Low-energy decay pathways of doubly charged charged silver clusters Ag n 2+ n = 9–24)

The low-energy dissociation channels of mass selected silver cluster ions Ag _n ²⁺ (n = 9–24) are determined by collision induced dissociation (CID) in a Penning trap. While all clusters of the size n ≥ 17 evaporate neutral monomers, most smaller clusters undergo asymmetric fission of the form Ag _n ²⁺ → Ag _n?3 ⁺ + Ag _{3} ⁺ . However, Ag ₁₅ ²⁺ and Ag ₁₁ ²⁺ emit monomers which indicates shell or odd-even effects. The observed fragmentation pathways are different from previous reports of measurements with sputtered Ag _n ²⁺ .     

Shell correction study of fission of doubly charged silver clusters

Fission of doubly charged silver clusters is investigated by the method of shell corrections. The following fission events are considered: Ag ₂₂ ²⁺ → Ag _n ⁺ + Ag _{22 ?n} ⁺ , (n=11, 10, 9, 8); Ag ₂₁ ²⁺ → Ag _n ⁺ + Ag _{21 ?n} ⁺ , (n=10, 9, 8, 7); Ag ₁₈ ²⁺ → Ag _n ⁺ + Ag _{18 ?n} ⁺ , (n=9, 8, 7, 6). It is found that the shell correction energy is comparable to or larger than the deformation energy of the liquid drop. Threshold energies for the fission events are calculated and compared with the experimental abundance spectra obtained by Katakuse et al. (1990). Correspondence between the calculated threshold energies with the shell corrections and the experimental abundance is very good, showing products from lower threshold fission channels yield more abundance. The threshold energies without the shell corrections are almost constant irrespective of the fission channels and cannot explain the experimental abundance. Abundance of some products are too small to be accounted for only by the threshold energies. The low abundance of those products may be explained by the presence of competing fission channels that have similar minimal energy paths. It is found in fission of Ag ₁₈ ²⁺ that the shell correction overwhelms the Coulomb energy and the fission channel to Ag₈ + Ag ₁₀ ²⁺ is preferred over the fission channel to Ag ₈ ⁺ + Ag ₁₀ ⁺ .     

Photoelectron spectroscopy of transition metal-sulfur cluster anions

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The M_nS _m ^? cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (n _max) is generally expressed as n _max =2m ? 1 in each series of the number of S atoms (m). For Fe?S and Mn?S, it was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag?S, the EAs of Ag₁S_m are small and around 1 eV, whereas those of Ag_nS_m (_n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag₂S unit is preferentially formed with increasing the number of Ag atoms. For Fe?S and Mn?S, the PES spectra of Fe_nS _m ^? /Mn_nS _m ^? show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to Fe_nS _n ^? /Mn_nS _n ^? has little effect on the electronic property of Fe_nS_n/Mn_nS_n. The PES spectra imply that the Fe_nS_n cluster is the structural framework of these clusters, as similarly as the determined structure of the Fe_nS_n cluster in nitrogenase enzyme.     

Formation mechanism of nanostructured Ag films from Ag2O particles using a sonoprocess

Nanostructured Ag films composed of nanoparticles and nanorods can be formed by the ultrasonication of ethanol solutions containing Ag₂O particles. The present work examined the formation process of these films from ethanol solutions by two different agitation methods, including ultrasonication and mechanical stirring. The mass-transfer process from Ag₂O particles to ethanol solvent is accelerated by the mechanical effects of ultrasound. Ag⁺ ions and intermediately reduced Ag clusters were released into the ethanol. These Ag⁺ ions and Ag clusters provide absorption bands at 210, 275 and 300 nm in UV-vis spectra. These bands were assigned to the absorption of Ag⁺, Ag ₄ ²⁺ and Ag_n (n?≈?3). The Ag_n clusters that readily grow to become Ag nanoparticles were formed due to the surface reaction of Ag₂O particles with ethanol under ultrasonication. The reactions of Ag⁺ ions in ethanol to form Ag nanomaterials (through the formation of Ag ₄ ²⁺ clusters) were also accelerated by ultrasonication.     

