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We present the first experimental vibrational spectra of gaseous oxidized derivatives of C60 in protonated and radical cation forms, obtained through infrared multiple-photon dissociation spectroscopy using the FELIX free-electron laser. Neutral C60O has two nearly iso-energetic isomers: the epoxide isomer in which the O atom bridges a CC bond that connects two six-membered rings and the annulene isomer in which the O atom inserts into a CC bond connecting a five- and a six-membered ring. To determine the isomer formed for C60O+ in our experiment a question that cannot be confidently answered on the basis of the DFT-computed stabilities alone we compare our experimental IR spectra to vibrational spectra predicted by DFT calculations. We conclude that the annulene-like isomer is formed in our experiment. For C60OH+, a strong OH stretch vibration observed in the 3 μm range of the spectrum immediately reveals its structure as C60 with a hydroxyl group attached, which is further confirmed by the spectrum in the 400-1600 cm-1 range. We compare the experimental spectra of C60O+ and C60OH+ to the astronomical IR emission spectrum of a fullerene-rich planetary nebula and discuss their astrophysical relevance.

Fragmentation is an important decay mechanism for polycyclic aromatic hydrocarbons (PAHs) under harsh interstellar conditions and represents a possible formation pathway for small molecules such as H2, C2H2, C2H4. Our aim is to investigate the dissociation mechanism of superhydrogenated PAHs that undergo energetic processing and the formation pathway of small hydrocarbons. We obtain, experimentally, the mass distribution of protonated tetrahydropyrene (C16H15 , py+5H+) and protonated hexahydropyrene (C16H17+, py+7H+) upon collision induced dissociation (CID). The IR spectra of their main fragments are recorded by infrared multiple-photon dissociation (IRMPD). Extended tight-binding (GFN2-xTB) based molecular dynamics simulations are performed in order to provide the missing structure information in experiment and identify fragmentation pathways. The pathways for fragmentation are further investigated at a hybrid-density functional theory (DFT) and dispersion corrected level. A strong signal for loss of 28 mass units of py+7H+ is observed both in the CID experiment and the MD simulation, while py+5H+ shows negligible signal for the product corresponding to a mass loss of 28. The 28 mass loss from py+7H+ is assigned to the loss of ethylene (C2H4) and a good fit between the calculated and experimental IR spectrum of the resulting fragment species is obtained. Further DFT calculations show favorable kinetic pathways for loss of C2H4 from hydrogenated PAH configurations involving three consecutive CH2 molecular entities. This joint experimental and theoretical investigation proposes a chemical pathway of ethylene formation from fragmentation of superhydrogenated PAHs. This pathway is sensitive to hydrogenated edges (e.g. the degree of hydrogenation and the hydrogenated positions). The inclusion of this pathway in astrochemical models may improve the estimated abundance of ethylene., 11 pages, 6 figures

The so-called aromatic infrared bands are attributed to emission of polycyclic aromatic hydrocarbons. The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, i.e. with respect to their size, structure, and charge state. Laboratory spectra of members of these classes are needed to compare them to observations and to benchmark quantum-chemically computed spectra of these species. In this paper we present the experimental infrared spectra of three different PAH dications, naphthalene$^{2+}$, anthracene$^{2+}$, and phenanthrene$^{2+}$, in the vibrational fingerprint region 500-1700~cm$^{-1}$. The dications were produced by electron impact ionization of the vapors with 70 eV electrons, and they remained stable against dissociation and Coulomb explosion. The vibrational spectra were obtained by IR predissociation of the PAH$^{2+}$ complexed with neon in a 22-pole cryogenic ion trap setup coupled to a free-electron infrared laser at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic density-functional theory calculations for both singly and doubly charged states of the three molecules. The experimental band positions showed excellent agreement with the calculated band positions of the singlet electronic ground state for all three doubly charged species, indicating its higher stability over the triplet state. The presence of several strong combination bands and additional weaker features in the recorded spectra, especially in the 10-15~$\mu$m region of the mid-IR spectrum, required anharmonic calculations to understand their effects on the total integrated intensity for the different charge states. These measurements, in tandem with theoretical calculations, will help in the identification of this specific class of doubly-charged PAHs as carriers of AIBs., Comment: Accepted for publication in A&A

