Mass-selected Cluster Ions Research Articles

Water-Hydrocarbon Interactions in Anionic Pyrene Monohydrate.

Interactions between water and polycyclic aromatic hydrocarbons are essential in many aspects of chemistry, from interstellar and atmospheric processes to interfacial hydrophobicity and wetting phenomena. Despite their growing importance, the intermolecular potentials of the water-hydrocarbon interactions are underdeveloped compared to the water-water potentials, and there are similarly few experimental probes that are sensitive to the details of the water-hydrocarbon potential. We present a combined experimental and computational study of anionic pyrene monohydrate, one of the simplest water/hydrocarbon clusters. The action spectrum in the OH region of the mass-selected cluster ion provides a rigorous benchmark for intermolecular potentials and computational methodologies. We identify missing intermolecular interactions and shortcomings in conventional dynamics calculations by comparing experimental data to density functional theory and classical molecular dynamics calculations. Kinetic trapping is prevalent, even for one water molecule and one pyrene molecule, leading to slow equilibration in conventional molecular dynamics calculations, even on nanosecond time scales and at low temperatures (50 K). At constant energy, temperature fluctuations for the pair of molecules are substantial. Immersing the system in a bath of soft spheres and employing parallel tempering alleviates kinetic trapping and dampens temperature fluctuations, bringing the system closer to the thermodynamic limit. With such augmented sampling, a simple, flexible water model reproduces the line width and the asymmetric broadening of the symmetric OH stretching mode, which we assign to spectral diffusion. In the OH stretching region, dynamics calculations predict a more intense antisymmetric peak than experiments observe but do not predict the bimodal split symmetric peak that the experiments show. Our work suggests that electronic polarization, missing in the empirical force field, is responsible for the first discrepancy and that quantum nuclear effects, captured neither in density functional theory nor in classical dynamics, may be responsible for the second.

Isolating the Contributions of Specific Network Sites to the Diffuse Vibrational Spectrum of Interfacial Water with Isotopomer-Selective Spectroscopy of Cold Clusters.

Decoding the structural information contained in the interfacial vibrational spectrum of water requires understanding how the spectral signatures of individual water molecules respond to their local hydrogen bonding environments. In this study, we isolated the contributions for the five classes of sites that differ according to the number of donor (D) and acceptor (A) hydrogen bonds that characterize each site. These patterns were measured by exploiting the unique properties of the water cluster cage structures formed in the gas phase upon hydration of a series of cations M+·(H2O)n (M = Li, Na, Cs, NH4, CH3NH3, H3O, and n = 5, 20-22). This selection of ions was chosen to systematically express the A, AD, AAD, ADD, and AADD hydrogen bonding motifs. The spectral signatures of each site were measured using two-color, IR-IR isotopomer-selective photofragmentation vibrational spectroscopy of the cryogenically cooled, mass selected cluster ions in which a single intact H2O is introduced without isotopic scrambling, an important advantage afforded by the cluster regime. The resulting patterns provide an unprecedented picture of the intrinsic line shapes and spectral complexities associated with excitation of the individual OH groups, as well as the correlation between the frequencies of the two OH groups on the same water molecule, as a function of network site. The properties of the surrounding water network that govern this frequency map are evaluated by dissecting electronic structure calculations that explore how changes in the nearby network structures, both within and beyond the first hydration shell, affect the local frequency of an OH oscillator. The qualitative trends are recovered with a simple model that correlates the OH frequency with the network-modulated local electron density in the center of the OH bond.

Gas-phase N2 adsorption on mass-selected hydrogen-bonded cluster ions

Gas-phase N2 adsorption on protonated tryptophan and dipeptides, hydrogen-bonded with methanol, was investigated as a model for molecular cloud formation using a tandem mass spectrometer containing a cold ion trap. The number of N2 molecules adsorbed on H+Trp(CH3OH)m and H+Trp-Gly(CH3OH)m (m = 0–3) decreased with increasing number of methanol molecules. The surface of the gas-phase ions was covered by the CH3 groups of methanol upon solvation, which rendered the solvated cluster ions inactive toward N2 adsorption. The lack of N2 adsorption on unsolvated H+Gly-Trp was ascribed to its compact conformation, which was driven by the intramolecular interactions.

