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.     

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.     

Multiple H 3 + fragment production in single collision of fast H n + clusters with He atoms

The production of H ₃ ⁺ ions resulting from single collisions of mass-selected ionic hydrogen clusters, H _n ⁺ (n=9, 25, 31), with helium at high velocity (1.55 times the Bohr velocity) has been studied. A strong double H ₃ ⁺ ion production resulting from one incident cluster is observed. Moreover, evidence for a triple H ₃ ⁺ fragment production is presented forn=25 and 31. Thus, in this energy range, the collision gives rise to multifragmentation processes. The formation of H ₃ ⁺ ions takes place in the fragmentation of the multicharged cluster resulting from the collision.     

Reaction dynamics of Na n + in collision with molecular oxygen

Reaction dynamics of sodium cluster ions, Na _n ⁺ (n = 2–9), in collision with molecular oxygen, O₂ was investigated by measuring the absolute dissociation cross sections and the branching fractions by using a tandem mass spectrometer equipped with several octapole ion guides. The mass spectrum of the product ions show that the dominant reaction channels are production of oxide ions, Na_kO_i (i =1, 2), and intact ions, Na _p ⁺ (p < n). With increase in the collision energy, the cross section for the production of the oxide ions decreased, while that for the production of the intact ions increased. The collision-energy dependences of the cross section for the oxide formation reveals that electron harpooning from the molecule to Na _n ⁺ preludes the oxideion formation. On the other hand, the collision-energy dependences of the cross sections for the intact ion formation is explained by a hard-sphere-collision model similar to the collisional dissociation of Na _n ⁺ by rare-gas impact.     

Charge exchange in alkali cluster collisions

Collisional charge exchange between mass selected alkali cluster ions and Cs has been studied and cross sections have been determined for the processes Na _n ⁺ + Cs and K _n ⁺ + Cs, withn=1–21 andn=1–14, respectively. A strong dependence of the cross sections on the energy defect as well as on cluster size and collision energy is found. The results are analysed by a coupled two state density matrix model, taking account of the relaxation of electronic amplitudes due to interaction with the nuclear motion in the cluster.     

Collision induced fragmentation of small sodium cluster ions

The basic mechanisms of collision induced fragmentation of small sodium cluster ions (Na _n ⁺ n < 9) at keV collision energy are investigated by measuring the velocity vectors of the two fragments employing a new type of coincidence experiment. The results suggest that in most of the cases the lost of one Na atom can be interpreted as relevant of an impulsive mechanism. On the other hand, the lost of one Na⁺ ion seems to require transition to an electronically excited state of the cluster.     

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.     

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.     

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.     

The Jellium—Atom quasimolecular model for the investigation of charge transfer in alkali-cluster-ion/alkali-atom interactions: the Na 19 + /Na system

The potential energy curves of the Na ₁₉ ⁺ /Na quasimolecular (Jellium—Atom)⁺ system determined by Guissani and Sidis [3] are used to calculate the vibrational energy levels of a sodium atom stuck to a Na ₁₉ ⁺ Jellium-like cluster and to determine excitation and charge transfer cross sections in Na ₁₉ ⁺ + Na collisions in the 0.2 10⁷ cm/s < v < 1.5 10⁷ cm/s velocity range.     

The two-centre shell model for the fission of metallic clusters

We calculate potential energies for charged and neutral jellium clusters which fragment in two pieces, in the framework of the liquid drop model plus Strutinsky shell corrections obtained from the two-centre harmonic oscillator. We consider the symmetric fragmentation of Na ₄ ⁺ ₂ ⁺ , Na ₁ ⁺ ₈ ⁺ , and Na₃₈. Good agreement is found with results obtained by self-consistent methods, which are much more involved.     

Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, He _n ⁺ ,n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He ₄ ⁺ and He ₅ ⁺ also one alternative structure was considered. The low-lying excited states can be classified into two categories: the electronic transition can occur either within the central He ₂ ⁺ or He ₃ ⁺ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≈0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He ₂ ⁺ and He ₃ ⁺ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He ₂ ⁺ or He ₃ ⁺ are placed as “chromophores” into the centre of a larger He _n ⁺ cluster. The large difference in the vertical excitation energy of the strong transition should enable an experimental decision of the question whether the cluster ions have He ₂ ⁺ or He ₃ ⁺ cores.     

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⁺ .     

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.     

