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The fragmentation of the CO molecule by O 7+ ion impact is investigated in two different energy regimes by fragment ion momentum spectroscopy. The improved resolution of the present kinetic energy release measurement together with application of a time-dependent wavepacket dynamics method used in conjunction with new high-level computations of a large number of dication potential energy curves enables one to unambiguously assign each line to an excited state of the transient molecular dication produced during the collision. This is the first direct experimental evidence of the limitations of the Coulomb explosion model to reproduce the molecular fragmentation dynamics induced by ion impact. Electron removal due to a capture process is shown to transfer less excitation to the target than direct ionization. At low collision velocity, the three-body interaction between the projectile and the two fragments is also clearly highlighted.

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 þ and C 118 þ are the dominant molecular fusion products. We use molecular dynamics simulations to show that C 59 þ and C 58 þ ions-effectively produced in prompt knockout processes with He 2þ -react rapidly with C 60 to form dumbbell C 119 þ and C 118 þ . 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. DOI: 10.1103/PhysRevLett.110.185501 PACS numbers: 61.48.Àc, 31.15.xv, 36.40.Qv Photodriven intracluster reactions have been observed in clusters of fullerenes [1,2] and in, e.g., clusters of methanol, water, dimethyl ether, and acetic acid molecules [3]. Clusters of fullerenes are of particular interest in this context as C 60 and C 70 recently have been observed in space [4,5] at temperatures indicating that they may reside on grains [4] or are in the solid phase (fullerite) [6]. The question of how fullerenes form in space and elsewhere [7][8][9] is still open and here we report experimental observations on specific very efficient fullerene growth processes inside small pieces of fullerite material. These ion-impact induced growth processes in which so-called dumbbell fullerene systems are formed are inherently different from the ones induced by photons [1,2] for reasons that will be discussed in the following.The lowest energy barrier for formation of covalently bound dumbbell C 120 from two neutral C 60 molecules is roughly one or two electron volts [10]. Thus, two C 60 molecules in their ground states may, in principle, form such a C 120 system when the center-of-mass kinetic energy is larger than this barrier. However, molecular dynamics simulations by Jakowski, Irle, and Morokuma [11] have shown that, in practice, much larger kinetic energies are needed to form dumbbell C 120 (* 60 eV) or single-cage C 120 (*100 eV) efficiently [11]. The reason is that at least one C 60 cage needs to rearrange to form covalent bonds with the other cage on very short, picosecond, time scales. This is highly unlikely in a single encounter at low kinetic energy, as the transferred energy is redistributed over the fullerene molecule and a critical energy needs to be localized in a specific bond in order to break it. On the other hand, the reverse reaction (dissociation) may proceed at much lower energies but after many vibrations and on much longer time scales. For C 60 -C 60 collisions at kinetic energies above 60/100 eV covalent bond formation becomes much more likely but will then also give internally hot dumbbell/single-cage C 120 . Single-cage C 120 will fragment through sequences of C 2 emissions yielding molecular mass distributions with an even number of carbon atoms [1] as C 2 loss is the lowest-energy dissociation channel at about 10 eV. The dumbbell C 120 ! C 60 þ C 60 dissociation energy is only a couple of eV [1...