Removal of Organic Contamination from Graphene with a Controllable Mass-Selected Argon Gas Cluster Ion Beam

Since the discovery of graphene, organic surface contamination has posed difficulties both for accurate characterization of the material’s intrinsic properties and for development of graphene devices. In this study, we investigate the use of a mass-selected argon gas cluster ion beam for removing organic contaminants, such as residual poly(methyl methacrylate) (PMMA), from single-layer graphene. The influence of cluster ion size, energy, and ion dose has been investigated to identify the important factors for minimizing damage to the graphene layer during the cleaning process. Raman spectroscopy was used to analyze the variation in the D-peak and G-peak intensity ratio, an indicator of damage to the graphene lattice, as a function of ion beam dose and kinetic energy per atom (E/n) in the cluster ions. Using a mass-selected 5 keV Ar5000 beam with a dose of 5.0 ions/nm2, we were able to demonstrate removal of polymer residue and other carbonaceous material from single-layer chemical vapor deposition (CVD)-g...

(Keynote) Effects of Catalytic Site Size on Activity of Model Electrodes Prepared By Mass-Selected Cluster Deposition

Model size-selected electrodes were prepared by deposition of mass-selected metal cluster ions on electrode substrates, cleaned and characterized in ultra-high vacuum (UHV). After analysis by X-ray photoelectron spectroscopy (XPS), the electrodes were transferred and mated to a small electrochemical cell without air exposure, and then studied by cyclic voltammetry. Electrodes with small Ptn (n = 1 - 16) on glassy carbon and indium tin oxide substrates were studied. The ethanol oxidation reaction (EOR), the oxygen reduction reaction (ORR), and carbon oxidation by water were all found to depend strongly on cluster size, with peak currents varying by factors of up to 1000. For oxidation reactions, the activity was found to be anticorrelated with Pt core level binding energies measured by XPS, i.e., the dominant factor appears to be the electronic properties of the metal centers. For ORR, the dominant factor appears to be the physical size of the active site for O2 binding.

Formation of Covalently Bonded Polycyclic Hydrocarbon Ions by Intracluster Polymerization of Ionized Ethynylbenzene Clusters

Here we report a detailed study aimed at elucidating the mechanism of intracluster ionic polymerization following the electron impact ionization of van der Waals clusters of ethynylbenzene (C8H6)n generated by a supersonic beam expansion. The structures of the C16H12, C24H18, C32H24, C40H30, and C48H36 radical cations resulting from the intracluster ion-molecule addition reactions have been investigated using a combination of mass-selected ion dissociation and ion mobility measurements coupled with theoretical calculations. Noncovalent structures can be totally excluded primarily because the measured fragmentations cannot result from noncovalent structures, and partially because of the large difference between the measured collision cross sections and the calculated values corresponding to noncovalent ion-neutral complexes. All the mass-selected cluster ions show characteristic fragmentations of covalently bonded molecular ions by the loss of stable neutral fragments such as CH3, C2H, C6H5, and C7H7. The population of the C16H12 dimer ions is dominated by structural isomers of the type (C6H5)-C≡C-CH(•+)CH-(C6H5), which can grow by the sequential addition of ethynylbenzene molecules, in addition to some contributions from cyclic isomers such as the 1,3- or 1,4-diphenyl cyclobutadiene ions. Similarly, two major covalent isomers have been identified for the C24H18 trimer ions: one that has a blocked cyclic structure assigned to 1,2,4- or 1,3,5-triphenylbenzene cation, and a second isomer of the type (C6H5)-C≡C-C(C6H5)═CH-CH(•+)CH-(C6H5) where the covalent addition of further ethynylbenzene molecules can occur. For the larger ions such as C32H24, C40H30, and C48H36, the major isomers present involve the growing oligomer sequence (C6H5)-C≡C-[C(C6H5)═CH]n-CH(•+)CH-(C6H5) with different locations and orientations of the phenyl groups along the chain. In addition, the larger ions contain another family of structures consisting of neutral ethynylbenzene molecules associated with the blocked cyclic isomer ions such as the diphenylcyclobutadiene and triphenylbenzene cations. Low-energy dissociation channels corresponding to evaporation of ethynylbenzene molecules weakly associated with the covalent ions are observed in the large clusters in addition to the high-energy channels corresponding to fragmentation of the covalently bonded ions. However, in small clusters only high-energy dissociation channels are observed corresponding to the characteristic fragmentation of the molecular ions, thus providing structural signatures to identify the product ions and establish the mechanism of intracluster ionic polymerization.

