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     

Dissociation channels of silver bromide cluster Ag<Subscript>2</Subscript>Br,silver cluster Ag<Subscript>3</Subscript> and their ions studied by using alkali metal target

Various dissociation channels of silver bromide cluster ion Ag₂Br⁺ and silver cluster ion Ag₃ ⁺ were observed in high-energy collisionally-activated dissociation (CAD) using a Cs target. The fragment patterns of the high-energy CAD were compared with those of the metastable dissociation and low-energy CAD. The difference in the fragment patterns between the high-energy CAD and the other dissociation methods was explained in terms of the internal energy distributions. The dissociation mechanisms of neutral silver bromide cluster Ag₂Br and silver cluster Ag₃ were also investigated by charge inversion mass spectrometry using the Cs target. While the fragment ions AgBr^- and Ag₂ ^- were dominantly observed in the charge inversion spectrum of Ag₂Br⁺, the undissociated ion Ag₃ ^- was observed as a predominant peak in the case of Ag₃ ⁺. The dissociation behavior of Ag₂Br^* can be explained on the basis of the calculated thermochemical data. Contrary to this, the predominant existence of the undissociated Ag₃ ^- cannot be explained by the reported thermochemical data. The existence of undissociated Ag₃ ^- suggests that the dissociation barrier is higher than the internal energy of Ag₃ ^* (theoretical: 1.03 eV, experimental: 2.31 eV) estimated from the ionization potentials of Ag₃ and Cs.     

Formation of higher-order fullerene dianions in collisions with Na atoms

We have measured electron capture cross sections in collisions between higher order fullerene anions C_n ^- (n=76, 78, 82, 84, 86, 90 and 96) and Na atoms. The ions were produced in an electrospray ion source (ESI) and accelerated to an energy of 50 keV. The measured cross section for dianion formation is three times larger for C₉₆ than that for C₆₀. The latter cross section was earlier found to be 36 ?². The dramatic increase of the cross section with fullerene size is explained by means of the curve crossing model for electron transfer.     

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.     

Electron capture in the ion-cluster collision: effect of the electronic interaction

We investigate the electron capture occurring in the collision between an ion A⁺ and a cluster A_n (n = 5). The process has been modelled within the Hubbard Hamiltonian,which takes into account the intrasite U electron correlation. An exact procedure has been numerically applied which involves all the excited states to examine the time evolution of the system during the collision. We have applied the model to the sodium case. We have investigated the time evolution of the electron population during the collision on the projectile versus the kinetic energy of the projectile. It displays some oscillations which means that the electron exchanges between the ion and the cluster occurs alternatively in one direction and the other. We also vary U and examine its influence on the dynamics of the oscillation of the average population. Finally the cross section is derived versus the energy and U. Received 29 November 2000     

Photodissociation spectroscopy of the chromium trimer ion

The electronic and geometric structures and photodissociation dynamics of the chromium trimer ion, Cr₃ ⁺, were investigated by photodissociation spectroscopy in the photon-energy range from 1.32 to 5.52 eV. The branching fractions of the product ions, Cr⁺ and Cr₂ ⁺, exhibit stepwise changes at the threshold energies for dissociation into Cr⁺+Cr₂, Cr+Cr₂ ⁺, Cr⁺+2Cr, and Cr^*+Cr₂ ⁺. It is noted that Cr₂ ⁺ is produced even above the threshold for atomization; the excess energy is redistributed to produce a fragment atom, Cr^*, in an excited state. The photodissociation action spectrum is well explained by a mixture of simulated spectra for two nearly-degenerate structural isomers identified by density functional calculations: those having a metastable C_2v structure and the most stable structure slightly distorted from the C_2v one. The barrier height between the two isomers which is lower than the zero-point energy suggests that Cr₃ ⁺ has an intrinsically floppy structure.     

Molecular picture of excited states and fragmentation paths of the Na 5 F 4 cluster

The stability against fragmentation and possible relaxation of the lowest excited states of the Na₅F₄ cluster (representative of cubic non stoechiometric clusters with an excess sodium atom, also called sodium-tail) is investigated by means of one-electron pseudopotential calculations with particular reference to photoabsorption processes from the ground state. Whereas the equilibrium configuration of the ground state has C_3v symmetry, the doubly degenerate 1²E excited state is affected by a conical intersection and a Jahn-Teller effect associated with the rotation of the sodium tail around the C₃-axis. This yields a “Mexican hat" topology for the lowest sheet with three equivalent C_s minima. Alternatively the 2²A₁ state has a minimum retaining the C_3v symmetry. The dissociation paths of the cluster along the C₃-axis into respectively Na₄F_{4 +} Na and Na₄F_{3 +} NaF are also investigated. Among the former paths, the excited states are found adiabatically stable with respect to the products. However in the A₁ symmetry, fragmentation into NaF exhibits an interesting avoided crossing between configurations correlated respectively with Na₄F₃ ^{+ +} NaF^- and Na₄F_{3 +} NaF. Such interaction, similar to the well-known charge exchange processes in elementary molecules might induce non adiabatic predissociation of the 2²A₁ state. This mechanism is invoked to explain the differences between R2PI and depletion spectra, correlated with the dissociation or relaxation of the excited states. Received 24 March 2000 and Received in final form 11 July 2000     

A theoretical study on electron transfer in low-energy collisions of a collinear meta-stable with a $ {\rm{Na}}_3^ - $ {\rm{Na}}_3^ - " border="0"> Na

Electron transfer in the collisions of a with a Na is theoretically studied. It is assumed that the target is collinear (D _h ) and that its electronic state is meta-stable triplet state. Adiabatic potential energy surfaces and non-adiabatic couplings of the system are calculated by using a semi-empirical diatomics-in-molecules (DIM) method. The positions of (avoided)-crossings of potential surfaces are investigated and the non-adiabatic couplings between two different electronic states are calculated. An avoided crossing is found in the region where the separation between the target and projectile is relatively large (10–15 bohr). A dynamical calculation demonstrates that this crossing causes charge transfer between the target and projectile. Another intersection at a smaller separation changes the targets spin state (from triplet state to singlet state or vice versa). The cross-sections for charge and spin transfer reaction are estimated at the collision energy of 6.8 keV. It is found that the charge transfer cross-section is extremely enhanced when the target cluster ion is in its meta-stable triplet state comared to the case where the cluster is the ground singlet state.     

