Multi electron capture processes in slow collisions between X<Superscript>7+</Superscript> (X = N,O and Ne) ions with C<Subscript>60</Subscript>

Electron capture processes in collision between slow X⁷⁺ (X = N, O and Ne) ions and C₆₀ fullerene have been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of the multicharged C₆₀ ^r+ recoil ions and their fragments C_m ⁱ⁺ and the final charge state of the outgoing projectiles X^(q-s)+ ( ). The collision velocity is about 0.4 a.u. The partial cross-sections σ_r ^s , corresponding to r electrons transferred to the projectile with only s electrons stabilized, have been measured. Cross-sections for collisions “inside” and those “outside” the C₆₀ cage have been separated by analyzing the kinetic energy of the outgoing projectile. The mean final charge state for frontal collisions has been measured to 3.1, 2.6 and 2.5 for N⁷⁺, O⁷⁺ and Ne⁷⁺ respectively. These results show the importance of the core effect on the stabilisation processes of captured electrons.     

Fragmentation of neutral Cn clusters (n = 9): experimental and theoretical investigations

We report on experimental and theoretical efforts designed to understand the fragmentation of small carbon clusters. Experimentally, a new detection system for high velocity fragments has been recently developed allowing the fragmentation of high velocity clusters to be totally recorded [1]. Results for the branching ratios of deexcitation of C₅ and C₉ formed by electron capture in high velocity collisions are presented. Theoretically, the dissociation dynamics of C₅ has been investigated using a kinematical model based on the statistical theory of Weisskopf. In this model various structural quantities (geometries, dissociation energies, harmonic frequencies), are required for both the parent cluster and the fragments. They have been calculated within DFT and coupled-cluster formalisms for C_n up to n = 9. In all cases, a strong correlation between measured branching ratios and calculated dissociation energies is observed.     

Doubly charged clusters - neutral atom charge transfer: the role of the Coulombic repulsion

We have studied experimentally the collisional charge transfer between a neutral atom and a multicharged metal-atom cluster. The charge transfer cross section measured for Na ₃₁ ^{+ +} + Cs is in the range of 400 ?². The time-of-flight mass analysis of the singly charged collision products demonstrates that an energy of about 0.5 eV is deposited in the cluster fragment during the charge transfer collision. This effect can be interpreted as a charge transfer to an excited state of the metal cluster. The measured cross section for Na ₃₁ ^{+ +} + Cs is larger than the one for Na ₃₁ ⁺ + Cs collisions. This difference between these two systems is due to the existence, for the first one, of a Coulombic repulsion term in the collision output channel. Received 24 October 2000     

Combined experimental and theoretical study of small aluminum oxygen clusters

We report a combined experimental and computational investigation of small Al_nO_m species (n ≤20, m ≤ 12), produced in a laser vaporization cluster source. The oxygen content in the clusters was tuned by varying the oxygen concentration in the carrier gas. Ionization energies are bracketed using different ionizing photon energies in the energy range between 5.37 and 7.89 eV. Among the singly doped Al_nO species, Al₃O and Al₁₅O are found to have relatively low ionization energies, which can be related to the magic character of the corresponding cations. Peculiarly low ionization energies also are observed for specific oxygen rich species (m > 1), suggesting the formation of ionically bound subunits. The structures and ionization energies of singly doped Al_nO^0,+ (n = 1 - 7) clusters were determined using density functional theory (B3LYP/6-311+G(d)). Electronic supplementary material Supplementary Online Material     

Intensity and detuning dependence of fine structure changing collisions in a 85 Rb magneto-optical trap

In an experimental study, the multi-ionisation of metallic clusters (Na_n) has been analysed in collisions with light ions in low charge states (H⁺, He⁺, He²⁺, O³⁺) at collision velocities below 1 a.u. Cluster ions are produced in charge states up to 5+. The average charge of the nano-particles is found to increase linearly with the variation of projectile velocity and the square of the effective projectile charge, well in agreement with the electronic stopping power of the bulk material. A fraction of 50% to 30% of the total projectile energy loss (decreasing with velocity) is transferred into vibrational modes in good agreement with recent theoretical predictions. Received 8 November 2000 and Received in final form 26 January 2001     

Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters

Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO ₂) _q ⁻ ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO₂) _N () clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO ₂) _{N −1}CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO ₂) _{N −1}CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster. Received 11 January 2000 and Received in final form 10 April 2000     

Experimental evidence for molecular ultrafast dissociation in O<Subscript>2</Subscript> clusters

Resonant Auger spectra of O₂ clusters excited at the O1s edge are reported. After excitation to the repulsive 1s^-13σ^* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular ultrafast dissociation processes within the clusters or on their surface.     