Microsolvation of the acetanilide cation (AA+) in a nonpolar solvent: IR spectra of AA+–Lnclusters (L = He, Ar, N2; n ≤ 10)

Infrared photodissociation (IRPD) spectra of mass-selected cluster ions of acetanilide (N-phenylacetamide), AA(+)-Ln, with the ligands L = He (n = 1-2), Ar (n = 1-7), and N2 (n = 1-10) are recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I-III) ranges of AA(+) in its (2)A'' ground electronic state. Cold AA(+)-Ln clusters are generated in an electron impact ion source, which predominantly produces the most stable isomer of a given cluster ion. Systematic vibrational frequency shifts of the N-H stretch fundamentals (νNH) provide detailed information about the sequential microsolvation process of AA(+) in a nonpolar (L = He and Ar) and quadrupolar (L = N2) solvent. In the most stable AA(+)-Ln clusters, the first ligand forms a hydrogen bond (H-bond) with the N-H proton of trans-AA(+) (t-AA(+)), whereas further ligands bind weakly to the aromatic ring (π-stacking). There is no experimental evidence for complexes with the less stable cis-AA(+) isomer. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra. The calculated binding energies of De(H) = 720 and 1227 cm(-1) for H-bonded and De(π) = 585 and 715 cm(-1) for π-bonded Ar and N2 ligands in t-AA(+)-L are consistent with the observed photofragmentation branching ratios of AA(+)-Ln. Comparison between charged and neutral AA((+))-L dimers indicates that ionization switches the preferred ion-ligand binding motif from π-stacking to H-bonding. Electron removal from the HOMO of AA(+) delocalized over both the aromatic ring and the amide group significantly strengthens the C[double bond, length as m-dash]O bond and weakens the N-H bond of the amide group.

Synthesis and Characterization of Gold Clusters Ligated with 1,3-Bis(dicyclohexylphosphino)propane.

In this multidisciplinary study the chemical reduction synthesis of novel gold clusters in solution was combined with high-resolution analytical mass spectrometry (MS) to gain insight into the composition of the gold clusters and how their size, ionic charge state, and ligand substitution influences their gas-phase fragmentation pathways. Ultrasmall cationic gold clusters ligated with 1,3-bis(dicyclohexylphosphino)propane (dcpp) were synthesized for the first time and introduced into the gas phase using electrospray ionization (ESI). Mass-selected cluster ions were fragmented by employing collision-induced dissociation (CID) and the product ions were analyzed using MS. The solutions were found to contain the multiply charged cationic gold clusters Au9 L4 3+ , Au13 L5 3+ , Au6 L3 2+ , Au8 L3 2+ , and Au10 L4 2+ (L=dcpp). The gas-phase fragmentation pathways of these cluster ions were examined systematically by employing CID combined with MS. In addition, CID experiments were performed on related gold clusters of the same size and ionic charge state but capped with 1,3-bis(diphenylphosphino)propane (dppp) ligands containing phenyl functional groups at the two phosphine centers instead of cyclohexane rings. It is shown that this relatively small change in the molecular substitution of the two phosphine centers in diphosphine ligands (C6 H11 versus C6 H5 ) exerts a pronounced influence on the size of the species that are preferentially formed in solution during reduction synthesis as well as the gas-phase fragmentation channels of otherwise identical gold cluster ions. The mass spectrometry results indicate that in addition to the length of the alkyl chain between the two phosphine centers, the substituents at the phosphine centers also play a crucial role in determining the composition, size, and stability of diphosphine-ligated gold clusters synthesized in solution.

Velocity distribution of mass-selected nano-size cluster ions

The paper reports on an experimental investigation of the ion velocity distribution function (IVDF) for mass-selected copper cluster ions with a diameter in the range 5–10 nm. The cluster ion beam is produced by a dc magnetron-based gas aggregation source. The measurements are carried out by means of a quadrupole mass filter combined with a basic retarding field analyser. The most probable cluster velocities are in the range 80–180 m s−1; decreasing with cluster size and increasing with buffer gas (Ar) flow. Experimental results are compared with the theoretical model of Smirnov et al. Noted discrepancies are explained by the interaction of clusters with buffer gas atoms outside the aggregation chamber during the passage from the orifice towards the detector. This interaction is not included in the theoretical model. Both positively and negatively charged clusters are investigated. The density ratio of positively to negatively charged clusters in the beam is evaluated from the measured IVDFs and compared with the theoretical model.

Collision-Induced Dissociation Studies on Fe(O2)n+ (n = 1–6) Clusters: Application of a New Technique Based on Crossed Molecular Beams

Gas-phase oxygen-rich iron oxide clusters Fe(O2)n(+) (n = 1-6), are produced in a molecular beam apparatus. Their stability and structure are investigated by measuring the fragmentation cross-sections from collision-induced-dissociation experiments. For this purpose, two different techniques have been employed. The first one relies on the measurement of the fragments resulting after collisional activation and subsequent dissociation of mass selected cluster ions in a molecular beam passing through a cell filled with noble gas atoms. The second one is a new approach that we introduce and is based on crossed molecular beams to measure the fragmentation cross-sections, in a more efficient manner without mass selection of the individual clusters. The cross-sections obtained with the different techniques are compared with each other as well as with theoretical ones resulting from the application of a simple geometrical projection model. Finally, the general trends observed are compared with results for other Fe-molecule clusters available in the literature.

Microsolvation of the 4‐Aminobenzonitrile Cation (ABN+) in a Nonpolar Solvent: IR Spectra of ABN+Ln (L=Ar and N2, n≤4)

IR photodissociation (IRPD) spectra of mass-selected cluster ions of 4-aminobenzonitrile (ABN(+)) with up to four Ar and N2 ligands are recorded over the spectral range of the N-H stretching vibrations (ν(s/a)) of ABN(+) in its (2)B1 ground electronic state. ABN(+)-L(n) clusters are produced in an electron impact cluster ion source, which predominantly generates the most stable isomer of a given cluster ion. Vibrational frequency shifts of ν(s/a) provide information about the sequential microsolvation process of ABN(+) in a nonpolar solvent. In ABN(+)-(N2)n, the first two ligands fill a first subshell by forming hydrogen bonds to the acidic protons of the amino group, whereas further ligands bind more weakly to the aromatic ring (π bonds). Although the preferred cluster growth sequence in ABN(+)-Ar(n) is similar, several isomers are observed because the hydrogen bonds are only slightly stronger than the π bonds. Quantum chemical calculations at the M06-2X/aug-cc-pVTZ level confirm the cluster growth sequence derived from the IR spectra and provide further details of the intermolecular potential. The calculated binding energies of D0(H)=532 and 895 cm(-1) for hydrogen-bonded and D0(π)=512 and 530 cm(-1) for π-bonded Ar and N2 ligands are consistent with the observed photofragmentation branching ratios. Comparison between ABN(+)-L(n) and the corresponding clusters with the aniline cation demonstrates that the NH protons of the amino group become slightly more acidic upon H→CN substitution at the para position. Comparison between charged and neutral ABN((+))-L dimers indicates that ionization switches the preferred ion-ligand binding motif from π to hydrogen bonding.

Metastable fragmentation of photoionized styrene cluster ions: Implications for cluster ion structure

Styrene clusters are produced by supersonic expansion of a styrene/Ar mixture, photoionized with 193 or 248nm light, and analyzed in a reflectron time-of-flight mass spectrometer. The styrene cluster ions Stn+, n≤10, show an alternation in intensity that favors cluster ions containing an even number of molecules. Mass-selected cluster ions, n=2–8, are allowed to undergo metastable fragmentation, and the fragment ions are separated from the parent ions in the reflectron. For n=4–8, fragmentation proceeds exclusively by elimination of a series of styrene molecules. Loss of as many as five styrene molecules is observed for n=6–8. For n=4, the St+ fragment signal is very small, and for n=3 it is nonexistent. A robust signal for St2+ indicates the major product channel in both cases. These results are interpreted in terms of formation of covalently bound styrene dimer ions within the ionized clusters, especially n=3 and 4. The present fragmentation results are also discussed in relation to earlier work. Internal energy considerations show that there should be sufficient energy present to cause fragmentation of the structures proposed in the prior work, but no fragments corresponding to such fragmentation are observed. Thus the present work does not support the cluster ion structures proposed earlier, though it also does not completely rule them out.