Charge transfer in the collision system He<Superscript>2+</Superscript> + C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack>: Theory and experiment

Fullerenes are a direct link between atoms with discrete electronic energy levels and solids with a band structure and a well-defined surface. In this paper, we report on a quantum mechanical treatment of charge transfer and ionization in the ion-ion collision system ³He²⁺ + C ₆₀ ⁺ . This approach considers under- and over-barrier transitions through the one-dimensional barrier between the collision partners. The calculated cross sections for charge transfer compare favorably with experimental data measured in the center-of-mass energy range from 27 to 196 keV employing the crossed beams technique.     

Photoinduced dissociation reactions of silver fluoride cluster ions

Photoinduced dissociation in the ultraviolet region has been investigated for Ag _nF _n-1 ⁺ cluster ions. Photodissociation spectrum of Ag₂F⁺ in the energy of 3.8–5.6 eV exhibits several sharp bands corresponding to the transition to electronically excited states. In this dissociation, only the Ag₂ ⁺ ion was observed as a fragment ion. Theoretical calculation indicates that the parent Ag₂F⁺ ion has a linear Ag-F-Ag equilibrium geometries in the ground and excited states. Since conformational changes by excitation of bending vibration are necessary for the fragmentation of an F atom, this indicates that production of Ag₂ ⁺ from Ag₂F⁺ is a result of internal conversion and following conformational changes.     

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     

Decay pathways of small gold clusters

The decay pathway competition between monomer and dimer evaporation of photoexcited cluster ions Au ⁺ _n, n = 2-27, has been investigated by photodissociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature in time-resolved measurements of the dissociation. For the smaller clusters, simple fragment spectra were used. As in the case of the other copper-group elements, even-numbered gold cluster ions decay exclusively by monomer evaporation, irrespective of their size. For small odd-size gold clusters, dimer evaporation is a competitive alternative, and the smaller the odd-sized clusters, the more likely they decay by dimer evaporation. In this respect, Au ⁺ ₉ shows an anomalous behavior, as it is less likely to evaporate dimers than its two odd-numbered neighbors, Au ⁺ ₇ and Au ⁺ ₁₁. This nonamer anomaly is typical for copper-group cluster ions M ⁺ ₉ (M = Cu, Ag, Au) and a similar behavior is found in the anionic heptamers M ^- ₇. It is discussed in terms of the well-known electronic shell closing at n _e = 8 atomic valence electrons. Received 2 November 2000     

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     

Radiative charge transfer and radiative association of protons colliding with Na at low energies

Using the fully quantum-mechanical approach, the radiative charge transfer for H⁺ + Na(3s) collisions has been investigated. The charge transfer emission spectra are analyzed at resonant and non-resonant collision energies. The radiative association cross sections, obtained by subtracting the radiative charge transfer part from total radiative decay cross sections calculated by the optical potential method, are presented in the energy range 10⁻⁶-1 eV.     

Hindered Coulomb explosion of embedded Na clusters — stopping,shape dynamics and energy transport

We investigate the dynamical evolution of a Na₈ cluster embedded in Ar matrices of various sizes from N=30 to 1048. The system is excited by an intense short laser pulse leading to high ionization stages.We analyze the subsequent highly non-linear motion of cluster and Ar environment in terms of trajectories, shapes, and energy flow. The most prominent effects are: temporary stabilization of high charge states for several ps, sudden stopping of the Coulomb explosion of the embedded Na₈ clusters associated with an extremely fast energy transfer to the Ar matrix, fast distribution of energy throughout the Ar layers by a sound wave. Other ionic-atomic transfer and relaxation processes proceed at slower scale of few ps. The electron cloud is almost thermally decoupled from ions and thermalizes far beyond the ps scale.     

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     

Dimer dissociation energies of small odd-size clusters

The dimer dissociation energies of gold cluster ions Au ⁺ _n , n = 9, 11, 13, 15 have been determined with an extension of a recently developed model-independent method. Monomer-dimer decay pathway branching ratios provide the energy dependent process which is needed in this method. The measured values are D ₂ ( Au ⁺ ₉ ) = 3.66(8)(9) eV, D ₂ ( Au ⁺ ₁₁ ) = 4.27(11)(8) eV, D ₂ ( Au ⁺ ₁₃ ) = 4.50(9)(7) eV and D ₂ ( Au ⁺ ₁₅ ) = 4.29(10)(6) eV. Received 13 May 2002 / Received in final form 22 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: manuel.vogel@uni-mainz.de     

Differential cross sections for charge transfer in collisions of He+, Ne+ and Ar+ with Kr

Differential charge transfer cross sections for collisions of He⁺, Ne⁺ and Ar⁺ with Kr were measured at collision energies below 500 eV. A remarkable fraction of these collisions (2–30 %) occurs with large momentum transfer and small impact parameters. These close collisions lead to an excitation of the product particles, the measured reaction channels are strongly endothermic. In the system Ar⁺+Kr one reaction channel may be described in terms of a curve crossing.