Investigation of the initial reactions of the Calcote mechanism for soot formation

The first three reactions of the Calcote mechanism for soot formation, that is, C₃H ₃ ⁺ +C₂H₂→C₅H ₅ ⁺ , C₅H ₅ ⁺ →C₅H ₃ ⁺ H₂, and C₅H ₃ ⁺ +C₂H₂→C₇H ₅ ⁺ , have been studied based on chemi-ions withdrawn directly from a premixed methane-oxygen flame by supersonic molecular beam sampling. The first reaction is reversible and involves the formation of a specific encounter complex sensitive to pressure and ion kinetic energy. The second reaction appears to require large amounts of internal energy in the C₅H ₅ ⁺ ion to proceed. The third reaction is reversible; however, in contrast to the initiating reaction, the C₅H ₃ ⁺ ion formed from the [C₇H ₅ ⁺ ]* complex exhibits a much lower reactivity. The conclusions are based on ion-molecule reactions as well as collision activation mass spectrometry of isolated chemi-ions. In addition, the product distributions as functions of pressure and ion kinetic energy were studied.     

Photoionization/fragmentation of endohedral fullerenes

Photoionizationlfragmentation of endohedral fullerenes was investigated by use of laser-de sorption time-offlight (LD-TOF) mass spectroscopy. The velocity distribution of the parent ion (LaC ₈₂ ⁺ ) was found to be bimodal, as has previously been shown for laser desorbed C ₆₀ ⁺ . The 0 fragment ions have velocity distributions corresponding predominantly to the fast parent ion distribution. The LD-TOF mass spectra taken with a relatively low laser fluence were independent of the delay time of the extraction pulse, showing only a monotonically decreasing pattern of LaC _2n ⁺ (as n decreased). However, with higher laser fluence, it was shown that the mass distributions drastically changed from the monotonically decreasing pattern to that of C _2n ⁺ and LaC _2n ⁺ with magic numbers. Based on these findings, a plausible photoionization/fragmentation mechanism is presented and discussed.     

Influence of the organic layer thickness in (Metal-Assisted) secondary ion mass spectrometry using Ga+ and C 60 + projectiles

This article investigates the influence of the organic film thickness on the characteristic and molecular ion yields of polystyrene (PS), in combination with two different substrates (Si, Au) or gold condensation (MetA-SIMS), and for atomic (Ga⁺) and polyatomic (C ₆₀ ⁺ ) projectile bombardment. PS oligomer (m/z ～ 2000 Da) layers were prepared with various thicknesses ranging from 1 up to 45 nm on both substrates. Pristine samples on Si were also metallized by evaporating gold with three different thicknesses (0.5, 2, and 6 nm). Secondary ion mass spectrometry was performed using 12 keV atomic Ga⁺ and C ₆₀ ⁺ projectiles. The results show that upon Ga⁺ bombardment, the yield of the fingerprint fragment C₇H ₇ ⁺ increases as the PS coverage increases and reaches its maximum for a thickness that corresponds to a complete monolayer (～3.5 nm). Beyond the maximum, the yields decrease strongly and become constant for layers thicker than 12 nm. In contrast, upon C ₆₀ ⁺ bombardment, the C₇H ₇ ⁺ yields increase up to the monolayer coverage and they remain constant for higher thicknesses. A strong yield enhancement is confirmed upon Ga⁺ analysis of gold-metallized layers but yields decrease continuously with the gold coverage for C ₆₀ ⁺ bombardment. Upon Ga⁺ bombardment, the maximum PS fingerprint ion yields are obtained using a monolayer spin-coated on gold, whereas for C ₆₀ ⁺ , the best results are obtained with at least one monolayer, irrespective of the substrate and without any other treatment. The different behaviors are tentatively explained by arguments involving the different energy deposition mechanisms of both projectiles.     

