split trajectories of the component atoms under the polyatomic impact.     

Relationships between cluster secondary ion mass intensities generated by different cluster primary ions

Measurements are described to evaluate the constitution of secondary ion mass spectra for both monatomic and cluster primary ions. Previous work shows that spectra for different primary ions may be accurately described as the product of three material-dependent component spectra, two being raised to increasing powers as the cluster size increases. That work was for an organic material and, here, this is extended to (SiO₂) _t OH⁻ clusters from silicon oxide sputtered by 25 keV Bi _n ⁺ cluster primary ions for n = 1, 3, and 5 and 1 ≤ t ≤ 15. These results are described to a standard deviation of 2.4% over 6 decades of intensity by the product of a constant with a spectrum, H _SiOH/*, and a power law spectrum in t. This evaluation is extended, using published data for Si _t ⁺ sputtered from Si by 9 and 18 keV Au⁻ and Au₃⁻, with confirmation that the spectra are closely described by the product of a constant with a spectrum, H _Si^*, and a simple spectrum that is an exponential dependence on t, both being raised to appropriate powers. This is confirmed with further published data for 6, 9, 12, and 18 keV Al⁻ and Al₂⁻ primary cluster ions. In all cases, the major effect of intensity is then related to the deposited energy of the primary ion at the surface. The constitution of SIMS spectra, for monatomic and cluster primary ion sources, is shown, in all cases, to be consistent with the product of a constant with two component spectra raised to given powers.     

Time of flight secondary ion mass spectrometric determination of molecular weight distributions of low polydispersity poly(dimethyl siloxane) with polyatomic primary ions

This work reports a comparison of oligomer and fragment ion intensities resulting from primary ion bombardment with several primary ion sources (Bi_n⁺, C₆₀⁺, and Cs⁺) at various energies in secondary ion mass spectrometry (SIMS). Although the use of polyatomic primary ions are of great interest due to increased secondary ion efficiency and yield, we demonstrate that monatomic primary ions result in increased oligomer ion yield for polymers prepared as submonolayer films on silver substrates. The enhancement of oligomer secondary ion yield with monatomic ions is evidence that monatomic primary ions have a shallower sampling depth than polyatomic ions, resulting from a collision cascade that is less energetic at the sample surface. The results are also consistent with a lower degree of fragmentation of the resultant secondary ions, which is observed when evaluating the fragmentation data and the spectral data.     

Formation of gold cluster cations by direct laser vaporization

Gold cluster cations, Au⁺_n, with n= 1 to 6 have been produced by direct laser vaporization of gold metal in a Fourier transform ion cyclotron resonance mass spectrometer. Wavelength studies at λ = 532, 355, and 266 nm show similar results. However, the intensities and intensity ratios of the cluster ions strongly depended on laser power at the two longer wavelengths. Ionization energies of Au⁺ , Au⁺₂, and Au⁺₃ have also been estimated.     

Use of C60 cluster projectiles for sputter depth profiling of polycrystalline metals

We have investigated the merits of fullerene cluster ions as projectiles in time‐of‐flight secondary neutral mass spectrometry (ToF‐SNMS) sputter depth profiling of an Ni:Cr multilayer sample similar to the corresponding NIST depth profiling standard. It is shown that sputter erosion under bombardment with C₆₀⁺ ions of kinetic energies between 10 and 20 keV provides good depth resolution corresponding to interface widths of several nanometres. This depth resolution is maintained during the complete removal of the multilayer stack with a total thickness of 500 nm. This finding is in contrast to the case where atomic Ga⁺ projectile ions of comparable kinetic energy are used, demonstrating the unique features of cluster projectiles in sputter depth profiling. Copyright © 2004 John Wiley & Sons, Ltd.     