Reactions within fluorobenzene-ammonia heterocluster ions: experiment and theory.

Reactions occurring within gas phase fluorobenenze-ammonia heterocluster cations (FC(6)H(5)-(NH(3))(n=1-4)) have been studied through the use of a triple quadrupole mass spectrometer as well as employing density functional theory (DFT). Collision induced dissociation (CID) experiments were conducted in which mass selected cluster ions are accelerated into a cell containing argon gas and the resulting products then subsequently mass analyzed. Two dominate reaction channels are observed. The first is simple evaporative loss of neutral ammonia from the cluster ion. The second involves a substitution reaction occurring within the cluster ion to form the aniline cation, C(6)H(5)NH(2)(+), where the reactivity was found to vary as a function of cluster size. DFT calculations have been performed to both help analyze the structure and the reactivity of these cluster ions. Pronounced differences in activation energies were found that provide an explanation for the observed variation of reactivity as a function of cluster size. An ad hoc model based upon the Arrhenius equation was developed to fit both the experimental collision energy dependence of the reaction and the observed lowering of the reaction barrier to aniline formation as a function of cluster size.

Photofragmentation of mass-selected titanium oxide cluster cations

In this paper we present the photofragmentation spectra of mass-selected positive titanium oxide cluster ions TixOy+. The clusters are generated by the combination of laser ablation of a titanium target and the supersonic nozzle expansion of oxygen and are detected by time-of-flight mass spectrometry. Small clusters are mass-selected and photodissociated at a wavelength of 308 nm. The recorded photofragmentation spectra indicate that for all parent clusters the main fragment is TiO+ and, in some cases, Ti2O3+ is also observed. This is consistent with the assumption that small TixOy+ clusters are built from a TiO+ core with TiO2 building blocks.

Zeeman splitting and Faraday rotation associated with Mn+ 7PJ ← 7S3 absorption: Photon-trap spectroscopy and quantum-theory analysis

A novel experimental method is presented for detection of a trace of optical absorption of mass-selected ions stored in an ion trap. Photon-trap spectroscopy (a generalized scheme of cavity ring-down spectroscopy) is the key technique, where a storage lifetime of photons in a high-Q cavity provides a ppm-level high sensitivity. Experiment was performed on the manganese ion, Mn+, to record 7P2,3,4 ← 7S3 absorption spectra at around 260 nm. Zeeman splitting and Faraday rotation of the polarization plane were observed as well in the presence of a magnetic field. The high-resolution spectra show complex structures, which are fully interpreted by the quantum theory incorporating hyperfine interaction due to the Mn nuclear spin (I = 5/2). The present technique provides a novel approach to electronic and magnetic structures of mass-selected atomic, molecular, and cluster ions.

Microscopic studies of atom–water collisions

The influences of water molecules surrounding biological molecules during irradiation with heavy particles (atoms, ions) are currently a major subject in radiation science on a molecular level. In order to elucidate the underlying complex reaction mechanisms, we have initiated a joint experimental and theoretical investigation with the aim to make direct comparisons between experimental and theoretical results. As a first step, studies of collisions of a water molecule with a neutral projectile (C atom) at high velocities ( ≥ 0.1 a.u.), and with a charged projectile (proton) at low velocities ( ≤ 0.1 a.u.) have been studied within the microscopic framework. In particular, time-dependent density functional theory (TDDFT) was applied to the valence electrons and coupled non-adiabatically to molecular dynamics (MD) for ionic cores. Complementary experimental developments have been carried out to study projectile interactions with accelerated ( ≤ 10 keV) and mass-selected cluster ions. The first size distributions of protonated water cluster ions H +(H 2O) n ( n = 2–39) produced using this new apparatus are presented.

Reactions within p-difluorobenzene/methanol heterocluster ions: a detailed experimental and theoretical investigation.