Isomerization of linear C3H 3 + in its reaction with acetylene,and collisional stabilization of the [C5H 5 + ]* collision complex in a quadrupole ion trap mass spectrometer

The isomerization of linear C₃H ₃ ⁺ in its reaction with acetylene to cyclic C₃H ₃ ⁺ was studied with a quadrupole ion trap mass spectrometer. The reaction of linear C₃H ₃ ⁺ with ¹³C₂H₂ shows that isomerization takes place via a [C₅H ₅ ⁺ ]^* activated complex that is unstable relative to disproportionation back into the cyclic and linear forms of C₃H ₃ ⁺ and acetylene. The formation of carbon-13 labeled cyclic and linear C,Hi indicates that isomerization involves skeletal exchange. Collisional stabilization of the [C₅H ₅ ⁺ ]^* collision complex was achieved at a helium pressure of approximately 1 mtorr.     

Kinetic energy release of clusters after multiphoton ionisation with time of flight and electrostatic analyser techniques

A new technique is presented which allows direct observation of initial kinetic energies in multiphoton ionisation-fragmentation processes of molecules and clusters and provides an unambiguous determination of metastable decay channels. Results are presented for the unimolecular loss of a monomer from clusters (C₆H₆) ₈ ⁺ to (C₆H₆) ₁₂ ⁺ and for the reaction C₆H ₆ ⁺ →C₄H ₄ ⁺ +C₂H₂. We also observe a significant amount of probably collision induced fragmentation processes (C₆H₆) _n ⁺ →(C₆H₆) _n?x ⁺ + (C₆H₆) _x withx much larger than 1.     

Chemisorption and chemical reactions on size selected clusters

Low energy ion beam techniques have been used to perform a detailed study of the reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ with a range of simple molecules (D₂, CH₄, O₂, C₂H₄, CO and N₂). The reactions were studied over a center of mass collision energy range from 0.2eV up to 7eV. Activation barriers for chemisorption onto the clusters were deduced from the experimental results. The activation barriers for chemisorption on Al ₂₅ ⁺ and Si ₂₅ ⁺ are generally similar and show a qualitative correlation with the electronic properties of the reactant molecule. However, the products of the chemical reactions of Al ₂₅ ⁺ and Si ₂₅ ⁺ which result from cluster fragmentation are quite different. Si ₂₅ ⁺ shows a tendency to undergo fission as observed in a number of recent studies of the dissociation of the bare clusters.     

Matrix-assisted laser desorption/ionization tandem reflectron time-of-flight mass spectrometry of fullerenes

Atandem reflectron time-of-flight mass spectrometer developed in our laboratory provides a unique opportunity to investigate the collision-induced dissociation of fullerene ions formed by matrix-assisted laser desorption/ionization (MALDI). Specifically, this opportunity arises from the ability to utilize high energy collisional activation (normally available only on tandem sector instruments by using continuous ionization techniques) for ions formed by pulsed laser desorption, whereas most MALDI time-of-flight instruments record product ion mass spectra of ions formed by metastable or postsource decay. In this study we investigate the products of mass-selected and collisionally activated C ₆₀ ⁺ and C ₇₀ ⁺ ions by using different target gases over a range of target gas pressures. In general, heavier target gases produce more extensive fragmentation and improve the mass resolution of lower mass ionic products because a greater portion of these ions are formed by single collisions. Additionally, the tandem time-of-flight instrument utilizes a nonlinear (curved-field) reflectron in the second mass analyzer that enables high energy collision-induced dissociation spectra to be recorded without scanning or stepping the reflectron voltage.     

Surface-Induced dissociation from a liquid surface

Mass-selected projectile ions in the tens of electronvolt energy range undergo surface-induced dissociation upon collision with a liquid perfluorinated polyether (PFPE) surface. The efficiency of translational-to-vibrational (T-V) energy transfer is similar to that observed for a fluorinated self-assembled monolayer (SAM) surface. The thermometer ion W(CO)^’ was used to detenrrine an average T-V conversion efficiency of 18% in the collision energy range of 30–50 eV. The surface can be bombarded for several hours without displaying any change in the scattered ion products. Ion-surface reactions occur with some projectiles and are analogous to those seen with the fluorinated SAM surface. For example, WF _? ⁺ (m=1–5) and W(CO)_nF _? ⁺ (n=1–2, m=1–2) are generated upon collisions of W(CO) ₆ ⁺ with the PFPE liquid surface. The ion-surface reactions observed suggest that F atoms and/or CF₃ groups are accessible for reaction while the oxygen atoms lie below the outermost surface layer. Chemical sputtering of the liquid surface also occurs and yields common fluorocarbon fragment ions, including CF ₃ ⁺ , C₂F ₅ ⁺ , and C₃F ₇ ⁺ and the oxygenated product CFO⁺. The liquid surface is remarkably free of hydrocarbon impurities. Collisions of the pyrazine and benzene molecular ions, both probes for hydrocarbon impurities, resulted in very little protonated pyrazine or protonated benzene.     

