Although fullerenes have long been hypothesized to occur in interstellar environments, their actual unambiguous spectroscopic identification is of more recent date. 1-4 C 60 , C 70 and C + 60 now constitute the largest molecular species individually identified in the interstellar medium (ISM). Fullerenes have significant proton affinities and it was suggested that C 60 H + is likely the most abundant interstellar analogue of C 60 . 5 We present here the first laboratory infrared (IR) spectrum of gaseous C 60 H + . Symmetry breaking relative to C 60 produces an IR spectrum that is much richer than that of C 60 . The experimental spectrum is used to benchmark theoretical spectra indicating that the B3LYP density functional with the 6-311+G(d,p) basis set accurately reproduces the spectrum. Comparison with IR emission spectra from two planetary nebulae, SMP LMC56 and SMC16, that have been associated with high C 60 abundances, indicate that C 60 H + is a plausible contributor to their IR emission.Buckminsterfullerene C 60 is undoubtedly one of the most iconic molecules of our time. Since its discovery in 1985, 6 its physico-chemical properties have been extensively characterized, including its ion chemistry and spectroscopic properties. IR spectra have been reported in condensed and gas phases, 7-10 and spectra for ionized forms are available as well. 4,8,11,12 The high cosmic abundance of carbon combined with the high stability of fullerenes 13 initiated a quest for their detection in inter-and circumstellar environments. [14][15][16][17] This search culminated in the identifications of neutral C 60 and C 70 in a young planetary nebula (Tc1) 1 based on diagnostic IR features. Accurate gas-phase laboratory spectra in the near-IR range led to the identification of C + 60 as carrier of two of the diffuse interstellar bands near 9600Å. 4 The question of whether or not fullerenes can form in H-rich regions of the interstellar medium (ISM) has been under debate. 1,3 Hydrogenation produces stable fullerene derivatives and partially hydrogenated fullerenes (fulleranes) have been suggested to occur in circumstellar envelopes and in the ISM 3,18,19 . On the other hand, hydrogenation and the concomitant change in orbital hybridization from sp 2 to sp 3 reduces the stability of the fullerene cage, which under the conditions of the ISM would likely lead to dehydrogenation and restoration of the original fullerene 20 or to breakdown of the carbon cage. 5 However, in this latter paper, Kroto also noted that protonation does not compromise cage stability and hypothesized that "protonated C 60 is likely to be the most abundant fullerene analogue," analogous to high abundances of protonated carbon monoxide, HCO + .Ion chemistry studies 21 have determined the proton affinity (PA) of C 60 at 860 kJ/mol. This relatively high value, just above the PA of ammonia, makes C 60 H + (Figure 1a) one of the most relevant stable fullerene derivatives and underpins Kroto's statement above. However, a) Corresponding author: j.oomens@science.ru.nl t...

Gas-phase coronene cations (C H 24 12 + ) can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule toward a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations C H n 24 12 H + + [ ] show that molecules with odd numbers of additional hydrogen atoms (n H ) are dominant, with particularly high relative intensity for "magic numbers" n H =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 C H n 24 12 H + + [ ] with n H =3-11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer. For weakly hydrogenated systems (n H = 3, 5) multiple-photon absorption mainly leads to loss of H atoms (and/or H 2 ). With increasing n H , C 2 H 2 loss becomes more relevant. For n H =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 to each n H . This confirms the concept of hydrogenation occurring along very specific sequences. Moreover, the atomic sites participating in the first 11 steps of this hydrogenation sequence are clearly identified.

The so-called aromatic infrared bands (AIBs) are attributed to emission of polycyclic aromatic hydrocarbons (PAHs). The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, that is to say 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 (IR) 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 (EI) 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 (DFT) 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 µm 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.

With the detection of C 60 , C 70 , and + C 60 in the interstellar medium, fullerenes are currently the largest molecules identified in space. The relatively high proton affinities of C 60 and C 70 support the hypothesis that protonated fullerenes may also be abundant in the interstellar matter. Here, we present the first experimental vibrational spectrum of C 70 H + , recorded in the gas phase. The attachment of a proton to C 70 causes a drastic symmetry lowering, which results in a rich vibrational spectrum. As compared to C 60 , where all C-atoms are equivalent due to the icosahedral symmetry, C 70 belongs to the D 5h 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 C 60 H + and C 70 H + to IR emission spectra from planetary nebulae, which suggests that a mixture of these fullerene analogs could contribute to their IR emission.

We present the development and performance of an optically detected magnetic resonance (ODMR) spectrometer. The spectrometer represents advances over similar instruments in three areas: (i) the exciting light is a tunable laser source which covers much of the visible light range, (ii) the optical signal is analyzed with a spectrograph, (iii) the emitted light is detected in the near-infrared domain. The need to perform ODMR experiments on single-walled carbon nanotubes motivated the present development and we demonstrate the utility of the spectrometer on this material. The performance of the spectrometer is critically compared to similar instruments. The present development opens the way to perform ODMR studies on various new materials such as molecules and luminescent quantum dots where the emission is in the near-infrared range and requires a well-defined excitation wavelength and analysis of the scattered light.

The 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 the SWCNT diameter, and it follows predictions based on quantum confinement effects. Saturation under high microwave power (up to 10 W) irradiation allows the spin-relaxation time for triplet states to be determined. Our study sensitively discriminates whether the lowest optically active state is populated from an excited state on the same nanotube or through Förster exciton energy transfer from a neighboring nanotube.

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We present the first experimental vibrational spectra of gaseous oxidized derivatives of C 60 in protonated and radical cation forms, obtained through infrared multiple-photon dissociation spectroscopy using the FELIX free-electron laser. Neutral C 60 O 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 C 60 O + 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 C 60 OH + , a strong OH stretch vibration observed in the 3 μm range of the spectrum immediately reveals its structure as C 60 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 C 60 O + and C 60 OH + to the astronomical IR emission spectrum of a fullerene-rich planetary nebula and discuss their astrophysical relevance.