The reactivity of p-difluorobenzene/methanol cluster ions has been investigated by using triple quadrupole mass spectrometry and DFT calculations. The present study was performed in light of a recent investigation of p-difluorobenzene/methanol (P = F-C(6)H(4)-F and M = CH(3)OH) heterocluster ions where the solvent-catalyzed formation of p-fluoroanisole (A = CH(3)O-C(6)H(4)-F) was observed in P(M)(2)(+) clusters and not in PM(+) clusters. The results of our mass selected cluster ion study and theoretical calculations confirm that a single extra molecule of methanol can lower the reaction activation energy barrier in agreement with previous work for smaller clusters (PM(+) and P(M)(2)(+)). However, we also observe that P(M)(3)(+) and P(M)(4)(+) clusters undergo evaporative loss of neutral methanol to establish the P(M)(2)(+) cluster before reacting. P(M)(n>4)(+) clusters are capable of reacting through multiple pathways, in some cases generating a 1,4-dimethoxybenzene (B = CH(3)O-C(6)H(4)-OCH(3)) product via two separate substitution reactions within the same cluster ion. DFT calculations were employed to model the structures of the parent cluster ions, and transition state calculations were used to evaluate the activation energy for the p-fluoroanisole-forming substitution reaction. The calculations suggest that the reaction proceeds through a transition state containing a six-member hydrogen-bonded ring involving a reacting methanol and a second methanol that significantly lowers the activation energy.

Cluster reactivity experiments: employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions.

Mass spectrometry is the most widely used tool in the study of the properties and reactivity of clusters in the gas phase. In this article, we demonstrate its use in investigating the molecular-level details of oxidation reactions occurring on the surfaces of heterogeneous catalysts via cluster reactivity experiments. Guided ion beam mass spectrometry (GIB-MS) employing a quadrupole-octopole-quadrupole (Q-O-Q) configuration enables mass-selected cluster ions to be reacted with various chemicals, providing insight into the effect of size, stoichiometry, and ionic charge state on the reactivity of catalyst materials. For positively charged tungsten oxide clusters, it is shown that species having the same stoichiometry as the bulk, WO(3)(+), W(2)O(6)(+), and W(3)O(9)(+), exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene (C(3)H(6)), suggesting the formation of propylene oxide (C(3)H(6)O), an important monomer used, for example, in the industrial production of plastics. Furthermore, the same stoichiometric clusters are demonstrated to be active for the oxidation of CO to CO(2), a reaction of significance to environmental pollution abatement. The findings reported herein suggest that the enhanced oxidation reactivity of these stoichiometric clusters may be due to the presence of radical oxygen centers (W-O) with elongated metal-oxygen bonds. The unique insights gained into bulk-phase oxidation catalysis through the application of mass spectrometry to cluster reactivity experiments are discussed.

Isolating the spectra of cluster ion isomers using Ar-“tag” -mediated IR-IR double resonance within the vibrational manifolds: Application to NO2−⋅H2O

We demonstrate a method for isolating the vibrational predissociation spectra of different structural isomers of mass-selected cluster ions based on a population-labeling double resonance scheme. This involves a variation on the "ion dip" approach and is carried out with three stages of mass selection in order to separate the fragment ion signals arising from a fixed-frequency population-monitoring laser and those generated by a scanned laser that removes population of species resonant in the course of the scan. We demonstrate the method on the Ar-tagged NO(2) (-)H(2)O cluster, where we identify the spectral patterns arising from two isomers. One of these structures features accommodation of the water molecule in a double H-bond arrangement, while in the other, H(2)O attaches in a single ionic H-bond motif where the nominally free OH group is oriented toward the N atom of NO(2) (-). Transitions derived from both the NO(2) (-) and H(2)O constituents are observed for both isomers, allowing us to gauge the distortions suffered by both the ion and solvent molecules in the different hydration arrangements.

Cluster size dependences of bombardment effects using mass-selected gas cluster ion beams

Bombardment effects induced by mass-selected gas cluster ion beams (GCIB) were studied. In this study, a mass-selected GCIB system using a strong permanent magnet was employed and the cluster size effects of sputtering by GCIB irradiation were studied. The cluster size varied between 500 and 10,000 atoms/cluster. The sputtering yields of Au with mass-selected Ar-GCIB decreased with an increase in the cluster size because of a decrease in the energy per atom. The threshold energy for the sputtering of Au by Ar-GCIB was approximately 0.6eV/atom, which was significantly lower than that by Ar monomer ions (20eV). Although the energy per atom of the cluster ions was low, dense energy deposition occurred near the surface and surface Au atoms were effectively ejected. When the number of Ar atoms transferred by the clusters and the energy per atom were equivalent, the number of sputtered Au atoms was also same.