Polyatomic Projectiles Research Articles

Yields and ionization probabilities of sputtered In n particles under atomic and polyatomic Au m− ion bombardment

The emission of neutral and charged atoms and clusters from a polycrystalline indium surface under bombardment with 5 and 10 keV Au, Au 2, Au 3 and Au 5 projectiles was investigated. Single photon laser postionization was utilized for the detection of sputtered neutral particles. Secondary ions were detected without the laser under otherwise exactly the same experimental conditions. The relative cluster yields were found to be enhanced under polyatomic projectile bombardment, more so the larger the number of atoms in the sputtered cluster. The ionization probability strongly increases with increasing cluster size, but is essentially independent of the projectile impact energy. At a fixed impact energy, the ionization probability of sputtered monomers was found to decrease with increasing number of constituent gold atoms per projectile, but there was no detectable effect for sputtered dimers and larger clusters.

Molecular secondary ion formation under cluster bombardment: A fundamental review

A brief review is given regarding the application of cluster ion beams as desorption probes in molecular SIMS. The general observation is that the efficiency of secondary ion formation, particularly that of complex molecular species, is significantly enhanced if polyatomic projectiles are employed instead of atomic species. Apart from the sensitivity increase, cluster bombardment also appears to allow for molecular depth profiling studies without the accompanying damage accumulation normally associated with atomic projectiles. A few fundamental aspects are addressed in an attempt to highlight the physics behind these observations. It appears that much of the benefit associated with cluster bombardment is connected to the fact that these projectiles give access to very high sputter yields which are not accessible with atomic primary ions.

Kinetic energy distributions of neutral In and In 2 sputtered by polyatomic ion bombardment

Kinetic energy distributions of neutral In monomers and In 2 dimers sputtered from a polycrystalline indium surface under bombardment with 5 keV/atom Au 1 − and Au 2 − projectiles have been investigated by means of laser postionization time-of-flight mass spectrometry. Results show that 5 keV Au 1 bombardment leads to results in full compliance with linear cascade sputtering theory. For polyatomic ion bombardment, we find a clear transition to a collisional spike dominated emission process. The spike contribution appears as a low-energy part in the sputtered flux which increases with increasing projectile nuclearity and energy. We show that, the velocity spectrum associated with the low-energy contribution is virtually identical for sputtered monomers and dimers. This finding has important implications with respect to the particle emission mechanism under polyatomic projectile bombardment.

Caesium sputter ion source compatible with commercial SIMS instruments

A simple design for a caesium sputter cluster ion source compatible with commercially available secondary ion mass spectrometers is reported. This source has been tested with the Cameca IMS 4f instrument using the cluster Si n − and Cu n − ions, and will shortly be retrofitted to the floating low energy ion gun (FLIG) of the type used on the Cameca 4500/4550 quadruple instruments. Our experiments with surface characterization and depth profiling conducted to date demonstrate improvements of analytical capabilities of the SIMS instrument due to the non-additive enhancement of secondary ion emission and shorter ion ranges of polyatomic projectiles compared to atomic ions with the same impact energy.

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.

Semi-quantitative characterisation of binary salt mixtures with static secondary ion mass spectrometry (S-SIMS)

The use of static secondary ion mass spectrometry for the semi-quantitative characterisation of major components by means of molecule specific ion signals has been investigated for binary salts. The calibration of the peak intensities as a function of the local concentration has been based on reference mixtures. The preparation of the mixtures has been proven to be a major bottleneck. Five different approaches have been tried out. Fast freezing with subsequent cryo-lyophylisation is the preferred method to obtain adequate molecular mixtures for the empirical calibration of relative peak intensities in S-SIMS as a function of the local concentration. The applicability has been verified in two different laboratories and three instrumental S-SIMS configurations. Monoatomic as well as polyatomic primary ions have been used. Specifically, comparative work involved the use of Ga + primary ions in a time-of-flight (TOF) S-SIMS and ReO 4 − polyatomic projectiles in an ion trap and a triple quadrupole instrument, used in the single mass selection mode. In all cases, the local content can be determined with an experimental uncertainty of 10–20% using the molecular (adduct) ion signal intensities in either the positive or negative ion detection mode.

Sputtering of indium usingAumprojectiles: Transition from linear cascade to spike regime

We have investigated the yields and emission velocity distributions of neutral In atoms and In2 dimers sputtered from a pure indium surface under bombardment with Aum − m=1,2 ,3 projectile ions. It is shown that 5-keV Au1 bombardment leads to results in full compliance with linear cascade sputtering theory. All polyatomic projectiles are found to generate an additional low-energy contribution to the sputtered flux, which increases with increasing projectile nuclearity and energy and completely dominates the spectra under 10-keV Au3 bombardment. Analysis shows that this contribution cannot be explained in terms of thermal spike sputtering models. Instead, the results indicate a spike emission mechanism, which closely resembles a free expansion of a supercritically heated subsurface volume.

Use of monoatomic and polyatomic projectiles for the characterisation of polylactic acid by static secondary ion mass spectrometry

The application of polyatomic primary ions is a strongly developing branch of static secondary ion mass spectrometry (S-SIMS), since these projectiles allow a significant increase in the secondary ion yields to be achieved. However, the different limitations and possibilities of certain polyatomic primary ions for use on specific functional classes of samples are still not completely known. This paper compares the use of monoatomic and polyatomic primary ions in S-SIMS for thin layers of polylactic acid (PLA), obtained by spin-coating solutions on silicon wafers. Bombardment with Ga+, Xe+ and SF5+ primary ions allowed the contribution of the projectile mass and number of atoms in the gain in ion yield and molecular specificity (relative importance of high m/z and low m/z signals) to be assessed. Samples obtained by spin-coating solutions with increasing concentration showed that optimal layer thickness depended on the primary ion used. In comparison with the use of Ga+ projectiles, the yield of structural ions increased by a factor of about 1.5 to 2 and by about 7 to 12 when Xe+ and SF5+ primary ion bombardment were applied, respectively. A detailed fragmentation pattern was elaborated to interpret ion signal intensity changes for different projectiles in terms of energy deposition and collective processes in the subsurface, and the internal energy of radical and even-electron precursor ions.

Comparison of primary monoatomic with primary polyatomic ions for the characterisation of polyesters with static secondary ion mass spectrometry

Static secondary ion mass spectrometry (S-SIMS) emerges as one of the most adequate methods for the surface characterisation of polymers with an information depth of essentially one monolayer. The continuing search for increased analytical sensitivity and specificity has led to exploring the use of polyatomic primary ions as an alternative to the traditionally applied monoatomic projectiles. As part of a systematic investigation on polyatomic bombardment of organic and inorganic solids, this paper focuses on selected polyesters. Mass spectra and ion yields are compared for layers deposited on silicon wafers by spincoating solutions with different concentrations of poly(epsilon-caprolactone) (PCL), poly(butylene adipate) (PBA) and poly(ethylene adipate) (PEA). Accurate mass measurements have been used to support the assignment of the ions and link the composition of the detected ions to the analyte structure. Use of polyatomic projectiles increases the yield of structural ions with a factor of +/-15, +/-30 and +/-10 for PCL, PBA and PEA, respectively, in comparison to bombardment with Ga+ primary ions, while the molecular specificity is improved by the detection of additional high m/z ions.

Comparison of Ga+ and SF5+ primary ions for the molecular speciation of oxysalts in static secondary ion mass spectrometry (S-SIMS)

Speciation of inorganic compounds by means of static secondary ion mass spectrometry (S-SIMS) aims at deriving the molecular composition of the analyte in the solid. The recent use of polyatomic primary ions, instead of the traditionally applied monatomic or diatomic projectiles, potentially offers an increase in secondary ion intensity and an increase in molecular specificity. This paper focuses on the comparison of SF5+ and Ga+ primary ion bombardment for the molecular speciation of inorganic oxysalts. The polyatomic primary ions are found to increase the total ion yield by a factor of 4–10. Furthermore, the relative intensities of the most structure-specific signals are increased in comparison with the low m/z peaks under SF5+ bombardment relative to Ga+. Although the formation of oxide fragments is promoted with polyatomic projectiles, the mass spectra significantly gain in information concerning the molecular specificity.

