Abstract
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.
Highlights
Interest in polycyclic aromatic hydrocarbons (PAHs) has many roots, including their role in the formation of soot,[1] of H2 in the interstellar medium,[2,3,4] as well as their contribution to the diffuse interstellar bands and aromatic infrared bands.[4,5,6] The highly symmetrical, planar coronene molecule (C24H12, D6h symmetry) is a strong candidate for many unidentified interstellar bands.[7]
Prominent mass peaks are due to bare coronene cluster ions, Corm+, m = 1 through 5
(H2)n13C12C23H12+ and (H2)nH12C24H12+ remain unresolved, their contributions are disentangled by the analysis software which considers the complete pattern of isotopologues.[45]
Summary
Interest in polycyclic aromatic hydrocarbons (PAHs) has many roots, including their role in the formation of soot,[1] of H2 in the interstellar medium,[2,3,4] as well as their contribution to the diffuse interstellar bands and aromatic infrared bands.[4,5,6] The highly symmetrical, planar coronene molecule (C24H12, D6h symmetry) is a strong candidate for many unidentified interstellar bands.[7]. The perfectly superimposed stack (or sandwich) does not form a local minimum.[20,21,23,25] Using an empirical potential for the repulsion–dispersion interaction, Rapacioli et al found that the twisted sandwich (TS) represents the most stable structure at À0.98 eV.[20] Most other studies conclude that the parallel displaced (PD) dimer constitutes the global energy minimum but do not agree on the direction and magnitude of the horizontal displacement.[11,12,18,19,21,22,23,24,25,26] Published experimental work on neutral or charged coronene clusters does not provide structural information.[31,32,33,34,35]
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