High energy collisions of protonated water clusters

We have measured attenuation cross sections and fragmentation cross sections for protonated water clusters H(H₂O)_n ⁺ (n = 1 to 100) colliding with noble gas atoms (He and Xe) at a laboratory energy of 50 keV. In collisions with He, a transparency effect in the attenuation cross section was observed. For the case of fragmentation in collisions with Xe, a strong enhancement of small clusters was observed which we attribute to multifragmentation. Received 30 November 2000     

The influence of O and C doping on the ionization potentials of Li-clusters

The influence of doping of Li-clusters by electronegative O and C atoms on the ionization potentials was investigated. Experimentally, we report ionization potentials for bare Li_n clusters deduced from photoionization efficiency spectra. The values are compared with the results for Li_nO and Li_nC clusters. Observed differences are largely attributed to a quantum size effect caused by the segregated molecular part around the impurity, which changes the electron work function. Theoretically, the Fermi and exchange-correlation energies which enter the work function, are calculated in the frame of the augmented plane wave (APW) method by taking explicitly into account the presence of the molecular core. The other contribution to the work function, the moment of the double layer at the cluster surface, is computed by solving the corresponding Poisson's equation. Received 9 September 1999 and Received in final form 7 February 2000     

Hyperthermal cluster-surface scattering

Using molecular-dynamics simulation, we study the processes occurring after impact of clusters on a rigid wall. Comparing the impact of model clusters consisting of 13 atoms, or of 13 diatomic molecules with varied bond strength, the systematics in the results of the collision process are investigated. Four regimes of impact-induced cluster fragmentation are identified: intact reflection, shattering into large fragments, complete fragmentation, and molecule dissociation. The effect of the number of degrees of freedom activated in the collision on the translational and internal energies of the reflected fragments is discussed in detail. As a rule, with increasing number of degrees of freedom which can be activated in the collision, the translational energy sinks. On the other hand, for weak intramolecular bonding, intramolecular vibrations are easily excited at small impact energies, reducing the resulting translational energy. The presence of even a very weak attractive well epsilon_w at the surface has a major influence on the sticking behavior of the clusters — and hence also on the absolute reflected energies — even at impact energies E₀ ≫ epsilon_w.     

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M _n ⁺ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and v_m, the maximum cross-section and the corresponding velocity. For small size clusters (), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and v_m parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures. Received 13 April 2000 and Received in final form 29 June 2000     

Cold Ar 3 + generated by ionization of argon clusters in large helium clusters: Temperature dependence of the photofragmentation

Ionized argon clusters were generated by electron impact ionization of neutral argon clusters embedded in large neutral helium clusters. Photofragmentation spectroscopy of Ar ₃ ⁺ and Ar ₃ ⁺ He produced in this way demonstrates the strong influence of vibrational excitation on the photodissociation dynamics, and indicates the low internal energy of the latter cluster.     

Predictions of geometrical structures and ionization potentials for small barium clusters Ba

The geometrical structure of ground state Ba_n clusters (n =2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2( n =2-6), DFT (LSDA)( n =2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations as well as some isomers have been investigated. The sizes n =4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron, the pentagonal bipyramid and the icosahedron. The growth behavior from Ba₇ to Ba₁₃ appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of Ba₁₃ which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated for n =2-5 at the CI level are seen to be in quite good agreement with recent measures. Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998     

Chirality, defects, and disorder in gold clusters

Theoretical and experimental information on the shape and morphology of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. An effective theoretical approach to determine the lowest-energy configuration (global minimum) and the structures of low energy isomers (local minima) of clusters is to combine genetic algorithms and many-body potentials (to perform global structural optimizations), and first-principles density functional theory (to confirm the stability and energy ordering of the local minima). The main trend emerging from structural optimizations of bare Au clusters in the size range of 12-212 atoms indicates that many topologically interesting low-symmetry, disordered structures exist with energy near or below the lowest-energy ordered isomer. For example, chiral structures have been obtained as the lowest-energy isomers of bare Au₂₈ and Au₅₅ clusters, whereas in the size-range of 75-212 atoms, defective Marks decahedral structures are nearly degenerate in energy with the ordered symmetrical isomers. For methylthiol-passivated gold nanoclusters [Au₂₈(SCH₃)₁₆ and Au₃₈(SCH₃)₂₄], density functional structural relaxations have shown that the ligands are not only playing the role of passivating molecules, but their effect is strong enough to distort the metal cluster structure. In this work, a theoretical approach to characterize and quantify chirality in clusters, based on the Hausdorff chirality measure, is described. After calculating the index of chirality in bare and passivated gold clusters, it is found that the thiol monolayer induces or increases the degree of chirality of the metallic core. We also report simulated high-resolution transmission electron microscopy (HRTEM) images which show that defects in decahedral gold nanoclusters, with size between 1-2 nm, can be detected using currently available experimental HRTEM techniques.     