FTMS studies of sputtered metal cluster ions (IV): size-selective effects in the chemistry of Fe n + with NH3 and Pd n + with D2 or C2H4

Fe _n ⁺ and Pd _n ⁺ clusters up ton=19 andn=25, respectively, are produced in an external ion source by sputtering of the respective metal foils with Xe⁺ primary ions at 20 keV. They are transferred to the ICR cell of a home-built Fourier transform mass spectrometer, where they are thermalized to nearly room temperature and stored for several tens of seconds. During this time, their reactions with a gas leaked in at low level are studied. Thus in the presence of ammonia, most Fe _n ⁺ clusters react by simply adsorbing intact NH₃ molecules. Only Fe ₄ ⁺ ions show dehydrogenation/adsorption to Fe₄(NH) _m ⁺ intermediates (m=1, 2) that in a complex scheme go on adsorbing complete NH₃ units. To clarify the reaction scheme, one has to isolate each species in the ion cell, which often requires the ejection of ions very close in mass. This led to the development of a special isolation technique that avoids the use of isotopically pure metal samples. Pd _n ⁺ cluster ions (n=2...9) dehydrogenate C₂H₄ in general to yield Pd _n (C₂H₂)⁺, yet Pd ₆ ⁺ appear totally unreactive. Towards D₂, Pd ₇ ⁺ ions seem inert, whereas Pd ₈ ⁺ adsorb up to two molecules.     

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.     

Fragmentation of clusters induced by collision with a solid surface: comparison of antimony and bismuth cluster ions

Mass-selected antimony cluster ions Sb _n ⁺ (n = 3-12) and bismuth cluster ions Bi _{ntn} ⁺ (n = 3-8) are allowed to collide with the surface of highly oriented pyrolytic graphite at energies up to 350 eV. The resulting fragment ions are analysed in a time-of-flight mass spectrometer. Two main fragmentation channels can be identified. At low impact energies both Sb _n ⁺ and Bi _n ⁺ cluster ions lose neutral tetramer and dimer units upon collision. Above about 150 eV impact energy Sb ₃ ⁺ becomes the predominant fragment ion of all investigated antimony clusters. The enhanced stability of these fragment clusters can be explained in the framework of the polyhedral skeletal electron pair theory. In contrast, Bi _n ⁺ cluster scattering leads to the formation of Bi ₃ ⁺ , Bi ₂ ⁺ and Bi+ with nearly equal abundances, if the collision energy exceeds 75 eV. The integral scattering yield is substantially higher in this case as compared to Sb _n ⁺ clusters.     

Reactivity of positively charged cobalt cluster ions with CH4, N2, H2, C2H4, and C2H2

Reactivity of positively charged cobalt cluster ions (Co _n ⁺ ,n=2?22), produce by laser vaporization, with various gas samples (CH₄, N₂, H₂, C₂H₄, and C₂H₂) were systematically investigated by using a fast-flow reactor. The reactivity of Co _n ⁺ with the various gas samples is qualitatively consistent with the adsorption rate of the gas to cobalt metal surfaces. Co _n ⁺ highly reacts with C₂H₂ as characterized by the adsorption rate to metal surfaces, and it indicates no size dependence. In contrast, the reactions of Co _n ⁺ with the other gas samples indicate a similar cluster size dependence; atn=4, 5, and 10?15, Co _n ⁺ highly reacts. The difference can be explained by the amount of the activation energy for chemisorption reaction. Compared with neutral cobalt clusters, the size dependence is almost similar except for Co ₄ ⁺ and Co ₅ ⁺ . The reactivity enhancement of Co ₄ ⁺ and Co ₅ ⁺ indicates that the cobalt cluster ions are presumed to have an active site for chemisorption atn=4 and 5, induced by the influence of positive charge.     

