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Register a new userLaboratory formation and photo-chemistry of fullerene/anthracene cluster cations
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Besides buckminsterfullerene (C60), other fullerenes and their derivatives may also reside in space. In this work, we study the formation and photo-dissociation processes of astronomically relevant fullerene/anthracene (C14H10) cluster cations in the gas phase. Experiments are carried out using a quadrupole ion trap (QIT) in combination with time-of-flight (TOF) mass spectrometry. The results show that fullerene (C60, and C70)/anthracene (i.e., [(C14H10)nC60]+ and [(C14H10)nC70]+), fullerene (C56 and C58)/anthracene (i.e., [(C14H10)nC56]+ and [(C14H10)nC58]+) and fullerene (C66 and C68)/anthracene (i.e., [(C14H10)nC66]+ and [(C14H10)nC68]+) cluster cations, are formed in the gas phase through an ion-molecule reaction pathway. With irradiation, all the fullerene/anthracene cluster cations dissociate into mono$-$anthracene and fullerene species without dehydrogenation. The structure of newly formed fullerene/anthracene cluster cations and the bonding energy for these reaction pathways are investigated with quantum chemistry calculations. Our results provide a growth route towards large fullerene derivatives in a bottom-up process and insight in their photo-evolution behavior in the ISM, and clearly, when conditions are favorable, fullerene/PAH clusters can form efficiently. In addition, these clusters (from 80 to 154 atoms or ~ 2 nm in size) offer a good model for understanding the physical-chemical processes involved in the formation and evolution of carbon dust grains in space, and provide candidates of interest for the DIBs that could motivate spectroscopic studies.
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The formation and evolution mechanism of fullerenes in the planetary nebula or in the interstellar medium are still not understood. Here we present the study on the cluster formation and the relative reactivity of fullerene cations (from smaller to larger, C$_{44}$ to C$_{70}$) with anthracene molecule (C$_{14}$H$_{10}$). The experiment is performed in the apparatus that combines a quadrupole ion trap with a time-of-flight mass spectrometer. By using a 355 nm laser beam to irradiate the trapped fullerenes cations (C$_{60}$$^+$ or C$_{70}$$^+$), smaller fullerene cations C$_{(60-2n)}$$^+$, n=1-8 or C$_{(70-2m)}$$^+$, m=1-11 are generated, respectively. Then reacting with anthracene molecules, series of fullerene/anthracene cluster cations are newly formed (e.g., (C$_{14}$H$_{10}$)C$_{(60-2n)}$$^+$, n=1-8 and (C$_{14}$H$_{10}$)C$_{(70-2m)}$$^+$, m=1-11), and slight difference of the reactivity within the smaller fullerene cations are observed. Nevertheless, smaller fullerenes show obviously higher reactivity when comparing to fullerene C$_{60}$$^+$ and C$_{70}$$^+$. A successive loss of C$_2$ fragments mechanism is suggested to account for the formation of smaller fullerene cations, which then undergo addition reaction with anthracene molecules to form the fullerene-anthracene cluster cations. It is found that the higher laser energy and longer irradiation time are key factors that affect the formation of smaller fullerene cations. This may indicate that in the strong radiation field environment (such as photon-dominated regions) in space, fullerenes are expected to follow the top-down evolution route, and then form small grain dust (e.g., clusters) through collision reaction with co-existing molecules, here, smaller PAHs.
We report the first observation of cations, dications and trications of large clusters of adamantane. Cluster formation was initiated near 0 K in helium droplets and ionization was achieved with one or more collisions with energetic He species (He$^{star}$, He$^{+}$ or He$^{star}$$^{-}$). The occurrence of Coulomb explosion appeared to discriminate against the formation of small multiply charged clusters. High resolution mass spectrometry revealed the presence of magic number m/z peaks that can be attributed to the packing of adamantane molecules into cluster structures of special stability involving preferred arrangements of these molecules. These abundance anomalies were seen to be independent of charge state. While some dehydrogenation of adamantane and its clusters was seen as well, no major transformations into adamantoids or microdiamonds were observed.
Helium tagging in action spectroscopy is an efficient method for measuring the absorption spectrum of complex molecular ions with minimal perturbations to the gas phase spectrum. We have used superfluid helium nanodroplets doped with corannulene to prepare cations of these molecules complexed with different numbers of He atoms. In total we identify 13 different absorption bands from corannulene cations between 5500 {AA} and 6000 {AA}. The He atoms cause a small, chemically induced redshift to the band positions of the corannulene ion. By studying this effect as a function of the number of solvating atoms we are able to identify the formation of solvation structures that are not visible in the mass spectrum. The solvation features detected with the action spectroscopy agree very well with the results of atomistic modeling based on path-integral molecular dynamics simulations. By additionally doping our He droplets with D$_2$, we produce protonated corannulene ions. The absorption spectrum of these ions differs significantly from the case of the radical cations as the numerous narrow bands are replaced by a broad absorption feature that spans nearly 2000 {AA} in width.
We provide answers to long-lasting questions in the puzzling behavior of fullerene-fullerene fusion: Why are the fusion barriers so exceptionally high and the fusion cross sections so extremely small? An ab initio nonadiabatic quantum molecular dynamics (NA-QMD) analysis of C$_{60}$+C$_{60}$ collisions reveals that the dominant excitation of an exceptionally giant oblate-prolate H$_g(1)$ mode plays the key role in answering both questions. From these microscopic calculations, a macroscopic collision model is derived, which reproduces the NA-QMD results. Moreover, it predicts analytically fusion barriers for different fullerene-fullerene combinations in excellent agreement with experiments.
Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be strongly processed by vacuum ultraviolet photons. Here, we report experimental studies on the ionization and fragmentation of coronene (C24H12), ovalene (C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to synchrotron radiation in the range of 8--40 eV. The results show that for small PAH cations such as coronene, fragmentation (H-loss) is more important than ionization. However, as the size increases, ionization becomes more and more important and for the HBC cation, ionization dominates. These results are discussed and it is concluded that, for large PAHs, fragmentation only becomes important when the photon energy has reached the highest ionization potential accessible. This implies that PAHs are even more photo-stable than previously thought. The implications of this experimental study for the photo-chemical evolution of PAHs in the interstellar medium are briefly discussed.
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