Electron impact ionization and dissociation of neutral and charged fullerenes

Three different types of electron impact ionization experiments have been performed, involving neutral and charged C₆₀ and C₇₀. 1) We have determined absolute partial ionization cross sections for formation of parent ions C ₆₀ ^z+ and C ₇₀ ^z+ in charge states up to z = 4, and of singly and multiply charged fragments of size n ≥ 44 and n ≥ 50 from C₆₀ and C₇₀ neutral precursors, respectively. 2) Previous appearance energy measurements of C₇₀ have been improved and extended to z = 5; ionization energies are found to depend linearly on the charge state of the precursor, in agreement with theoretical predictions. 3) A beam of mass selected C ₆₀ ²⁺ has been crossed with an intense electron beam; the induced reactions (fragmentation, post- ionization, and dissociative post-ionization) have been analyzed.     

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.     

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.     

Metastable decay of rare gas cluster ions: mechanism and kinetics

Metastable decay of cluster ions has been discovered only recently. It was noted that one has to take this metastable decay into account when using mass spectrometry to probe neutral clusters, because ion abundance anomalies in mass spectra of rare gas and molecular clusters are caused by delayed metastable evaporation of monomers following ion production. Moreover, it was found that(i) the individual metastable reaction rates k depend strongly on cluster size and cluster ion production pathways and that(ii) there exists experimental evidence (k=k(t)) and a theoretical prediction that a given mass selected cluster ion generated by electron impact ionization of a nozzle expansion beam will comprise a range of metastable decay rates. In addition, it was discovered that metastable Ar cluster ions which lose two monomers in the μs time regime decay via sequential decay series Ar _n ⁺ *→Ar _n?1 ⁺ *→Ar _n?2 ⁺ * with cluster sizes 7≤n≤10 andn=3 (similar results were obtained recently in case of N₂ cluster ions). Conversely, the dominant metastable decay channel of Ar ₄ ⁺ * into Ar ₂ ⁺ was found to proceed predominantly via a single step fissioning process.     

Stability of multiply charged silver clusters

Using the gasaggregation technique it is possible to generate metal clusters in narrow size distributions and to vary their mean size by adjusting the cell parameters. The high intensity of this source allows to detect besides singly charged clusters also multiply charged ones. Ag _n ²⁺ and Ag _n ³⁺ are observed forn≧9 andn≧31, respectively; i.e. at values well below the critical sizes reported for spheres.     

Applying ZEKE-spectroscopy to clusters

In this paper we describe the application of a special form of electron photodetachment spectroscopy, zero electron kinetic energy (ZEKE) spectroscopy, to the study of cold metal cluster anions. From this study we have obtained vibrational and thus structural information of several neutral and charged clusters, e.g. Au ₂ ^?/0 , Ag ₃ ^?/0 , Sn ₂ ^?/0 and Al ₃ ^?/0 . In addition, in separate measurements of the relative photodetachment cross section of Au ₂ ^? , we have discovered sharp resonances which are due to electronic autodetachment from a short lived excited state.     

Photoemission from tin and lead cluster anions

Photoelectrons from mass-identified jet-cooled tin and lead cluster anions (Sn _n ^? , Pb _n ^? ) are detached by ultraviolet laser light (hν=3.68 eV). The photoelectron energy spectra give the detachment energies of ground state cluster anions (electron affinities) as well as excitation energies of neutral clusters in the geometry of the anions. The energy spectra for Sn _n ^? are dominated by flat thresholds with ann-dependence similar to that of other group IV clusters. In contrast, for Pb _n ^? we find pronounced narrow lines close to threshold, generally followed by a 0.3–1.4 eV gap which indicates closed-shell behaviour of Pb _n ^? for nearly alln.     

Formation of negatively-charged cluster ions in thermal energy collisions with state-selected rydberg atoms

Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO₂) _k ^? (4≦k≦25) and (O₂) _k ^? (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO₂) _m and (O₂) _m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s ³ P _{2, 0}) Penning ionization. For both CO ₂ ^? and O₂-clusters, the negative and the positive ion mass spectra are very different. For (CO₂) _k ^Emphasis>/? cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF ₆ ^? formation in collisions with SF₆. For (O₂) _k ^? cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O ₄ ^? as dominant species).     

