Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C$_{16}$H$_{10+m}^+$, $m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.Comment: 10 pages, 5 figure

Structures and stabilities of mixed clusters of fullerene and coronene molecules

We have performed molecular dynamics simulations on the formation of mixed molecular clusters of buckminsterfullerene and coronene, $(\mathrm{C}_{24}\mathrm{H}_{12})_n(\mathrm{C}_{60})_{N-n}$. We report on our findings on the structures and their relative stabilities for cluster sizes $N=5$ and 13 and for all possible combinations of the two species within these sizes, including the pure clusters of each type. Generally, we see that the two species mix rather poorly and that compactly bound clusters are favoured over spatially extended ones. For a given ratio of coronene and fullerene, clusters with one or two coronene stacks tend to be more stable than those with a larger number of stacks. In the case of small clusters, the coronene and fullerene molecules tend to separate into two different cluster parts. For larger clusters, this is often but not always the case.Comment: 6 pages, 6 figure

The Largest Fullerene

Fullerenes are lowest energy structures for gas phase all-carbon particles for a range of sizes, but graphite remains the lowest energy allotrope of bulk carbon. This implies that the lowest energy structure changes nature from fullerenes to graphite or graphene at some size and therefore, in turn, implies a limit on the size of free fullerenes as ground state structures. We calculate this largest stable single shell fullerene to be of size $N=1\times10^4$, using the AIREBO effective potential. Above this size fullerene onions are more stable, with an energy per atom that approaches graphite structures. Onions and graphite have very similar ground state energies, raising the intriguing possibility that fullerene onions could be the lowest free energy states of large carbon particles in some temperature range

Protonated Clusters of Neon and Krypton

We present a study of cationic and protonated clusters of neon and krypton. Recent studies using argon have shown that protonated rare gas clusters can have very different magic sizes than pure, cationic clusters. Here we find that neon behaves similarly to argon, but that the cationic krypton is more similar to its protonated counterparts than the lighter rare gases are, sharing many of the same magic numbers.Comment: 5 pages, 5 figures, accepted for publication in Journal of The American Society for Mass Spectrometr

Bond breaking and making in mixed clusters of fullerene and coronene molecules following keV-ion impact

We have performed classical molecular dynamics simulations of 3 keV Ar + $(\mathrm{C}_{24}\mathrm{H}_{12})_n(\mathrm{C}_{60})_{m}$ collisions where $(n,m)=(3,2), (1,4), (9,4)$ and $(2,11)$. The simulated mass spectra of covalently bound reaction products reproduce the main features of the corresponding experimental results reported by Domaracka et al., PCCP, 2018, 20, 15052. The present results support their conclusion that molecular growth is mainly driven by knockout where individual atoms are promptly removed in Rutherford type scattering processes. The so formed highly reactive fragments may then bind with neighboring molecules in the clusters producing a rich variety of growth products extending up to sizes containing several hundreds of atoms, and here we show examples of such structures. In addition, knocked out atoms may be absorbed such that e.g. hydrogenated coronene and fullerene molecules are formed.Comment: 7 pages, 7 figure

Magic Sizes of Cationic and Protonated Argon Clusters

There has long been a discrepancy between the size distributions of Ar$_n^+$ clusters measured by different groups regarding whether or not magic numbers appear at sizes corresponding to the closure of icosahedral (sub-)shells. We show that the previously observed magic cluster size distributions are likely the result of an unresolved Ar$_n$H$^+$ component, that is, from protonated argon clusters. We find that the proton impurity gives cluster geometries that are much closer to those for neutral rare gas clusters, which are known to form icosahedral structures, than the pure cationic clusters, explaining why the mass spectra from protonated argon clusters better matches these structural models. Our results thus show that even small impurities, e.g.\ a single proton, can significantly influence the properties of clusters.Comment: 5 pages, 4 figures, published in Physical Review

Knockout driven fragmentation of porphyrins

We have studied collisions between tetraphenylporphyrin cations and He or Ne at center-of-mass energies in the 50-110 eV range. The experimental results were interpreted in view of Density Functional Theory calculations of dissociation energies and classical Molecular Dynamics simulations of how the molecules respond to He/Ne impact. We demonstrate that prompt atom knockout strongly contributes to the total destruction cross sections. Such impulse driven processes typically yield highly reactive fragments and are expected to be important for collisions with any molecular system in this collision energy range, but have earlier been very difficult to isolate for biomolecules.Comment: 6 pages, 5 figure

The Structure of Coronene Cluster Ions Inferred from H2 Uptake in the Gas Phase

Mass spectra of helium nanodroplets doped with H2 and coronene feature anomalies in the ion abundance that reveal anomalies in the energetics of adsorption sites. The coronene monomer ion strongly adsorbs up to n = 38 H2 molecules indicating a commensurate solvation shell that preserves the D6h symmetry of the substrate. No such feature is seen in the abundance of the coronene dimer through tetramer complexed with H2; this observation rules out a vertical columnar structure. Instead we see evidence for a columnar structure in which adjacent coronenes are displaced in parallel, forming terraces that offer additional strong adsorption sites. The experimental value for the number of adsorption sites per terrace, approximately six, barely depends on the number of coronene molecules. The displacement estimated from this number exceeds the value reported in several theoretical studies of the bare, neutral coronene dimer