Molecular orbital analysis of the bonding in high nuclearity gold cluster compounds

Extended Hückel molecular orbital calculations on high nuclearity gold clusters of the general type [Au(AuPH₃)_n]^x+ have demonstrated that they can be classified into two broad topological classes according to the three-dimensional disposition of the peripheral gold atoms. If they lie approximately on a sphere they are characterised by a total of 12n + 18 valence electrons, but if they adopt a toroidal or eliptical arrangement the total electron count is 12n + 16. The computed energy differences between alternative polyhedral geometries is generally small and accounts for the stereochemical non-rigidity of the gold cluster compounds in solution. Detailed aspects of the structures of the high nuclearity gold cluster compounds have been interpreted in terms of molecular orbital calculations on clusters derived from the centred chair [Au₇(PH₃)₆]⁺ by edge- and face-capping with Au(PH₃)⁺ fragments.     

Characterization of synthetic N-acetylcysteine conjugates by positive- and negative-ion 252Cf plasma desorption mass spectrometry

N-Acetylcysteine and nine N-acetylcysteine conjugates of synthetic origin were characterized by positive- and negative-ion plasma desorption mass Spectrometry. For sample preparation the electrospray technique and the nitrocellulose spin deposition technique were applied. The fragmentation of these compounds, which are best seen as S-substituted desaminoglycylcysteine dipeptides, shows a similar behaviour to that of linear peptides. In the positive-ion mass spectra intense protonated molecular ion peaks are observed. In addition, several sequence-specific fragment ions (A⁺, B⁺, [Y + 2H]⁺, Z⁺), immonium ions (I⁺) and a diagnostic fragment ion for mercap-turic acids (R_M⁺) are detected. The negative-ion mass spectra exhibit deprotonated molecular ions and in contrast only one fragment ion corresponding to side-chain specific cleavage ([R_XS]^?) representing the xenobiotic moiety. In the case of a low alkali metal concentration on the target, cluster molecular ions of the [nM + H]⁺ or [nM - H]^? ion type (n = 1-3) are observed. The analysis of an equimolar mixture of eight N-acetylcysteine conjugates shows different quasi-molecular ion yields for the positive- and negative-ion spectra.     

Surface-induced dissociation and chemical reactions of C2D 4 + on stainless steel, carbon (HOPG), and two different diamond surfaces

Surface-induced interactions of the projectile ion C₂D₄⁺ with room-temperature (hydrocarbon covered) stainless steel, carbon highly oriented pyrolytic graphite (HOPG), and two different types of diamond surfaces (O-terminated and H-terminated) were investigated over the range of incident energies from a few eV up to 50 eV. The relative abundance of the product ions in dependence on the incident energy of the projectile ion [collision-energy resolved mass spectra, (CERMS) curves] was determined. The product ion mass spectra contained ions resulting from direct dissociation of the projectile ions, from chemical reactions with the hydrocarbons on the surface, and (to a small extent) from sputtering of the surface material. Sputtering of the surface layer by low-energy Ar⁺ ions (5–400 eV) indicated the presence of hydrocarbons on all studied surfaces. The CERMS curves of the product ions were analyzed to obtain both CERMS curves for the products of direct surface-induced dissociation of the projectile ion and CERMS curves of products of surface reactions. From the former, the fraction of energy converted in the surface collision into the internal excitation of the projectile ion was estimated as 10% of the incident energy. The internal energy of the surface-excited projectile ions was very similar for all studied surfaces. The H-terminated room-temperature diamond surface differed from the other surfaces only in the fraction of product ions formed in H-atom transfer surface reactions (45% of all product ions formed versus 70% on the other surfaces).     

Characterization of the fluxes of neutral and positively charged clusters (Agn and Agn+; n≤4) produced by argon ion sputtering of silver

The emission of neutral and positively charged silver clusters during sputtering of a polycrystalline silver target by 5 keV Ar⁺ ion bombardment has been studied and the sputter ejected silver flux has been characterized. As a result, the silver flux is found to be strongly dominated byneutral clusters rather than cluster ions. The contribution of neutral clusters in the overall silver flux decreases rapidly and monotonically with increasing cluster size n and decreases, in addition, with decreasing bombarding energy. The well known alternation of the secondary ion intensities of Ag _n ⁺ as a function of cluster size (higher intensities for odd n) is found to be correlated with the effective ionization potentials of the corresponding sputtered neutral clusters.     

Effects of Alkali Metal Ion Cationization on Fragmentation Pathways of Triazole-Epothilone

The collisionally activated dissociation mass spectra of the protonated and alkali metal cationized ions of a triazole-epothilone analogue were studied in a Fourier transform ion cyclotron resonance mass spectrometer. The fragmentation pathway of the protonated ion was characterized by the loss of the unit of C₃H₄O₃. However, another fragmentation pathway with the loss of C₃H₂O₂ was identified for the complex ions with Na⁺, K⁺, Rb⁺, and Cs⁺. The branching ratio of the second pathway increases with the increment of the size of alkali metal ions. Theoretical calculations based on density functional theory (DFT) method show the difference in the binding position of the proton and the metal ions. With the increase of the radii of the metal ions, progressive changes in the macrocycle of the compound are induced, which cause the corresponding change in their fragmentation pathways. It has also been found that the interaction energy between the compound and the metal ion decreases with increase in the size of the latter. This is consistent with the experimental results, which show that cesiated complexes readily eject Cs⁺ when subject to collisions.     

Cyclopropane intermediates in the rearrangement and fragmentation of olefinic molecular ions

Methyl loss from deuterium-labelled molecular ions of 4-methyl-2-pentene, 2-methyl-2-pentene and 1,1,2-trimethylcyclopropane has been investigated for metastable molecular ions and for molecular ions formed by charge exchange with COS⁺˙, XE⁺˙ and CO⁺˙. For metastable ion fragmentation reactions all three compounds exhibit very similar behavior and show specific and essentially equal loss of each of the original methyl groups as well as specific loss of a methyl where the hydrogens derive exclusively from the non-methyl hydrogens of the original molecules. The former results are interpreted in terms of interconversion of the three molecular ions through a ring-opened form of the trimethylcyclopropane molecular ion. The loss of the non-methyl hydrogens as CH₃ is interpreted in terms of isomerization to the 2,3-dimethyl-2-butene structure. With increasing internal energy direct allylic cleavage of the unrearranged methylpentene molecular ions increases in importance while the trimethylcyclopropane molecular ion shows an increased preference for loss of the C(2) methyl group. With increasing internal energy loss of the original non-methyl hydrogens as CH₃ decreases markedly in importance.     

Desorption due to charge exchange in low-energy collisions of organofluorine ions at solid surfaces

Collisions of organofluorine ions at a metal surface result in efficient emission of adsorbate species as gas-phase ions. The experiments are done at 120° scattering angle in a hybrid (BQ) mass spectrometer; the primary ions, mass-selected by a magnetic sector (B), are allowed to collide with a target at a selected kinetic energy in the tens of eV range and the emitted ions are mass-analyzed using a quadrupole mass filter (Q). It is proposed that the impinging ions undergo neutralization accompanied by desorption of hydrocarbon ions and that the amount of internal energy deposited in the desorbed ions is strongly dependent on the collision energy and affects their degree of fragmentation. Competing processes include reflection and fragmentation of the colliding particle, along with such ion/adsorbate reactions as hydrogen atom abstraction by the fluorinated ion. Small even-electron ions, such as [CHF₂]⁺ and [C₂H₂F]⁺ are more effective in promoting chemical sputtering of the surface adsorbate as compared to larger ions (e.g. [C₃F₅]⁺) and odd-electron ions (e.g. [C₂F₄]⁺˙ and [C₂HF₂]⁺˙). At low energies some odd-electron fluorinated ions undergo collision without any secondary ions being emitted from the surface. In these cases the parent ions are apparently neutralized, but without sufficient energy transfer to cause hydrocarbon ion desorption. Non-fluorinated organic ions yield fragment ions and ion/surface reaction products under the condition of these experiments, but do not cause significant desorption of hydrocarbon ions.     

Quantitation of major elements with secondary ion mass spectrometry by using M 2 + -molecular ions

A new quantitation method, based on the detection of M ₂ ⁺ molecular ions, is presented. It has been shown that M ₂ ⁺ molecular ions are formed by a recombination process between independently sputtered M and M⁺ particles. Based on this formation mechanism, it will be demonstrated that M ₂ ⁺ molecular ions can be used to quantitate major elements. The method will be used for quantitation of an Al _x Ga_1?x As multilayer. Furthermore, it will be shown that some matrix effects can be explained by the energy dependence of instrument transmission.     

Sputtered metal cluster ions: Unimolecular decomposition and collision induced fragmentation

Cluster ions are produced by ion bombardment of thick metal targets and mass selected in a Wien filter. The unimolecular decomposition of Al _n ⁺ , Cu _n ⁺ , Mo _n ⁺ , W _n ⁺ , and Pb _n ⁺ is investigated under UHV conditions. The time evolution of the decay allows a glimpse into the cluster formation/fragmentation process. Highly excited metal cluster ions decompose mainly by evaporating single neutral atoms with rates reaching 100%. The collision induced fragmentation (CIF) of stable mass selected metal cluster ions in a low pressure Ar and O₂ gas target will be compared to the unimolecular decay.     

Ionisation and appearance potentials of alkylketones

The ionisation potentials of a series of alkylketones have been measured by the electron-impact technique using the second derivative method. The ionisation efficiency curves were obtained with a conventional mass spectrometer and the data treated by means of a minicomputer. These results are compared with those given by photoelectron spectroscopy, which has enabled us to test our mass spectrometric method and to distinguish between the adiabatic and the vertical ionisation potentials. It has been observed that the electron-impact technique can lead to the adiabatic ionisation potential in favourable cases. We have determined the appearance potentials of fragmentations [A]⁺ = [RCO]⁺ and [B]⁺ = [CH₃CO]⁺, arising from the rupture of bonds in the α position with respect to CO, for a large series of methylalkylketones. The standard heats of formation of these ions have been calculated. The excess energy of the fragmentation reaction, corresponding to the kinetic shift, is found to be small or negligible for the decomposition giving ion [A]⁺, requiring a smaller activation energy and high in the case of ions [B]⁺. This kinetic shift becomes larger for an increase in the activation energy and the vibrational degrees of freedom of the molecular ion, as postulated by the quasi-equilibrium theory. An important fraction of the kinetic shift is due to the ‘competitive shift,’ i.e. the excess energy that must be supplied to the molecular ion for it to yield [B]⁺ ions in sufficient amounts to be detected by the mass spectrometer. This is confirmed by the claculation of the fragmentation rate constants for the 2-butanone molecular ion.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

Bare Clusters Derived from Protein Templates: Au25+, Au38+ and Au102+

A discrete sequence of bare gold clusters of well‐defined nuclearity, namely Au₂₅⁺, Au₃₈⁺ and Au₁₀₂⁺, formed in a process that starts from gold‐bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First‐principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed‐shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super‐atom electronic gap. This shell‐closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au_n⁺ (n=25, 38, and 102) clusters.     

An energy-resolved study of the fragmentation of ionized 1-Penten-3-ol

A detailed energy-resolved study of the fragmentation of CH₂?CHCH(OH)CD₂CD₃ (1-d₅) has been carried out using metastable ion studies and charge exchange techniques, combined with collision-induced dissociation studies to establish the structures of fragment ions. At low internal energies (metastable ions) the molecular ion of 1-d₅ rearranges to the 3-pentanone structure and fragments by loss of C₂H₅ or C₂D₅ leading to the acyl structure, [CH₃CH₂C?O]⁺ or [CD₃CD₂C?O]⁺, for the fragment ion. However, with increasing internal energy of the molecular ion this rearrangement process decreases rapidly in importance and loss of C₂D₅ by direct cleavage, leading to [CH₂?CHCH?OH]⁺, becomes the dominant fragmentation reaction. As a result the [C₃H₅O]⁺ ion seen in the electron impact mass spectrum of 1-penten-3-ol has predominantly the protonated acrolein structure.