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We report highly selective covalent bond modifications in collisions between keV alpha particles and van der Waals clusters of C(60) fullerenes. Surprisingly, C(119)(+) and C(118)(+) are the dominant molecular fusion products. We use molecular dynamics simulations to show that C(59)(+) and C(58)(+) ions--effectively produced in prompt knockout processes with He(2+)--react rapidly with C(60) to form dumbbell C(119)(+) and C(118)(+). Ion impact on molecular clusters in general is expected to lead to efficient secondary reactions of interest for astrophysics. These reactions are different from those induced by photons.

We find that the most stable fullerene isomers, C(70)-C(94), form efficiently in close-to central collisions between keV atomic ions and weakly bound clusters of more than 15 C(60)-molecules. We observe extraordinarily high yields of C(70) and marked preferences for C(78) and C(84). Larger even-size carbon molecules, C(96)-C(180), follow a smooth log-normal (statistical) intensity distribution. Measurements of kinetic energies indicate that C(70)-C(94) mainly are formed by coalescence reactions between small carbon molecules and C(60), while C(n) with n≥96 are due to self-assembly (of small molecules) and shrinking hot giant fullerenes.

We find that the most stable fullerene isomers, C70-C94, form efficiently in close-to central collisions between keV atomic ions and weakly bound clusters of more than 15 C60-molecules. We observe extraordinarily high yields of C70 and marked preferences for C78 and C84. Larger even-size carbon molecules, C96-C180, follow a smooth log-normal (statistical) intensity distribution. Measurements of kinetic energies indicate that C70-C94 mainly are formed by coalescence reactions between small carbon molecules and C60, while Cn with n≥96 are due to self-assembly (of small molecules) and shrinking hot giant fullerenes. [ABSTRACT FROM AUTHOR]

We have measured total and coincident (with outgoing charge-state analyzed projectiles) ionization and fragmentation spectra of C60 and C70 following collisions with Xe4+ and Kr4+ at v = 0.06 a.u. Intact positive fullerene ions in charge states up to five (C60 5+ and C70 5+) are produced with both projectiles and for Kr4++C70 collisions we even observe a small C70 6+ peak. The C60/70-2m 3+ (m=1–7) intensity distributions are predominantly associated with the stabilization of three electrons on the projectile (s=3) and are significantly different for Xe4+- and Kr4+-projectiles. On the other hand, we find remarkable similarities in the C3 +-C11 + multi-fragmentation pattern regardless of projectile species (mass) although they are associated with closer encounters in which the projectile is fully neutralized (s=4). Simple Monte Carlo calculations of nuclear and electronic loss processes show that both these contributions are very similar in glancing Xe4++C60 and Kr4++C60 collisions, suggesting that frontal (and more violent) collisions are strongly suppressed under the present experimental conditions. Nevertheless it is surprising that the more distant collisions (s=3) are projectile mass dependent, while the closer collisions (s=4) are not. This indicates that this simple approach (although it reproduces more advanced quantum mechanical calculations for slow collisions with singly charged atomic ions rather well) is not valid for a comprehensive description of the energy transfer processes in the present collision systems involving projectiles of higher charge states. [ABSTRACT FROM AUTHOR]

Clusters of C60 fullerenes are (multi)-ionised in collisions with O5+ projectiles at a collision energy of 100 keV. The dominant fragmentation channels are analysed by time-of-flight mass spectrometry for different cluster size distributions. Singly charged ${\rm C}^+_{60}$, ions are found to be the dominant fragments, in 25% connected with the loss of one or more C2-units. This result is explained by the large charge mobility in fullerene clusters. Doubly charged fragments, in form of ${\rm C}^{2+}_{60}$ and ${\rm C}^{2+}_{60-2m}$, contribute to the observed spectrum with less than 5–10% only and are mainly attributed to the fragmentation of dimers. Singly charged small carbon clusters in the size range n = 7 to 19 are formed with low kinetic energies indicating the importance of thermal dissociation processes. The present experiments confirm earlier conclusions on the charge mobility in fullerene clusters. [ABSTRACT FROM AUTHOR]

The production of carbon cluster ions by injecting C60 fullerene vapor into different electron cyclotron resonance (ECR) ion sources (MONO1001/2.45 GHz and CAPRICE/14 GHz) is described. The extracted ion mass spectra show a bimodal distribution, well known from collisions between electrons or ions and fullerenes. In addition to small carbon clusters and even-numbered fullerene ions, odd-numbered clusters with (n>30) are detected with low intensities. In particular, we have analyzed the mass spectra as a function of the rf power applied to the ECR plasma. Optimum power values are found for the production of individual carbon cluster ions, which increase with decreasing cluster size. Whereas at low power the production of fullerene ions dominates, the intensity of the very small carbon clusters is found to strongly increase with the injected power. This difference in the power dependence is used to vary and to determine the beam composition for ions with identical mass/charge ratios. [ABSTRACT FROM AUTHOR]

The present article reports on a recent study of the production of multiply charged fullerene beams based on an electron cyclotron resonance (ECR) ion source (ECRIS). As collision studies in fundamental physics are demanding intense beams of multiply charged ions of small molecules, clusters, and particularly of fullerenes, we have further developed the ion source ECRIS MONO1000 [P. Jardin et al., Rev. Sci. Instrum. 73, 789 (2002)], originally devoted to produce singly charged ions, towards the production of multiply charged fullerene beams. In this article, the test measurements performed at the Electrostatic Ion Storage Ring Århus rf power (ELISA) facility will be described. Typical mass spectra (from pure C60 and C70 powder) will be shown and the influence of several source parameters (rf power, support gas, gas pressure,...) will be discussed specifying the conditions necessary for an optimum ion source operation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present model calculations of kinetic energy releases and fission barriers in asymmetric fission of C60r+ ions, using a simple electrostatic model where the fragments are treated as conducting spheres. The kinetic energy releases are calculated using two different approaches for deducing the radii of the spheres. Both approaches give results in qualitative agreement with experimental results. The fission barriers, on the other hand, depend strongly on the model radii and the activation energies for neutral fragment emission. A comparison between the model calculations shows that the choice of the finite size of the smaller fragments become important as 9" increases and have large influences on the prediction for the C60r+ stability limit. The competition between neutral (evaporation) and charged-fragment emission (fission) are discussed within a static over-the-barrier model for electron transfer between conducting spheres. [ABSTRACT FROM AUTHOR]

We have investigated the time dependence of the fragmentation of protonated amino acids stored at 22 keV in the electrostatic ring ELISA. The ions were produced in an electrospray source and after bunching in a quadrupole trap they were excited by collisions in a Ne gas. As in earlier studies of metal clusters and fullerenes produced in "hot" ion sources we find that the dissociation of metastable molecules follows approximately a 1/t decay law until a time r after which the yield falls off much more rapidly. We interpret this reduction as a result of radiative cooling with a characteristic cooling time, τc ... Gr, where G is the magnitude of the exponent in an Arrhenius expression for the rate of the dominant fragmentation process. The values of τ obtained from fits to the data are in the range 9-17 ms corresponding to cooling times of a few hundred milliseconds, in good accord with the expected rate of cooling by emission from IR-active vibrations. The power-law behaviour for t < τ varies somewhat between the different amino acids, with powers between -0.9 and -1.1. We argue that this may be due to a competition between fragmentation channels with different Arrhenius parameters. [ABSTRACT FROM AUTHOR]

We show that the energetic processing of C 60 clusters by slow atomic projectiles leads to ultrafast ( < ps ) formation of large covalent carbon nanoparticles containing a few hundreds of atoms. The underlying mechanism is found to be due to impulse-driven collisions between the projectile and the nuclei of the molecules. Experimental findings are well reproduced by classical molecular dynamics simulations. The cross sections for molecular growth processes forming covalent systems which contain more than 60 carbon atoms are about 5 ⋅ 10 − 14 cm 2 representing more than 70% of the geometrical cross sections. This demonstrates the high efficiency of the underlying processes. The formed carbon nanoparticles contain both aromatic and aliphatic structures which have also been considered as dust components in space. [ABSTRACT FROM AUTHOR]

Highlights: [•] We compare statistical and non-statistical fragmentation of fullerenes and PAHs. [•] Non-statistical fragmentation is important in ∼100eV PAH-He collisions. [•] Non-statistical fragmentation gives highly reactive fragments. [Copyright &y& Elsevier]

Abstract: Electron capture processes were studied in glancing collisions between multiply charged C60 q+ ions and neutral C60 molecules at low collision energies where nuclear stopping dominates the interaction (typical energy: 10keV; velocity ). In addition, clusters of fullerenes were multiply ionized in collisions with highly charged slow ions and their fragmentation spectra were measured by applying multi-coincidence techniques in a second, separate experiment. Through the first experiment we show that non-fragmenting electron capture collisions do not produce free electrons in C60 q+–C60 collisions (q =1–5), i.e., the cross sections for transfer ionization processes are negligible in this charge state regime. This is in contrast to the case of atomic projectile ions where transfer ionization processes, as e.g., C q+ +C60 →C(q−r)+ +C60 r+ →C(q−r+1)+ +C60 r+ +e−, are strong for q >2. These results are rationalized by means of the classical over-the-barrier model for electron transfer between two C60 molecules, or between an atomic ion and a C60 molecule, where the molecules are modeled as conducting spheres. Further, the same model may also be used as a basis for understanding the present observations of limitations in the maximum numbers of charges, which might be transferred in non-fragmenting C60 q+–C60 collisions (q/2 for even q and (q +1)/2 for odd q) and in C q+–C60 collision (here up to q charges may be transferred). In the same experiment, we have further measured scattering angles, θ, and energy losses, ɛ, for the fullerene projectiles in C60 q+–C60 collisions and we have found low values for both θ and ɛ, which, however, increase with the number of C2-units lost from the projectile fullerene in electron capture collisions. The critical distances for electron transfer which are deduced from the C60 q+–C60 collision experiment and the Zettergren model are then used to explain the high charge mobility between the individual C60 molecules in charged (C60) n van der Waal''s clusters of fullerenes, which we observe in the second experiment. [Copyright &y& Elsevier]

Abstract: We have performed an experimental study of highly charged clusters of fullerenes produced in slow collisions with highly charged ions. Collision products are analysed with respect to their mass to charge ratio and their initial kinectic energy by a linear time-of-flight mass spectrometer. An event-by-event data registration mode allows to study the correlation between the different detected fragments. In this paper, we focus on the kinetic energy releases (KER) in the fragmentation of charged dimers of fullerenes. The results are compared with theoretical values obtained from the conducting sphere model. [Copyright &y& Elsevier]

Clusters of C60 fullerenes are (multi)-ionised in collisions with O5+ projectiles at a collision energy of 100 keV. The dominant fragmentation channels are analysed by time-of-flight mass spectrometry for different cluster size distributions. Singly charged ${\rm C}^+_{60}$, ions are found to be the dominant fragments, in 25% connected with the loss of one or more C2-units. This result is explained by the large charge mobility in fullerene clusters. Doubly charged fragments, in form of ${\rm C}^{2+}_{60}$ and ${\rm C}^{2+}_{60-2m}$, contribute to the observed spectrum with less than 5–10% only and are mainly attributed to the fragmentation of dimers. Singly charged small carbon clusters in the size range n = 7 to 19 are formed with low kinetic energies indicating the importance of thermal dissociation processes. The present experiments confirm earlier conclusions on the charge mobility in fullerene clusters. [ABSTRACT FROM AUTHOR]

We have studied electron-transfer and collision-induced fragmentation processes in collisions between slow highly charged ions and C60 molecules. By using a position sensitive detector, located after a time-of-flight spectrometer, we obtain recoil energies of fragments resulting from post-collisional disintegration of multiply charged C60 ions. We deduce kinetic energy releases (KERs) for the cases in which multiply charged C60 ions emit either a single C2+ ion or a neutral C2-unit. We find that C584+ ions predominately are due to the emission of a neutral C2 molecule. Model calculations of KER-values resulting from C60r+ ions emitting a C2+ ion are presented, and compared with the corresponding experimental results for C60r+ with 5&les;r&les;9. [Copyright &y& Elsevier]

We report here an experiment concerning the operation of a conic electrode trap to study the decay of a single trapped ion (Arq+ with q=1, 3 and C603+) in a time range up to 50 ms. Ions to be trapped are produced by collisions of slow highly charged ions (56 keV Ar8+) with Ar gas or a C60 effusive gas jet. The operation of the trap is triggered by the coincident detection of a charge selected scattered projectile and the ejected electron number in order to select different recoil ion charges. The ion storage efficiency has been studied for one oscillation of the trapped ion as a function of the cone voltage. One of the final aims of these experiments is to measure on which time scale multiply charged clusters produced by impact of highly charged ions are stable. Since stability and lifetime of clusters are known to depend on their internal energy, we plan to vary this parameter by changing the initial projectile charge or/and by selecting different collision processes. [Copyright &y& Elsevier]