Metastable fragmentation of silver bromide clusters

The abundance spectra and the fragmentation channels of silver bromide clusters have been measured and analyzed. The most abundant species are Ag_nBr_{n - 1} ⁺ and Ag_nBr_{n + 1} ^- and Ag₁₄Br₁₃ ⁺ is a magic number, revealing their ionic nature. However, some features depart from what is generally observed for alkali-halide ionic clusters. From a certain size, Ag_nBr_{n - 1} ⁺ is no more the main series, and Ag_nBr _{n - 2, 3} ⁺ series become almost as important. The fast fragmentation induced by a UV laser makes the cations lose more bromine than silver ions and lead to more silver-rich clusters. Negative ions mass spectra contain also species with more silver atoms than required by stoichiometry. We have investigated the metastable fragmentation of the cations using a new experimental method. The large majority of the cations release mainly a neutral Ag₃Br₃ cluster. These decay channels are in full agreement with our recent ab initio DFT calculations, which show that Ag⁺-Ag⁺ repulsion is reduced due to a globally attractive interaction of their d orbitals. This effect leads to a particularly stable trimer (AgBr)₃ and to quasi-planar cyclic structures of (AgBr)_n clusters up to n = 6. We have shown that these two features may be extended to other silver halides, to silver hydroxides (AgOH)_n, and to cuprous halide compounds. Received 9 November 2000 and Received in final form 25 January 2001     

Size-selected, supported clusters: the interaction of carbon monoxide with nickel clusters

We report on the size-dependent chemical reactivity of nickel clusters with up to 30 atoms. Monodispersed Ni₃₀ clusters show a higher reactivity for CO dissociation than Ni₁₁ and Ni₂₀. Under our experimental conditions the smallest nickel clusters (Ni_x, x<4) produce nickelcarbonyl complexes. These results demonstrate that such small clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but also essentially because of the distinctive properties of different cluster sizes. In addition thermal desorption spectroscopy of CO shows that on average four molecules are weakly adsorbed per Ni₁₁ at saturation coverage. Using an isotopic mixture of ¹²CO and ¹³CO, infrared spectroscopy reveals the existence of a vibrational coupling interaction between the four COs. A semi-classical model of interacting dipoles is applied to correlate the observed vibrational frequency shifts with the arrangement of the COs on the cluster. This simple analysis favors a three-dimensional structure for the deposited clusters. Received: 23 March 1998/Accepted: 25 August 1998     

The aluminum arsenides Al<Subscript>m</Subscript>As<Subscript>n</Subscript> (m + n = 2–5) and their anions: Structures,electron affinities and vibrational frequencies

Geometries, electronic states and electron affinities of Al_mAs_n and Al_mAs _n (m+n=2–5) clusters have been examined using four hybrid and pure density functional theory (DFT) methods. Structural optimization and frequency analyses are performed using a 6-311+G(2df) one-particle basis set. The geometries are fully optimized with each DFT method independently. The three types of energy separations reported in this work are the adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vert), and the vertical detachment energy (VDE). The calculation results show that the singlet structures have higher symmetry than that of doublet structures. The best functional for predicting molecular structures was found to be BLYP, while other functionals generally underestimated bond lengths. The largest adiabatic electron affinity, vertical electron affinity and vertical detachment energy, obtained at the 6-311+G(2df)/BP86 level of theory, are 2.20, 2.04 and 2.27 eV (AlAs), 2.13, 1.94 and 2.38 eV (AlAs₂), 2.44, 2.39 and 2.47 eV (Al₂As), 2.09, 1.80 and 2.53 eV (Al₂As₂), 2.01, 1.57 and 2.36 eV (AlAs₃), 2.32, 2.11 and 2.55 eV (Al₂As₃), 2.40, 1.45 and 3.26 eV (AlAs₄), 1.94, 1.90 and 2.07 eV (Al₄As), respectively. However, the BHLYP method gives the largest values for EA_ad and EA_vert of Al₃As and EA_ad of Al₃As₂, respectively. For the vibrational frequencies of the Al_nAs_m series, the B3LYP method produces good predictions with the average error only about 10 cm^-1 from available experimental and theoretical values. The other three functionals overestimate or underestimate the vibrational frequencies, with the worst predictions given by the BHLYP method.     

Formation of small water cluster anions by attachment of very slow electrons at high resolution

Using a high resolution laser photoelectron attachment method, we have studied the formation of (H ₂ O) _q ^- (q = 2, 6, 7, 11, 15) cluster ions in collisions of slow free electrons (E = 1-80 meV) and Rydberg electrons (n = 12-300) with water clusters. Resonances at zero energy have been observed, the shapes of which are strongly dependent on cluster size. The results are discussed in terms of the formation of metastable negative ions. Received 8 March 1999     

The binding of alkali atoms to the surfaces of liquid helium and hydrogen

Alkali atoms have been shown previously to have only unstable binding states inside liquid⁴He. We calculate the equilibrium configurations and binding energies of single alkali atoms near the liquid-vapor interface of⁴He and³He. A simple interface model is used to predict the surface deformation due to the presence of the atoms. A more realistic density functional model yields somewhat higher energies in the case of⁴He. For all alkali atoms, we find the surface binding energies to be around 10 to 20 K. A similar analysis with atom-H₂ interactions finds that alkali atoms tend to submerge into liquid H₂, with the exception of Li.