Organic surfaces excited by low-energy ions: atomic collisions, molecular desorption and buckminsterfullerenes

This article reviews the recent progress in the understanding of kiloelectronvolt particle interactions with organic solids, including atomic displacements in a light organic medium, vibrational excitation and desorption of fragments and entire molecules. This new insight is the result of a combination of theoretical and experimental approaches, essentially molecular dynamics (MD) simulations and secondary ion mass spectrometry (SIMS). Classical MD simulations provide us with a detailed microscopic view of the processes occurring in the bombarded target, from the collision cascade specifics to the scenarios of molecular emission. Time-of-flight SIMS measures the mass and energy distributions of sputtered ionized fragments and molecular species, a precious source of information concerning their formation, desorption, ionization and delayed unimolecular dissociation in the gas phase. The mechanisms of energy transfer and sputtering are compared for bulk molecular solids, organic overlayers on metal and large molecules embedded in a low-molecular weight matrix. These comparisons help understand some of the beneficial effects of metal substrates and matrices for the analysis of molecules by SIMS. In parallel, I briefly describe the distinct ionization channels of molecules sputtered from organic solids and overlayers. The specific processes induced by polyatomic projectile bombardment, especially fullerenes, are discussed on the basis of new measurements and calculations. Finally, the perspective addresses the state-of-the-art and potential developments in the fields of surface modification and analysis of organic materials by kiloelectronvolt ion beams.

Self sputtering yields of silver under bombardment with polyatomic projectiles

Molecular dynamics (MD) have been employed to model self sputtering yields of an Ag(111) surface under bombardment with Agm (m=1,2,3,4) projectiles. It is shown that the effect of projectile orientation on the sputter yield decreases strongly with increasing m and becomes practically negligible under Ag4 impact. For polyatomic projectiles we observe a nonlinear yield enhancement which is discussed in terms of the collisional spike model of Sigmund and Claussen. It is shown that the MD simulation provides a unique method to determine the essential input parameters T0 and 〈ρ02〉 entering that model.

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.

Molecular dynamics simulations to explore the role of mass matching in the keV bombardment of organic films with polyatomic projectiles

Molecular dynamics simulations of the bombardment of a organic film on a gold substrate with Au and Au 2 projectiles have been performed in order to understand the role of mass matching in the mechanisms for energy transfer. There will be an effective energy transfer from the projectile to the substrate atoms if the atom impacts the substrate unimpeded by molecules in the organic film. When a projectile atom hits an organic molecule, the process of fragmentation absorbs its incident kinetic energy and it enters the substrate region with little energy left. With Au 2, there is a higher probability that one or more of the projectile atoms will hit a bare portion of the surface and initiate collision cascades with sufficient energy to result in ejection. Atoms in the top layer of the substrate lift off stable, intact molecules as they move out of the crystal.

Sputtering simulations of organic overlayers on metal substrates by monoatomic and clusters projectiles

This paper reviews our recent work on computer simulations of monoatomic and cluster bombardment of metal and organic surfaces. The investigated surfaces are irradiated with keV monoatomic (C, Ar, Ga) and polyatomic (C 60) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The analysis of the results reveals that the C 60 ion beam enables to perform chemical imaging with higher sensitivity, better depth resolution, and lower contamination than monoatomic projectiles with similar kinetic energy. For monoatomic projectiles, the development of a linear collision cascade is the predominant mechanism responsible for the ejection of particles. In contrast, strongly nonlinear, collective processes occur during C 60 bombardment. These nonlinear processes lead to the enhanced removal of material and the formation of a macroscopic crater. This paper presents theoretical insight into possible mechanisms responsible for the observed behavior with the emphasis on the phenomena important to the SIMS/SNMS community.

Molecular dynamics simulations to explore the role of mass matching in the keV bombardment of organic films with polyatomic projectiles

Molecular dynamics simulations of the bombardment of a organic film on a gold substrate with Au and Au 2 projectiles have been performed in order to understand the role of mass matching in the mechanisms for energy transfer. There will be an effective energy transfer from the projectile to the substrate atoms if the atom impacts the substrate unimpeded by molecules in the organic film. When a projectile atom hits an organic molecule, the process of fragmentation absorbs its incident kinetic energy and it enters the substrate region with little energy left. With Au 2 , there is a higher probability that one or more of the projectile atoms will hit a bare portion of the surface and initiate collision cascades with sufficient energy to result in ejection. Atoms in the top layer of the substrate lift off stable, intact molecules as they move out of the crystal.

Characterization of surface structure by cluster coincidental ion mass spectrometry

Coincidental ion mass spectrometry, CIMS, is a technique in which spatial and/or chemical relationships are investigated by collection and analysis of coincidental ion emission events. Practical application of CIMS is dependant upon two factors. One is that the primary ion be selected so as to maximize coincidental ion, CI, yields. Polyatomic projectiles are best suited for this type of analysis because, in certain cases, they not only enhance conventional secondary ion yields but CI yields as well compared to that of equal velocity atomic projectiles. The second factor is that data collection and analysis be as efficient as possible. Experiments were run in an event-by-event bombardment/detection mode. All secondary ions from each impact were recorded with subsequent offline analysis of coincidental ion emission. We report here the combination of cluster SIMS (22 keV Au 3 + projectiles) with a fresh approach to data analysis for evaluation of α-zirconium phosphate in both gel and crystalline states. Results presented here show that it is possible to distinguish between these two phases although the stoichiometry of the compound is the same.

Sputtering of indium using polyatomic projectiles

We have investigated the emission of neutral and charged particles from a polycrystalline indium surface under bombardment with Au, Au 2, Au 3, Au 5 and AuCs 2 projectiles with energies between 5 and 10 keV. Sputtered neutral species were postionized by means of saturated single photon ionization, thus characterizing the respective partial sputtering yields. The ionization probability of the emitted species was addressed by measuring the respective secondary ion signals under the same experimental conditions. It is seen that the relative cluster yields are enhanced under polyatomic projectile bombardment, while the ionization probability of In atoms seems to be largely unaffected.

Cluster secondary ion mass spectrometry: an insight into “super-efficient” collision cascades

Some collision cascades, induced by keV polyatomic projectiles, result in the emission of multiple secondary ions. Such coincidental ion emission implies that the ejecta originate from molecules co-located within a nano-volume perturbed by a single projectile impact, thus providing an approach for the chemical analysis of nano-domains. The relevance for the chemical analysis of nano-structures depends on the effectiveness of the projectile to cause co-emission of two or more analytically significant secondary ions. The experiments involved Au n m+ projectiles (1≤ n≤4; m=1, 2) on phenylalanine targets. We have measured the yields for events in which multiple ions were detected as a function of projectile characteristics. The data show that some collision cascades are “super-efficient”. For example, in a four-ion detection event, the yield for the phenylalanine molecular ion is two orders of magnitude larger from Au 4 impacts than from equal velocity Au + projectiles.

Simultaneous ejection of two molecular ions from keV gold atomic and polyatomic projectile impacts.

We present the first experimental data on the simultaneous ejection of two molecular ions from the impact of Au(+)(n) (1< or =n< or =4) with energies ranging between 17 and 56 keV. The yields from single phenylalanine (Ph) emission, coemission of two Ph ions, and emission of the Ph dimer were measured. Large increases (1 to 2 orders of magnitude) in coemitted ion yields were observed with increasing projectile energy and complexity. Correlation coefficients were calculated for the coemission of two Ph ions; their behavior suggests differences in emission pathways for bombardment by atomic and polyatomic projectiles.