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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Fragmentation by electron impact ionization and intracluster reactions of size selected (N2H4) n and (OCS) n clusters

The hydrogen-bonded (N₂H₄) _n clusters and the van der Waals (OCS) _n clusters are size selected in a scattering experiment with a He beam up to the cluster sizen=6. By measuring the angular distributions of the scattered clusters the complete fragmentation pattern of electron impact ionization is obtained. For Hydrazine the two main fragment masses are the protonated species (N₂H₄) _n?1H⁺ and with somewhat weaker intensities also the nominal ion mass (N₂H₄) _n ⁺ . The largest intensity is observed for the monomer ion N₂H ₄ ⁺ to which clusters up ton=5 fragment. For carbonylsulfide, completely different results are obtained. Aside from the fragments of the OCS monomer and the van der Waals cluster fragments (OCS) ₂ ⁺ and (OCS) ₃ ⁺ signals at mass S ₂ ⁺ , S ₃ ⁺ and S₂OCS⁺ are detected. This indicates a fast chemical reaction in the cluster according to: S + OCS → CO + S₂ which occurs for clusters of sizen ≥ 2. Peaks at S ₃ ⁺ and S₂OCS⁺ are seen for the first time forn ≥ 5 according to a further reaction of S₂ in the cluster.     

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.     

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.     

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.     

Magic numbers in sims of Mn similar to those of charged Ar clusters

Manganese cluster ions Mn _k ⁺ (k?60) have been produced by 7 keV Xe ion bombardment and analyzed by a double-focusing mass spectrometer. Discontinuous variations of intensity are found atk=5, 14, 16, 29, 34, 45 and 54. Most of these magic numbers coincide with or differ by only one from those observed in Ar _k ⁺ . The similarity in magic numbers between Mn _k ⁺ and Ar _k ⁺ indicates that the bonding nature in the charged Mn clusters is similar to that in the charged Ar clusters; The polarization force between a positive ion in the center of a cluster and surrounding neutral atoms is dominant binding force.     

Electronic processes induced by high energy H+, H 2 + and H 3 + -ions: A scaling relation

A scaling relation is proposed which interrelates measurable quantities in the field of atomic collision physics performed with high velocity H⁺, H ₂ ⁺ and H ₃ ⁺ -ions. The relation may be written as $$Q(H^ + ) - 2*Q(H_2^ + ) + Q(H_3^ + ) = 0,$$ whereQ denotes an excitation or ionization cross section or a total or differential secondary particle yield evaluated at the same projectile velocity. The scaling relation will be tested by comparison with experimental data of yields and spectra from ion-induced secondary electron emission measurements and with cross section data for excitation and ionization of atoms and molecules. In general very good agreement is observed for high projectile velocities (v>2 a.u.).     

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.     

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.     

A diatomics-in-molecules model for singly ionized neon clusters

A minimum-basis diatomics-in-molecules (DIM) model previously developed for singly-ionized argon clusters is applied to neon clusters, Ne _n ⁺ , forn=3, 4,...,22. A search for the global minimum energy of each cluster yields structures with the positive charge localised on a dimer-ion. This appears to be due largely to the linear unsymmetrical configuration which the model finds for Ne ₃ ⁺ . For this reason, the structures of the clusters at their minimum energy are different from those for Ar _n ⁺ computed with the same model. On the other hand, the behaviour of the charge distribution as a function of the geometrical configuration is similar to that for Ar _n ⁺ , as are the overall shapes of the potential energy surfaces. The results are discussed in terms of the charge distributions and the ratios of equilibrium properties of the dimers and dimer-ions which constitute the input to the model.     

Helium cluster ions RgHe x + (Rg=Ne,Ar and Kr,x≦14) formed in a drift tube cooled by liquid helium

Rare gas ions Ne⁺, Ar⁺ and Kr⁺ are injected into a drift tube which is filled with helium gas and cooled by liquid helium. Helium cluster ions RgHe _x ⁺ (Rg=Ne, Ar and Kr,x≦14) are observed as products. Information regarding the stability of RgHe _x ⁺ is obtained from drift field dependence of the size distribution of the clusters, and magic numbers are determined. The magic numbers arex=11 and 13 for NeHe _x ⁺ andx=12 for ArHe _x ⁺ and KrHe _x ⁺ . NeHe _x ⁺ , Ar⁺ and Kr⁺ are proposed as the core ions for NeHe ₁₃ ⁺ , ArHe ₁₂ ⁺ and KrHe ₁₂ ⁺ , respectively.     

A theoretical study of internal rearrangements and open channels for fission and mass exchanges in Xe n + clusters

In a previous work the equilibrium geometrical and electronic structures of Xe _n ⁺ clusters had been established using a non-empirical model hamiltonian. The same model is used to determine the energetic barriers between the nearly degenerate isomers; the movement of the neutral atoms around the Xe ₃ ⁺ or Xe ₄ ⁺ ionized linear cores are quite easy (ΔE?0.9 kcal/mole), the changes from a Xe ₃ ⁺ to a Xe ₄ ⁺ core are more difficult (ΔE?2.0 kcal/mole). The energetically possible fissions from a vertical photoionization \(Xe_n \xrightarrow{{h v}}Xe_n^{v + } \to Xe_p^ + + Xe_{n - p} \) forn≦19,p=1–9 and 12–14 and mass exchanges Xe _p ⁺ +Xe _q →Xe _p+m ⁺ +Xe _q?m (m=1,2,3) from relaxed Xe _p ⁺ clusters are given forp+m≦9 and 12–14 andq≦19. Surprisingly the reverse reactions are shown to occur for some values ofp andq. Numerous processes lead to Xe ₁₃ ⁺ , which is especially stable.