Analysis of optical absorption spectra of transition metal cluster ions by the spin-polarized DV-Xα method

Optical absorption spectra of cobalt cluster ions, Co _n ⁺ , and vanadium cluster ions, V _n ⁺ , were analyzed by a theoretical calculation based on the spin-polarized DV-Xα method, and their electronic and geometric structures were obtained. Relative absorption cross section associated with each electronic transition was calculated; the calculation enables a qualitative comparison of calculated spectrum with a measured one not only in its transition energy but also in its intensity profile. This analysis shows that Co ₄ ⁺ , Co ₃ ⁺ , and V ₄ ⁺ have, respectively, a tetrahedral structure with a bond distance of 2.00Å, an equilateral triangle with a bond distance of 2.30Å, and a distorted tetrahedral structure with five bonds having a distance of 2.34 Å and one of 2.89Å. The differences in the population between majority and minority spins (spin-difference) evaluated from the electronic structure thus obtained were 2.0, 1.7, and zero per atom in Co ₃ ⁺ , Co ₄ ⁺ , and V ₄ ⁺ , respectively. These spin differences indicate a ferromagnetic and an antiferromagnetic spin-coupling in the cobalt and vanadium cluster ions, respectively.     

Dynamic solvation effects on surface-impact dissociation of I 2 - (CO2)n

Surface-impact dissociation of I ₂ ^- (CO₂)n was studied by a molecular dynamics simulation in comparison with the experimental results. The branching fraction, ? _dis, of the I ₂ ^- dissociation was calculated as a function of the parent cluster size, n. This computational result reproduces the experimental one. We calculated a number of the I ₂ ^- dissociation events starting from given initial orientations. The most favorable molecular orientation obtained supports the wedge effect in which a CO₂ molecule located at the waist position of the I ₂ ^- core ion splits the I ₂ ^- bond as if a piece of wood is split by a mechanical thrust against a wedge. The time profile of the wedge action calculated for the I ₂ ^- (CO₂) impact shows that more than 20 % of the collision energy is converted to the vibrational energy of the I ₂ ^- .     

Bare phosphorus and binary phosphide cluster ions generated by laser ablation

Both positive and negative phosphorus cluster ions were generated from the laser ablation of a red phosphorus sample. The mass distribution of phosphorus cluster ions was found to be very sensitive to the power density of the ablation laser. The P ₇ ⁺ species exhibits the highest signal intensity in the recorded mass spectra of bare phosphorus cluster cations, as does P ₅ ^- among the anions. Their special structural stability can be attributed to their planar configuration and their aromatic character. As the phosphorus cluster size increases, the odd/even alternation of the signal intensity becomes more pronounced. For the P _n ⁺ species with n > 24, the relative abundance varies in the order of 8 and P _n ⁺ with n = 8k + 1 (k = 3–11) are more intense than their neighbors. For comparison, some binary phosphide cluster ions, including C_nP _m ^- , Si_nP _m ^- , B_nP _m ⁺ and Al_nP _m ⁺ , were produced as well. The mass distribution of binary phosphide cluster ions changes with different components. From analysis of the recorded mass spectra of the phosphide cluster ions, the larger clusters may be in a polyhedral configuration and tend to have all valence electrons paired.     

Observation and investigation of metastable decay series of (N2) n + cluster ions

N₂ cluster ions are produced by electron impact ionization of a supersonic N₂ cluster beam and analyzed with a double focussing sector field mass spectrometer. It is found that metastable N₂ cluster ions lose more than one N₂ molecule in the μs time regime and decay predominantly via sequential series (N₂) _n ⁺ *→(N₂) _n?1 ⁺ *→...→N ₂ ⁺ , evaporating a single monomer in each of these successive decay steps. The metastable decay rates determined in detail for cluster sizes 2≤n≤6 andn=20 lie between 1 and 10⁶s^?1. These rates(i) depend strongly on the time elapsed after ion formation and on the respective parent cluster ion size, and(ii) exhibit a quasiperiodic pattern in magnitude.     

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 ⁺ .     

On ionisation induced unimolecular dissociation of sodium clusters

Fragmentation of sodium cluster ions (Na _x ⁺ ,x<42) was studied via photoionisation of neutral precursors. Expansions of metal vapor out of cylindrical and conical nozzles yielded supersonic beams with differing cluster compositions. Measurements of photoionisation efficiency curves in the 3–6 eV range for both types of expansion allow quantitative separation of direct ionisation and unimolecular dissociation contributions to specific ion signals. Data for Na ₈ ⁺ and Na ₇ ⁺ are analysed to yield lower limits on bond energies. Results obtained for larger clusters are also discussed.