Bimetallic cluster cations ejected from a liquid metal ion source: Li-Na and Li-Mg

Cluster ions of alloys (Li-Na, Li-Mg) have been produced by a liquid metal ion source (LMIS), and analyzed by mass spectrometry. For the Li-Na system, bimetallic clusters with various compositions were formed, and dominant bimetallic species were Na₂Li⁺, NaLi⁺, NaLi ₂ ⁺ and NaLi ₈ ⁺ with this sequence of ion intensity. These clusters are systems containing 2 or 8 valence electrons except for NaLi⁺. For the Li-Mg, observed bimetallic clusters were limited to only three species (MgLi⁺, MgLi ₂ ⁺ and Mg₂Li⁺), but unexpectedly small multiply charged homonuclear clusters, Mg ₂ ²⁺ and Mg ₃ ²⁺ , were observed.     

On the role of dissociative ionization in the formation of argon dimer ions

The formation of Ar ₂ ⁺ ions has been investigated by means of the threshold photoelectron photoion coincidence (TPEPICO) technique. Two pathways for the formation of Ar ₂ ⁺ ions are important. One is a direct path via excitation of Rydberg states of Ar₂ with consecutive autoionization. The other path is dissociative ionization of larger argon clusters, in this case argon trimers. These two pathways lead to Ar ₂ ⁺ ions with different internal energy. The pathways are easily distinguished in the TPEPICO-TOF spectra by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar ₃ ⁺ → Ar ₂ ⁺ + Ar was measured as a function of the photon energy and compared to the KER expected from statistical theory. The agreement is satisfying and confirms that Ar ₃ ⁺ ions do indeed dissociate at the thermochemical threshold. At higher photon energy the excited²Π(3/2)g state of Ar ₃ ⁺ is also detected from a second component in the KER. By applying a kinetic energy discrimination it is possible to measure cluster ion spectra in the presence of larger clusters but essentially without interference from the latter.     

Formation of O 2 − in charge transfer collisions of fast molecular hydrogen ions with O2 molecules

Cross sections for the production of O ₂ ^? in charge transfer collisions of fast molecular hydrogen ions (H ₂ ⁺ , D ₂ ⁺ , H ₃ ⁺ , and D ₃ ⁺ of 10 to 140 keV kinetic energy) with O₂ molecules have been determined by means of a time-of-flight mass spectrometer analysing the slow negative product ions from the collisions. Within the measuring accuracy equivelocity H ₂ ⁺ and D ₂ ⁺ ions have the same cross sections for the generation of O ₂ ^? . The projectile velocity dependence curve of the cross section passes through a broad maximum with a peak value of about 6.5×10^?18 cm² around the Bohr velocity (25 keV/u) before showing an asymptotic decrease still within the limited energy range under investigation that is in inverse proportion to the square of velocity. Throughout the examined energy range H ₃ ⁺ ions yield a cross section which is about 1.4 times larger than that of H ₂ ⁺ ions of the same velocity. The fragment ion O^? has been found to appear with cross sections between 10^?19 and 10^?18 cm² upon collisional excitation in the energy range under investigation, with ever decreasing intensity when the energy of the positive hydrogen ions, the proton included, was increased.     

Electronic structures and stabilities of GeC n and Ge2C n in Hückel theory

Some recent results about Ge _p C _n ⁺ ions (p=1, 2;n < 6) produced in laser microprobe mass analyser experiments (LAMMA) show very marked alternations in the emission intensities I(Ge _p C _n ⁺ ) as a function of then andp parities. I(Ge _p C _n ⁺ ) are maxima for evenn. Thus, intensity maxima occur when the total atom numberm of the aggregates is odd for GeC _n ⁺ (m=n+1) and even for Ge₂C _n ⁺ (m=n+2). As a result, GeC _n ⁺ ions seem to behave as C _m ⁺ ions, whereas the behaviour of Ge₂C _n ⁺ ions is quite similar to that of Ge _p ⁺ ions formed in SIMS or vaporization experiments on pure germanium. It is well known (correspondence rule) that the parity effect in the emissions corresponds to alternations in the ion stabilities. These results are analysed from a model built in Hückel approximation with hybridization. Forp=1, the clusters are assumed to be insp hybridization as for C _m ⁺ ions, hence with linear shapes, and forp=2, they would rather be insp ² orsp ³ hybridization as for Ge _p ⁺ ions. Relative stabilities and distributions of the energy levels of the aggregates are then calculated. The relative stabilities given for Ge _p C _n ⁺ by this model show maxima for evenn as in experiments, and we have thus a good agreement between our calculation results and the experimental data. Moreover, we found that Ge₂C _n ⁺ would rather be insp ³ hybridization, that is under three dimensional shapes.     

Metastable decay of rare gas cluster ions: mechanism and kinetics

Metastable decay of cluster ions has been discovered only recently. It was noted that one has to take this metastable decay into account when using mass spectrometry to probe neutral clusters, because ion abundance anomalies in mass spectra of rare gas and molecular clusters are caused by delayed metastable evaporation of monomers following ion production. Moreover, it was found that(i) the individual metastable reaction rates k depend strongly on cluster size and cluster ion production pathways and that(ii) there exists experimental evidence (k=k(t)) and a theoretical prediction that a given mass selected cluster ion generated by electron impact ionization of a nozzle expansion beam will comprise a range of metastable decay rates. In addition, it was discovered that metastable Ar cluster ions which lose two monomers in the μs time regime decay via sequential decay series Ar _n ⁺ *→Ar _n?1 ⁺ *→Ar _n?2 ⁺ * with cluster sizes 7≤n≤10 andn=3 (similar results were obtained recently in case of N₂ cluster ions). Conversely, the dominant metastable decay channel of Ar ₄ ⁺ * into Ar ₂ ⁺ was found to proceed predominantly via a single step fissioning process.     

Metastable and collision-induced fragmentation studies of all C4H12Si+ isomers; a systematic study of structure-reactivity relations

Metastable ion (MI) and collision-induced dissociation (CID) mass spectra have been recorded and compared for all nine C₄H₁₂Si^+. isomers. The (Me)₄Si^+., t-BuSiH ₃ ^+. , s-BuSiH ₃ ⁺ , and (Me)₂EtSiH^+. isomers have unique MI and CID mass spectra. The MI mass spectra, including the kinetic energy release values, of (Me)(i-Pr)SiH ₂ ^+. and (Me)(n-Pr)SiH ₂ ^+. are identical, which implies isomerization. MI data also suggest that a fraction of the n-BuSiH ₃ ^+. ions rearrange into branched (Me)₂EtSiH^+. ions and a fraction of the n-BuSiH ₃ ^+. ions rearrange into branched s-BuSiH ₃ ^+. ions. A comparison with the isomeric C₅H ₁₂ ^+. pentanes reveals a crucial difference: H₂ loss occurs for n-BuSiH ₃ ^+. , i-BuSiH ₃ ^+. , s-BuSiH ₃ ^+. , (Me)(n-Pr)SiH ₂ ^+. , (Me)(i-Pr)SiH ₂ ^+. , and Et₂SiH ₂ ^+. , but not for any of the C₅Hi ₁₂ ^+. isomers. Generation of four- or five-membered silicon containing rings is suggested for H₂ loss from the C₄H₁₂Si^+. silanes.     

Coincidence studies of collisionally induced fission of C 60 2+

A direct measurement of collisionally induced fission of C ₆₀ ²⁺ has been performed. We have measured coincidences between various charged fragments resulting from collisions between C ₆₀ ²⁺ and He atoms. The measurements show that C ₆₀ ²⁺ not only emits C₂ units but also breaks up into larger, singly charged parts. In this paper, we report on coincidences between C _n ⁺ (2≦n≦9) and C _m ⁺ (42≦m≦48) fragment ions.     

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, He _n ⁺ ,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He ₄ ⁺ and He ₅ ⁺ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He ₂ ⁺ or He ₃ ⁺ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He ₂ ⁺ and He ₃ ⁺ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He ₂ ⁺ or He ₃ ⁺ are placed as “chromophores” into the centre of a larger He _n ⁺ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He ₂ ⁺ or He ₃ ⁺ cores.