Quantized losses from charged nitrogen clusters

Metastable decay of (N₂) _n ⁺ , formed in a supersonic jet and ionized by electron impact, has been analyzed forn≤50. The probability for decay of (N₂) _n ⁺ into (N₂) _n?x ⁺ , plotted versusx, exhibits pronounced oscillations. The “period” of these oscillations increases with increasing precursor sizen, but converges to an average value of approximately 4.7 beyondn=25.     

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.     

Sputtered metal cluster ions: Unimolecular decomposition and collision induced fragmentation

Cluster ions are produced by ion bombardment of thick metal targets and mass selected in a Wien filter. The unimolecular decomposition of Al _n ⁺ , Cu _n ⁺ , Mo _n ⁺ , W _n ⁺ , and Pb _n ⁺ is investigated under UHV conditions. The time evolution of the decay allows a glimpse into the cluster formation/fragmentation process. Highly excited metal cluster ions decompose mainly by evaporating single neutral atoms with rates reaching 100%. The collision induced fragmentation (CIF) of stable mass selected metal cluster ions in a low pressure Ar and O₂ gas target will be compared to the unimolecular decay.     

Electron attachment to oxygen and oxygen/ozone clusters studied in a high-resolution crossed beams experiment

Highly monochromatized electrons (with 30 meV FWHM) are used in a crossed beams experiment to investigate electron attachment to oxygen clusters (O₂)_n at electron energies from approximately zero eV up to 2 eV. At energies close to zero the attachment cross section for the reaction (O₂)_n +e → O ₂ ^? varies inversely with the electron energy, indicative of s-wave electron capture to (O₂)_n. Peaks in the attachment cross section present at higher energies can be ascribed to vibrational levels of the oxygen anion. The vibrational spacings observed can be quantitatively accounted for. In addition electron attachment to mixed oxygen/ozone clusters has been studied in the energy range up to 4 eV. Despite the initially large excess of oxygen molecules in the neutral clusters the dominant attachment products are undissociated cluster ions (O₃) _m ^? including the O ₃ ^? monomer while oxygen cluster ions (O₂) _n ^? appear with comparatively low intensity.     

Novel metal cluster complexes synthesized by matrix deposition of mass-selected clusters

Metal complexes produced by depositing size selected Fe and Ag cluster cations in N₂ and O₂ matrices respectively are studied by infrared spectroscopy. Unknown species such as Fe(N₂)x, Fe₃ (N₂)x and Ag₃(O₂)x are observed. The IR spectra of Ag⁺, Ag ₂ ⁺ and Ag ₉ ⁺ in excess O₂ indicate that no complexes involving molecular oxygen are formed. However, the strong silver cluster UV-visible absorptions detected in Ar matrices disappear in the oxygen matrices, suggesting that silver-oxygen complexes are formed with dissociated oxygen.     

ECP-CI study of electronic structure and geometry of small neutral and charged Ag n clusters; Predictions and interpretation of measured properties

The ground state geometries of small neutral Ag _n (n=2–9) and charged Ag _n ^± (n=2–9) clusters have been determined in the framework of the SCF procedure employing a relativistic pseudopotential accounting for core-valence correlation effects (RECP-CVC). Similarities and differences between neutral and charged clusters have been found. Large scale CI for 5s electrons only has been carried out for determining stabilities, ionization potentials (IP) and vertical detachment energies (VDE) of anions. A comparison between predicted and measured observables allows for the tentative structural assignments. In addition, the low lying energies of excited states for the neutral species at the anionic geometries have been calculated to account fully for geometrical and spectroscopic assignment to the photodetachment measurements.     

Photoelectron spectroscopy of silicon and germanium-fluorine binary cluster anions (SinF m − , GenF m − )

Electronic properties of silicon-fluorine and germanium-fluorine cluster anions (Si_nF _m ^? n = 1–9, m = 1–3, Ge_nF _m ^? ; n =1–9, m = 1–3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon/germanium rod in an He carrier gas mixed with a small amount of SiF₄ or F₂ gas. Comparison between photoelectron spectra of Si_nF^?/Ge_nF^? and Sin /Gen (n = 4–9) gives the insight that the doped F atom can remove one electron from the corresponding Sin _n ^? /Ge _n ^? cluster without any serious rearrangement of Sin/Gen framework, because only the first peak of Si _n ^? /Ge _n ^? , corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping