Fragmentation and ionization of C

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 (C605+ and C705+) are produced with both projectiles and for Kr4++C70 collisions we even observe a small C706+ peak. The C60/70-2m3+ (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

Fragmentation of anthracene C14H10, acridine C13H9N and phenazine C12H8N2 ions in collisions with atoms

International audienceWe report experimental total, absolute, fragmentation cross sections for anthracene C14H10, acridine C13H9N, and phenazine C12H8N2 ions colliding with He at center-of-mass energies close to 100 eV. In addition, we report results for the same ions colliding with Ne, Ar, and Xe at higher energies. The total fragmentation cross sections for these three ions are the same within error bars for a given target. The measured fragment mass distributions reveal significant contributions from both delayed (≫10−12 s) statistical fragmentation processes as well as non-statistical, prompt (∼10−15 s), single atom knockout processes. The latter dominate and are often followed by secondary statistical fragmentation. Classical Molecular Dynamics (MD) simulations yield separate cross sections for prompt and delayed fragmentation which are consistent with the experimental results. The intensity of the single C/N-loss peak, the signature of non-statistical fragmentation, decreases with the number of N atoms in the parent ion. The fragment intensity distributions for losses of more than one C or N atom are rather similar for C14H10 and C13H9N but differ strongly for C12H8N2 where weak C–N bonds often remain in the fragments after the first fragmentation step. This greatly increases their probability to fragment further. Distributions of internal energy remaining in the fragments after knockout are obtained from the MD simulations

Non-statistical fragmentation of PAHs and fullerenes in collisions with atoms

International audienceNon-statistical fragmentation processes may be important when Polycyclic Aromatic Hydrocarbon molecules (PAHs), fullerenes, or other large complex molecules collide with atoms and atomic ions. For collisions with hydrogen or helium this occurs for center-of-mass energies between a few tens to a few hundreds of electron volts and typically results in losses of single atoms. In such processes one forms much more reactive fragments than in statistical fragmentation, which instead are dominated by losses of C2- or C2H2-molecules (H-atoms) from fullerenes and PAHs, respectively. An enhanced reactivity has recently been demonstrated for van der Waals clusters of C60 molecules where prompt knockouts of single C-atoms from one of the fullerenes yield highly reactive C59+ fragments, which easily form covalent bonds with a C60 molecule inside the clusters

Nonstatistical fragmentation of large molecules

International audienceWe present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions