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that is, C₃H ₃ ⁺ +C₂H₂→C₅H ₅ ⁺ , C₅H ₅ ⁺ →C₅H ₃ ⁺ H₂, and C₅H ₃ ⁺ +C₂H₂→C₇H ₅ ⁺ , have been studied based on chemi-ions withdrawn directly from a premixed methane-oxygen flame by supersonic molecular beam sampling.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.     

Multiple H 3 + fragment production in single collision of fast H n + clusters with He atoms

The production of H ₃ ⁺ ions resulting from single collisions of mass-selected ionic hydrogen clusters, H _n ⁺ (n=9, 25, 31), with helium at high velocity (1.55 times the Bohr velocity) has been studied. A strong double H ₃ ⁺ ion production resulting from one incident cluster is observed. Moreover, evidence for a triple H ₃ ⁺ fragment production is presented forn=25 and 31. Thus, in this energy range, the collision gives rise to multifragmentation processes. The formation of H ₃ ⁺ ions takes place in the fragmentation of the multicharged cluster resulting from the collision.     

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.     

Photofragmentation of mass resolved carbon cluster ions

The results of a detailed study of the photodissociation of carbon cluster ions, C ₃ ⁺ to C ₂₀ ⁺ , are presented and discussed. The experiments were performed using internally cold cluster ions derived from pulsed laser evaporation of a graphite target rod in a helium buffer gas followed by supersonic expansion. The mass selected clusters were photodissociated using 248 nm and 351 nm light from an excimer laser. Photofragment branching ratios, photodissociation cross sections and data on the laser fluence dependence of photodissociation are reported. For almost all initial clusters, C _n ⁺ , the dominant photodissociation pathway was observed to be loss of a C₃ unit to give a C _n?3 ⁺ ion. This observation is interpreted as indicating that dissociation occurs by a statistical unimolecular process rather than by direct photodissociation. The photodissociation was found to be linear with laser fluence forn>5 with 248 nm and 351 nm light; quadratic forn=5 for 248 nm and 351 nm; and linear forn=4 at 248 nm. Dissociation energies for the carbon cluster ions implied by these results are discussed. The photodissociation cross sections were found to change dramatically with cluster size and with the wavelength of the photodissociating light.     

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.     

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.     

Liquid secondary ion mass spectrometry of several β-diketonates of cobalt(III) and chromium(III)

The positive, liquid secondary ion (LSI) mass spectra of six cobalt(III) and three chromium(III) (β-diketonates ligand = L^?) were examined in a 3-nitrobenzyl alcohol matrix. The complexes of both metals yield clean, matrix-free mass spectra, but there are important differences between them. The cobalt compounds show prominent peaks assignable to the molecular ion, CoL ₃ ⁺ , of the monomeric chelates, together with abundant dimeric ions, such as Co₂L ₄ ⁺ and Co₂L ₃ ⁺ ; in contrast, chromium complexes show protonated monomers, CrL₃H⁺, in addition to ionized monomers, CrL ₃ ⁺ , and only minor formation of dimeric ions. The collisionally-activated dissociation (CAD) mass spectrum of Co₂L ₄ ⁺ shows fragmentation to CoL ₂ ⁺ and Co₂L ₃ ⁺ . That of Co₂L ₃ ⁺ shows fragmentation only to dimeric ions, including Co₂L ₂ ⁺ and, for thienyl or phenyl substituted ligands, to Co₂L₂Ar⁺ or Co₂LAr⁺ (Ar = thienyl or phenyl). Neither Co₂L ₄ ⁺ nor Co₂L ₃ ⁺ dissociates to the CoL ₃ ⁺ ion. The LSI mass spectrum of a mixture of two different cobalt chelates shows dimeric ions containing both types of ligand, which can be explained by ion-molecule reactions in the selvedge region. The differing behaviors of the cobalt and chromium complexes is attributed to the relatively greater stability of the +2 oxidation state for cobalt than for chromium.     

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.     

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.     

Formation of helium cluster ions HHe x + (x≦14) and H3He x + (x≦13) in a very low temperature drift tube

The formation of cluster ions when hydrogen molecular ions H ₂ ⁺ and H ₃ ⁺ are injected into a drift tube filled with helium gas at 4.4 K has been investigated. When H ₂ ⁺ ions are injected, cluster ions HHe _x ⁺ (x≦14) are produced. No production of H₂He _x ⁺ ions is observed. When H ₃ ⁺ ions are injected, cluster ions HHe _x ⁺ (x≦14) are produced as well as H₃He _x ⁺ (x≦13), and very small signals corresponding to H₂He _x ⁺ (3≦x≦10) are observed. Information on the stability of HHe _x ⁺ and H₃He _x ⁺ is derived from the drift field dependence of the cluster size distributions. The cluster sizex=13 is found to be a magic number for HHe _x ⁺ , and for H₃He _x ⁺ ,x=10 and 11.     

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.     

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.     

A photoionization study of the ion-neutral complexes CH3CH +CH 3 · CH2CH3] and CH3CH2CH+CH 3 · CH3 in the gas phase: Formation,H-transfer and C—C bond formation between partners,and channeling of energy into dissociation

Photoionization mass spectrometry was used to investigate the dynamics of ion-neutral complex-mediated dissociations of the n-pentane ion (1). Reinterpretation of previous data demonstrates that a fraction of ions 1 isomerizes to the 2-methylbutane ion (2) through the complex CH₃CH⁺CH ₃ ^· CH₂CH₃ (3), but not through CH₃CH⁺CH₂CH ₃ ^· CH₃ (4). The appearance energy for C₃Hin ₇ ⁺ formation from 1 is 66 kJ mol^?1 below that expected for the formation of n-C₃H ₇ ⁺ and just above that expected for formation of i-C₃H ₇ ⁺ . This demonstrates that the H shift that isomerizes C₃H ₇ ⁺ is synchronized with bond cleavage at the threshold for dissociation to that product. It is suggested that ions that contain n-alkyl chains generally dissociate directly to more stable rearranged carbenium ions. Ethane elimination from 3 is estimated to be about seven times more frequent than is C-C bond formation between the partners in that complex to form 2, which demonstrates a substantial preference in 3 for H abstraction over C-C bond formation. In 1 → CH₃CH⁺CH₂CH₃ + CH₃ by direct cleavage of the C1–C2 bond, the fragments part rapidly enough to prevent any reaction between them. However, 1 → 2 → 4 → C4H ₈ ⁺ + CH₄ occurs in this same energy range. Thus some of the potential energy made available by the isomerization of n-C₄H₉ in 1 is specifically channeled into the coordinate for dissociation. In contrast, analogous formation of 3 by 1 → 3 is predominantly followed by reaction between the electrostatically bound partners.     

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.     

Observations of magic numbers in gas phase hydrates of alkylammonium ions

Hydration of alkylammonium ions under nonanalytical electrospray ionization conditions has been found to yield cluster ions with more than 20 water molecules associated with the central ion. These cluster ion species are taken to be an approximation of the conditions in liquid water. Many of the alkylammonium cation mass spectra exhibit water cluster numbers that appear to be particularly favorable, i.e., “magic number clusters” (MNC). We have found MNC in hydrates of mono- and tetra-alkyl ammonium ions, NH₃(C _m H_2m+1)⁺(H₂O) _n , m=1–8 and N(C _m H_2m+1) ₄ ⁺ (H₂O) _n , m=2–8. In contrast, NH₂(CH₃) ₂ ⁺ (H₂O) _n , NH(CH₃) ₃ ⁺ (H₂O) _n1 and N(CH₃) ₄ ⁺ (H₂O) _n do not exhibit any MNC. We conjecture that the structures of these magic number clusters correspond to exohedral structures in which the ion is situated on the surface of the water cage in contrast to the widely accepted caged ion structures of H₃O⁺(H₂O) _n and NH ₄ ⁺ (H₂O) _n .     

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.     

Collision-induced dissociation evidence for charge separation and “ball-and-chain” propagation in the addition of 1-butene to C 60 2+

The collision-induced dissociation of the adduct ions C₆₀(C₄H₈) ₂ ²⁺ and C₆₀(C₄H₈) ₃ ²⁺ formed by sequential reactions of C ₆₀ ²⁺ with 1-butene has been investigated by using a selected-ion flow tube (SIFT) apparatus. Experiments at 295 ± 2 K in 0.35 ± 0.02 torr of helium indicated that C ₆₀ ²⁺ adds at least five molecules of 1-butene in a sequential fashion with rates that decrease with the number of molecules added. Collision-induced dissociation experiments in which the downstream sampling nose cone of the SIFT was biased with respect to the flow tube indicated that the adduct ions C₆₀(C₄H₈) ₂ ²⁺ and C₆₀(C₄H₈) ₃ ²⁺ dissociate into C ₆₀ ^·+ and (C₄H₈) ₂ ^·+ and (C₄H₈) ₃ ^·+ , respectively. These observations provide evidence for the occurrence of charge separation in the derivatization of C₆₀ dications and support the “ball-and-chain” mechanism first proposed by Wang et al. in 1992 for the sequential multiple addition of 1,3-butadiene to C ₆₀ ²⁺ and C ₇₀ ²⁺ .     

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.     

Production of negatively charged fragments in collisions of swift positive hydrogen ions with molecules

A first detection and analysis of negatively charged fragments produced in collisions of fast (20–150 keV) positive hydrogen ions (H⁺, H ₂ ⁺ and H ₃ ⁺ ) with gas-phase molecules is presented. The fragments and their abundances were determined by means of a time-of-flight mass spectrometer. Negative ions did emerge from every investigated target molecule species, such as halomethanes, sulfur hexafluoride, propane and propene, but in all cases with distinctly lower probability (cross sections in the range 10^?20?10^?18 cm²) than positively charged fragments (approximately on the scale 10^?3 or even less). Another essential result is that stable collisionally induced negative fragments are mostly monatomic ions, whereas positive fragments are in their majority more complex polyatomic ions. Furthermore, we observed a direct electron capture from a positively charged but not totally stripped projectile (here: H ₂ ⁺ and H ₃ ⁺ ) into stable or very longlived states of the molecular ions SF ₆ ^? and O ₂ ^? , the latter with the largest cross section (10^?18?10^?17 cm²) found up to now.