Yields Of Cluster Ions Research Articles

Features of semiconductor irradiation with low-energy caesium ions

The paper considers the emission of secondary cluster ions when Si and GaAs surfaces are simultaneously sputtered with low-energy caesium ions (0.5–2 keV) and a pulsing beam of bismuth ions with an energy of 30 keV. It is shown that the observed mass spectra can be obtained if the caesium atoms implanted during irradiation form nano-islands (nanoparticles) at the irradiated surface. It was found that these Cs nanoparticles are formed under the surface due to the so-called Ostwald ripening process and come to the surface during the ongoing sputtering. The composition of the experimental mass spectra with 6–7 target and caesium atoms of a sufficiently high intensity leads to the conclusion that neither the recombination model nor the direct emission model can explain the observed emission. We explain the observed yield of cluster ions by the sputtering mode with the formation of “thermal spots”. These high-temperature spots resulting from collisions of primary bismuth ions with target atoms give rise to the generation of elastic waves in the target. These waves reach the surface and lead to a simultaneous escape of several atoms from the surface layer, or, in other words, to the formation of a secondary cluster.

Electron-loss and destruction processes in collision of MeV/atom carbon cluster ions with rare gases

Singly charged cluster-ion yield ϕ1 of MeV/atom carbon cluster ions, generated using He and Ne gases as a charge-changing gas in tandem accelerator, is analytically formulated by six cross sections on the basis of the rate equation. These cross sections for electron-loss process from a cluster were estimated with use of the time-dependent quantum theory, assuming binary collision treatment and independent electron model. The cross sections in ϕ1 estimated for MeV/atom Cn (n = 2–4) clusters were found to show the sub-linear dependence on the atom number n. The gas-pressure dependences of the ϕ1 values calculated for the 2.5 MeV C2+ and C4+ ions using Ne gas are in good agreement with the experimental data obtained at TIARA.

Energy Threshold of the Atomic and Cluster Sputtering of Some Elements under Bombardment with Cs, Rb, and Na Ions

The results obtained using secondary-ion mass spectrometry under the bombardment of Si(111) crystals are analyzed. It is shown that, as bombarding ion masses are increased in the case of the same energy, the relative yield of cluster ions and the threshold cluster sputtering energy increase with increasing number n; n is the number of atoms in the cluster (n = 1–5). The threshold sputtering energies are determined for atoms and clusters. The experimental data on the threshold particle sputtering energies are compared with the data of molecular-dynamics simulation.

Association and Proton Transfer Reactions With a Series of Organic Molecules.

In this study, we present reactions of with a series of analytes (A): acetone (C3H6O), methyl vinyl ketone (C4H6O), methyl ethyl ketone (C4H8O), and eight monoterpene isomers (C10H16) using a Selective Reagent Ionization Time-of-Flight Mass Spectrometer (SRI-ToF-MS). We studied the ion-molecule reactions at collision energies of 55 and 80 meV. The ketones, having a substantially lower proton affinity than NH3, produce only cluster ions (A) in detectable amounts at 55 meV. At 80 meV, no cluster ions were detected meaning that these adduct ions are formed by strongly temperature dependent association reactions. Bond energies of cluster ions and proton affinities for most monoterpenes are not known and were estimated by high level quantum chemical calculations. The calculations reveal monoterpene proton affinities, which range from slightly smaller to substantially higher than the proton affinity of NH3. Proton affinities and cluster bond energies allow to group the monoterpenes as a function of the enthalpy for the dissociation reaction . We find that this enthalpy can be used to predict the (A) cluster ion yield. The present study explains product ion formation involving ion chemistry. This is of importance for chemical ionization mass spectrometry (CIMS) utilizing as well as (H2O) as reagent ions to quantitatively detect atmospherically important organic compounds in real-time.

Secondary ion emission from a GaAs single crystal upon bombardment with Bi m + cluster ions

Secondary-ion mass spectra and energy distributions upon bombarding a gallium arsenide single crystal using Bim+(m = 1–5) cluster ions with energies of 2–12 keV are investigated. The gallium cluster ion yield grew nonadditively with the number of atoms in the cluster projectiles. A quasi-thermal component found in the energy spectra of secondary Ga+ and Ga2+ ions is indicative of the occurrence of the thermal spike mode upon cluster ion bombardment. The quasi-thermal component in the yield of atomic Ga+ ions upon bombardment with Bi2+–Bi5+–ions is 35–75%.