With the detection of C60, C70, and in the interstellar medium, fullerenes are currently the largest molecules identified in space. The relatively high proton affinities of C60 and C70 support the hypothesis that protonated fullerenes may also be abundant in the interstellar matter. Here, we present the first experimental vibrational spectrum of C70H+, recorded in the gas phase. The attachment of a proton to C70 causes a drastic symmetry lowering, which results in a rich vibrational spectrum. As compared to C60, where all C-atoms are equivalent due to the icosahedral symmetry, C70 belongs to the D5h point group and has five nonequivalent C-atoms, which are available as protonation sites. Combined analysis of the experimental spectrum and spectra computed at the density functional theory level enables us to evaluate the protonation isomers being formed. We compare the IR spectra of C60H+ and C70H+ to IR emission spectra from planetary nebulae, which suggests that a mixture of these fullerene analogs could contribute to their IR emission.

Photophysics of single-wall carbon nanotubes (SWCNTs) is intensively studied due to their potential application in light harvesting and optoelectronics. Excited states of SWCNTs form strongly bound electron-hole pairs, excitons, of which only singlet excitons participate in application relevant optical transitions. Long-living spin-triplet states hinder applications but they emerge as candidates for quantum information storage. Therefore knowledge of the triplet exciton energy structure, in particular in a SWCNT chirality dependent manner, is greatly desired. We report the observation of light emission from triplet state recombination, i.e. phosphorescence, for several SWCNT chiralities using a purpose-built spectrometer. This yields the singlet-triplet gap as a function of SWCNT diameter and it follows predictions based on quantum confinement effects. Saturation under high microwave power (up to 10 W) irradiation allows to determine the spin-relaxation time for triplet states. Our study sensitively discriminates whether the lowest optically active state is populated from an excited state on the same nanotube or through F\"orster exciton energy transfer from a neighboring nanotube., Comment: 11 pages, 4 figures

Gas-phase coronene cations (C24H12+) can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule towards a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations [C24H12+nH ]+ show that molecules with odd numbers of additional hydrogen atoms (nH) are dominant with particularly high relative intensity for ”magic numbers” nH = 5, 11, and 17, for which hydrogen atoms have the highest binding energies. Reaction barriers and binding energies strongly affect the hydrogenation sequence and its site specificity. In this contribution, we monitor this sequence experimentally by the evolution of infrared multiple-photon dissociation (IRMPD) spectra of gaseous [C24H12+nH]+ with nH = 3 − 11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer.For weakly hydrogenated systems (nH = 3, 5) multiple-photon absorption mainly leads to loss of Hatoms (and/or H2). With increasing nH, C2H2 loss becomes more relevant. For nH = 9, 11, the carbon skeleton is substantially weakened and fragmentation is distributed over a large number of channels. A comparison of our IRMPD spectra with density functional theory calculations clearly shows that only one or two hydrogenation isomers contribute for each nH. This confirms the concept of hydrogenationoccurring along very specific sequences. Moreover, the atomic sites participating in the first 11 steps of this hydrogenation sequence are clearly identified.

The mid-infrared emission spectra of a large variety of astronomical objects are dominated by the aromatic infrared bands (AIBs), which are now widely accepted to originate from gaseous polycyclic aromatic hydrocarbons (PAHs). It is believed that the astrophysically most relevant molecules are at least 40–50 carbon atoms in size. Still, the large majority of laboratory experiments have been performed on smaller PAHs, mainly for reasons of experimental limitations and availability. Here, we show that combination of atmospheric pressure chemical ionization (APCI) with a direct insertion probe (DIP) inlet gives efficient access to larger, ionic PAHs for action spectroscopy studies. We present the gaseous IR spectra of two astrophysically relevant large PAHs, hexa-peri-hexabenzocoronene ( C 42 H 19 + ) and dicoronylene ( C 48 H 21 + ) in their protonated form. Compared to their radical cation analogs, the protonated species have a lower dissociation threshold as they can expel a neutral hydrogen radical leaving behind the resonance-stabilized radical cation; provided that the mass spectrometer can resolve precursor and product ions at one amu difference, this generates good quality spectra under multiple-photon dissociation conditions. Quantum-chemical computations at the density functional level are used to support experiments. Despite the apparent similarity of different protonation isomers, their IR spectra are predicted to be remarkably distinct. This facilitates a straightforward identification of the isomers formed experimentally. For both species studied, protonation occurs on the peripheral CH moiety in the ’bay region’ of the molecules. We compare the spectra of the protonated species with those of their radical cation analogs reported previously and briefly discuss the astrophysical relevance.