Secondary Ion Emission from Water Ice at 10–130 K Induced by MeV N2+ Ions

Secondary ion emission from vapor-deposited water ice has been investigated in exploring the influence of phase transitions on the desorption yield of H2O ions, particularly, the hydronium H3O+ and the cluster ions (H2O)nH3O+. For 1.5 MeV N2+ ion bombardment on water ice at a temperature of ∼80 K, the sum of the yields of H3O+ and of the cluster ions represents 80% of the positive ion yield (i.e., 0.23 ions/impact) and 0.33% of the total H2O molecular sputtering yield. When the beam energy of N2+ is varied between 0.5 and 1.7 MeV, the shape of the cluster mass distributions does not change and the total cluster ion yield scales with the third power of the electronic energy loss. Time-of-flight mass spectra were taken during the warming up of the ice from 10 to 216 K. Up to 30 K, the cluster ion yields are approximately constant, whereas between 30 and 75 K, they decrease gradually by a factor 6.5 and remain constant until 130 K. This is in analogy with modifications in the ice structure: in the 38–78 K re...

Cluster size and velocity dependences of sputtering and secondary ion emission under gold cluster impact

Sputtering and ion emission rates have been measured from CsI and gold targets under the impact of Au n + ( n = 1–9) clusters at energies between 30 and 350 keV/atom. The two materials have similar behaviors in regard to the variations of the sputtering and anion emission yields with energy and cluster size. The sputtering and anion emission yields increase nonlinearly with the projectile size. The maximum anion yields are found at lower energies than the maximum sputtering yields which themselves occur substantially below the maximum energy losses. The variations with energy of the atomic ion yields differ from those of the cluster ion yields. The experimental results are in agreement with an ion emission from linear collision cascades and spike collisions, the relative contribution of these two processes depending on the size of the cluster projectile and of the emitted ion. In addition they show that the ion emission yield enhancements under cluster impact result from a more effective sputtering mechanism and not from an enhanced ionization of the ejected species.

A Comparative Study of Secondary Ion Emission from Water Ice under Ion Bombardment by Au+, Au3+, and C60+

Secondary ion emission from water ice has been studied using Au+, Au3+, and C60+ primary ions. In contrast to the gas phase in which the spectra are dominated by the (H2O)nH+ series of ions, the spectra from ice using all three primary ions are principally composed of two series of cluster ions (H2O)nH+ and (H2O)n+. Dependent on the conditions, the unprotonated series can dominate the spectra. Since in the gas phase (H2O)n+ is unstable with respect to the formation of the protonated ion series, the presence of the solid must provide a means to stabilize their formation. The cluster ion yields under Au+ bombardment are very low and can be understood in terms of sputtering on the borderline between linear cascade and thermal spike behavior. There is a 104 increase in yield across the whole spectrum compared to Au+ when Au3+ and C60+ species are used as primary ions. The character of the spectra differed between these two primary ions, but insights into the mechanism of secondary ion emission for both is discussed within an energy deposition framework provided by the fluid flow-based mesoscale energy deposition footprint (MEDF) model that predicts a cone-shaped zone of activation and emission. C60+ differs from Au3+ in that it delivers its energy closer to the surface, and it is argued this has consequences for the cluster ion distribution and yield. Increasing the ion dose by sputtering suppresses the yield of (H2O)n+ and increases the yield of the protonated ions in the small cluster region, whereas the yield in the large cluster regime is suppressed significantly. The three primary ions show rather different behavior, and this is discussed in the light of the sputtering models. Finally, negative ion spectra including cluster ions have been observed for the first time. C60+ delivers the highest yields, but these are less than 10 times the positive ion yields, probably because the O and OH fragment ions on which the clusters are based are easily neutralized by protons.

Secondary ion emission from Si bombarded with large Ar cluster ions under UHV conditions

Si was bombarded with size-selected 40 keV Ar cluster ions and positive secondary ions were measured using the time-of-flight technique under high and ultra-high vacuum (HV and UHV respectively) conditions. Si + ions were main species detected under the incidence of 40 keV Ar cluster ions, and the yields of Si cluster ions such as Si 4 + were also extremely high under both conditions. On the other hand, oxidized secondary ions such as SiO + were detected with high intensity only under the HV condition. The yield ratios of oxidized ions decreased in UHV to less than 1% of their values in HV. The effect of residual gas pressure on Si cluster ion yields is relatively low compared to oxidized ions, and the UHV condition is required to obtain accurate secondary ion yields.

TOF-SIMS characterisation of spark-generated nanoparticles made from pairs of Ir–Ir and Ir–C electrodes

This paper constitutes a comprehensive study on the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for determining the composition of spark-generated nanoparticles as well as for deriving information on their size. This kind of nano-sized matter is often used in animal studies to explore and quantify lung inflammatory response to inhalation or instillation of insoluble particles. Two iridium samples, Ir(#1) and Ir(#2), were generated with a pair of electrodes made of pure iridium. A mixed C(Ir) sample was produced with one electrode of carbon and the other of iridium. The primary particles of Ir had a size (“diameter”) Dp≤5nm, but they were agglomerated in chain-type aggregates with different mean electrical mobility diameter, depending on the spark frequency. The atomic fraction of Ir in the C(Ir) particles was less than 1% (Dp∼13nm). The generated particles were collected on cellulose acetate-nitrate filters. An Ir electrode with a polished surface served as a reference sample. SIMS analysis of negative secondary ions was performed under impact of 34keV Ar+. The mass spectra of the Ir samples revealed surprisingly high yields of cluster ions of the type IrkXY−, with k up to 20 and XY indicating combinations of oxygen, nitrogen and carbon. A detailed evaluation of cluster composition based on the isotopic pattern of Ir revealed a high oxygen content in the particles located at the surface of fresh samples. The true oxygen content of these “oxide” particles appears to be reduced by preferential sputtering. Depth dependent information was derived by sputter erosion in between a series of TOF mass spectra. Surprisingly, the (primary) oxide particles in Ir(#1) featured a very high cross-section for sputter ejection, reminiscent of monolayer removal. The particles uncovered by sputtering were much larger and mostly made of carbon-rich Ir particles (“carbides”), with the addition of some nitrogen. Combined with the high cluster ion yields, the results suggest that the Ir oxide was contained in flakes or chunks with sizes as small as 1nm or even less. The same or similar oxide species were present in Ir(#2), but piled up to thicker layers. Very similar Ir oxide and carbide particles were identified in the C(Ir) sample, but at a very low yield, roughly corresponding to the atomic fraction of Ir.

Is proton cationization promoted by polyatomic primary ion bombardment during time‐of‐flight secondary ion mass spectrometry analysis of frozen aqueous solutions?

Ion bombardment of pure water ice by Au+ monoatomic and Au3 + and C60 + polyatomic projectiles results in the emission of two series of water cluster ions-(H2O)n + and (H2O)nH+-with n ranging from 1 to >40. The cluster ion yields are very significantly higher under polyatomic ion bombardment than when using an Au+ primary ion. The yield of the protonated water species (H2O)nH+ is found to be enhanced by increasing ion fluence. C60 + bombardment results in a very dramatic increase in the (H2O)nH+ yield and decrease in the yield of (H2O)n +. Au3 + also significantly increased the yield of protonated species relative to the non-protonated but to a lesser extent than C60 +. Bombardment by Au+ also increased the yield of protonated species but to a very much smaller extent. The hypothesis that the protonated species may enhance the yield of [M+H]+ from solute molecules in solution has been investigated using two amino acids, alanine and arginine, and a nucleic base, adenine. The data suggest that the protons produced by the sputtering of water ice are depleted in the presence of these solutes and concurrently the yields of solute-related [M+H]+ and immonium secondary ions are greatly enhanced. These yield enhancements are analysed in the light of other possible contributors such as increased rates of sputtering under polyatomic beams and increased secondary ion yields as a consequence of solute dispersion. It is concluded that enhanced proton attachment is occurring in polyatomic sputtered frozen aqueous solutions.

Temperature dependence of secondary ion emission from tantalum produced by atomic and polyatomic gold projectiles

Temperature dependencies have been measured within a wide range of target temperatures of 300 K ≤ T ≤ 2400 K for secondary ion yields of Ta n +, Ta n O m +, Ta n Nb + and Ta n Au + ( n = 1–14, m = 1–3) under the bombardment of tantalum target with 12 keV atomic Au − and 18 keV polyatomic Au 3 − projectiles. It is demonstrated that yields of Ta n + ( n = 2–14) and Ta n Nb + ions increase with temperature for T ≤ 1700 K and then tend to become temperature independent. On the contrary, the yields of Ta n O m + and Ta n Au + ions slightly increase with temperature reaching their maxima in the range of 1000 K ≤ T ≤ 1500 K and then sharply decrease to zero at T ≈ 1700 and 2100 K, respectively. These trends are interpreted to indicate the redistribution of the sputtered flux between these different emission channels while sputtering conditions change with the target temperature. Oxygen presence on the surface at lower temperatures limits the yield of Ta n + clusters and stimulates that of Ta n O m +. Removing oxygen from the surface enhances the yield of Ta n + clusters and the disappearance of Ta n O m +. After clean surfaces are established in the range of 1700 K ≤ T ≤ 2400 K, the yield of the Ta n + and Ta n Nb + cluster ions becomes constant thus indicating that their ionization probability does not depend on the target temperature in this range. Some differences in the temperature dependencies obtained under the atomic and polyatomic ion bombardment are observed and interpreted as the indication of different efficiencies of the sputtering process since polyatomic projectiles sputter more material than atomic ones. This, in addition to better surface cleaning, enhances yields of cluster ions. For atomic ions Ta +, an additional emission channel, thermal evaporation/surface ionization, is identified at target temperatures T > 2300 K. No evaporated cluster ions are observed.

Cesium/xenon dual beam depth profiling with TOF-SIMS: measurement and modeling of M +, MCs +, and M 2Cs 2+ yields

Using Cs + in the TOF-SIMS dual beam mode offers a semi-quantitative solution to depth profiling. Specifically, the use of these alkali ions strongly increases negative ion yields, decreases the positive ones and allows the formation of MCs + and MCs 2 + clusters. Recently, Niehuis and Grehl [Proceedings SIMS XII (2000) 49] developed a new approach consisting of co-sputtering Xe and Cs in order to control the Cs surface concentration, thus allowing the optimization of elemental and cluster ion yields. We applied that technique on different well-defined samples (e.g. Si, SiO 2 and Al 2O 3) and we monitored positive ions (e.g. Si +, Al +, CsSi +, CsAl +, CsO +, Cs 2O +, Cs 2Si +, etc.) as a function of the sputtering beam Cs concentration. First, we observed the decrease of the elemental ions due to the work function lowering, as is predicted by the tunneling model. We then studied the behavior of the MCs + and the MCs 2 + clusters. The MCs + yield exhibits a maximum at a given Cs/Xe beam concentration ratio, depending on the studied element M and also on its chemical environment (e.g. Si and SiO 2), and on the energy of the Cs beam. In other words, it is hypothesized that this yield maximum is a consequence of the competition between the varying surface Cs coverage (direct concentration effect) and the decreasing ionization probability due to that varying Cs [Phys. Rev. Lett. 50 (1983) 127; Phys. Rev. B 29 (1984) 2311; K. Wittmaack, Proceedings SIMS VIII, (1992) 91]. Simple models based on the tunneling model were applied to interpret our results. The MCs 2 + signal behaves in a very different way. As shown by Gao [Y. Gao, Y. Marie, F. Saldi, H.N. Migeon, Proc. SIMS IX, (1994) 382], these clusters are predominant for electronegative elements and increase in a monotonous way with Cs beam concentration.

Cesium/xenon dual beam depth profiling with TOF-SIMS: measurement and modeling of M+, MCs+, and M2Cs2+ yields

Using Cs+ in the TOF-SIMS dual beam mode offers a semi-quantitative solution to depth profiling. Specifically, the use of these alkali ions strongly increases negative ion yields, decreases the positive ones and allows the formation of MCs+ and MCs2+ clusters. Recently, Niehuis and Grehl [Proceedings SIMS XII (2000) 49] developed a new approach consisting of co-sputtering Xe and Cs in order to control the Cs surface concentration, thus allowing the optimization of elemental and cluster ion yields. We applied that technique on different well-defined samples (e.g. Si, SiO2 and Al2O3) and we monitored positive ions (e.g. Si+, Al+, CsSi+, CsAl+, CsO+, Cs2O+, Cs2Si+, etc.) as a function of the sputtering beam Cs concentration. First, we observed the decrease of the elemental ions due to the work function lowering, as is predicted by the tunneling model. We then studied the behavior of the MCs+ and the MCs2+ clusters. The MCs+ yield exhibits a maximum at a given Cs/Xe beam concentration ratio, depending on the studied element M and also on its chemical environment (e.g. Si and SiO2), and on the energy of the Cs beam. In other words, it is hypothesized that this yield maximum is a consequence of the competition between the varying surface Cs coverage (direct concentration effect) and the decreasing ionization probability due to that varying Cs [Phys. Rev. Lett. 50 (1983) 127; Phys. Rev. B 29 (1984) 2311; K. Wittmaack, Proceedings SIMS VIII, (1992) 91]. Simple models based on the tunneling model were applied to interpret our results. The MCs2+ signal behaves in a very different way. As shown by Gao [Y. Gao, Y. Marie, F. Saldi, H.N. Migeon, Proc. SIMS IX, (1994) 382], these clusters are predominant for electronegative elements and increase in a monotonous way with Cs beam concentration.

Effects of sample preparation on ion yield in the study of inorganic salts by s-SIMS

The influence of sample preparation on the ion yield in s-SIMS for alkaline salts was investigated. Potassium chromate and permanganate, dissolved in water, have been spin-coated on silicon or gold substrate before being analyzed by ToF-SIMS under 15 keV 69 Ga + primary ion bombardment. Analysis of powders pressed on In foil was also performed. The results were taken as reference to allow comparison. Spin coating on Si substrate did not lead to a significant modification as compared to the reference. In contrast, the cluster ion yield was increased by at least one order of magnitude when a gold substrate was used, especially in the negative detection mode. Moreover, large cluster ions were observed in the high mass range (up to m/ z 1400) and anionized species were also detected.

Small cluster ions from source of negative ions by cesium sputtering

We investigated the delivery of small cluster ions using a source of negative ions by cesium sputtering (SNICS). The negative cluster ions of B n , C n , Si n , Co n , Cu n , Ge n , Au n , GeB n and SiB n have been extracted by SNICS. Adequate beam current of some small clusters was obtained by changing several parameters for cluster ion yield. After a comprehensive study of the operation parameters, such as target material selection, target geometry, sputtering voltage and current, the small cluster ion current can be increased by several orders of magnitude, with little change on the monomer ion yield.

Hydrogenated carbon clusters produced by highly charged ion impact on solid

The emission of small (hydrogenated) carbon cluster ions CnHm + (n =2-22) upon highly charged Xeq+ (q =20-44) impact on C84 surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation of a certain carbon cluster ion Cn + is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted as an entity in agreement with “equilibrium” and “shock wave” models.

Dependence of cluster ion emission from uranium oxide surfaces on the charge state of the incident slow highly charged ion

The cluster ion yields and cluster ion distribution for highly charged ion sputtering have been measured for a uranium oxide target for \(\), \(\) and \(\) incident ions. The cluster yields exhibit a power law dependence on the cluster size with exponents increasing from -4 to -2.4 with increasing primary ion charge from 44+ to 75+. The power law exponent is also correlated with the total sputter yield.

Cluster ion emission in the interaction of slow highly charged ions with surfaces

Cluster ion emission from a variety of surfaces upon impact of highly charged ions is investigated by time-of-flight secondary ion mass spectrometry. The yield of cluster ions as a function of cluster size for and surface follow a power law decline with exponent approaching the -2 limit of the ”equilibrium” and ”shock wave” cluster emission models. While the decline of the cluster ion emission with cluster size is an exponential decay for highly oriented pyrolytic graphite upon impact, the decline is more gradual than for impact, such that at the relative cluster yield is 1000 times higher.

Electronic sputtering process of SiO 2 under heavy ion bombardment

Secondary-ion yields have been measured for a SiO 2 target bombarded by Ag ions at impact energies of 1.4–5.3 MeV, where the electronic and nuclear stopping powers compete with each other. Singly charged cluster ions as well as multiply charged monoatomic ions were observed. Dominant species of the cluster ions were Si(SiO 2) x +, SiO(SiO 2) x + and SiO 2(SiO 2) x +. Oxygen rich positively charged species were not observed. All the cluster-ion yields were characterized by a power function of the electronic stopping power, of which exponent varies with cluster size. From this fact we can conclude that the cluster ions are produced directly through the collective process in the solid.