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Gas-phase electronic spectra of H2O@C+60 and D2O@C+60 are presented. These data were obtained by one-photon dissociation of weakly bound helium complexes synthesised in a 3 K ion trap. Mea- surements were recorded in the vicinity of the 2 Ag ,2 Bg ← X 2 Au electronic transitions of the C+60 cage. Two-colour hole burning experiments enabled nuclear spin isomer pure data to be obtained. The spectra are rich in structure with many absorptions attributed to internal excitation of the encapsulated molecule accompanying the C+60 electronic transition. The experimental data are com- plemented with density functional theory calculations using the B3LYP functional and 6-31++G∗∗ basis set.

Gas phase electronic spectra of pure carbon cations generated by laser vaporization of graphite in a supersonic jet and cooled to below 10 K and tagged with helium atoms in a cryogenic trap are presented. The measured C2n+–He with n from 6 to 14, are believed to be monocyclic ring structures and possess an origin band wavelength that shifts linearly with the number of carbon atoms, as recently demonstrated through N2 tagging by Buntine et al. ( J. Chem. Phys. 2021, 155, 214302). The set of data presented here further constrains the spectral characteristics inferred for the bare C2n+ ions to facilitate astronomical searches for them in diffuse clouds by absorption spectroscopy.

The 2Πg←X2Σu+ electronic spectrum of C5+ in the gas-phase with origin band at 513nm is reported following experiments in a cryogenic ion trapping instrument. Buffer gas-cooled C5+ ions, generated by laser vaporisation of graphite, were investigated using two action spectroscopy approaches. Laser-induced dissociation of weakly bound C5+−Hen complexes synthesised in the trap reveal a linear dependence of the absorption energies on n allowing prediction of those of the bare ion C5+. These results are confirmed in two colour experiments on C5+, by monitoring fragmentation into the C3+ + C2 product channel. The data are supplemented with high-level electronic structure calculations which support the assignment of D∞h symmetry to the ground electronic state. These laboratory results provide the requisite data needed for spectroscopic detection of this structure in terrestrial and extraterrestrial environments.

The assignment of several diffuse interstellar bands in the near-infrared to C60+ ions present at high abundance in space has renewed interest in the astrochemical importance of fullerenes and analogues. Many of the latter have not been produced in macroscopic quantities, and their spectroscopic properties are not available for comparison with astronomical observations. An apparatus has been constructed that combines laser vaporisation synthesis with spectroscopic characterisation at low temperature in a cryogenic trap. This instrument is used here to record the electronic absorptions of C60+ produced by laser vaporisation of graphite. These are detected by (helium tagged) messenger spectroscopy in a cryogenic trap. By comparison with spectra obtained using a sublimed sample of Buckminsterfullerene, the observed data show that this isomer is the dominant C60+ structure tagged with helium at m/z=724, indicating that the adopted approach can be used to access the spectra of other fullerenes and derivatives of astrochemical interest.

The detection of molecules in the interstellar medium relies on the comparison between observational and laboratory data recorded under relevant conditions. In this contribution, the use of cryogenic ion traps to obtain low temperature spectra of gas phase ions of astrophysical relevance is reviewed. By exploiting the unique properties of these devices, the requisite spectroscopic data that led to the identification of C+60 as the first carrier of the enigmatic diffuse interstellar bands could be measured. These experiments are discussed, along with electronic transitions of other fullerenes, and recent approaches to synthesise and characterise molecular ions of long-held astrochemical interest.

The electronic spectrum of the endohedral fullerene + He@C60 observed by messenger spectroscopy in a cryogenic ion trap is presented. The role played by the messenger tag in the adopted experimental method is evaluated by recording spectra of - + He@C He 60 n with n = 1–4. The results indicate a linear shift of ∼0.7 Å in the wavelengths allowing accurate gas phase values to be reported. The presence of the helium inside the cage shifts the absorption bands by 2–3 Å toward shorter wavelengths compared to + C60. The magnitude of this displacement will enable searches for the spectral signatures of this fullerene analogue in interstellar environments by absorption spectroscopy. The implications for potential astronomical detection are discussed.

Experiments in mass spectrometry carried out during the 1980s led to the discovery of the fullerenes. The underlying aim behind these was to understand the formation of carbon chains identified in the interstellar medium some years earlier by radioastronomy. The stability of the discovered C60 cage structure led immediately to speculation concerning its role in the context of the diffuse interstellar bands, a set of several hundred optical absorptions observed in the spectrum of reddened star light. The identities of the molecular carriers responsible for these have remained elusive for a century. In 2015, laboratory measurements carried out using a cryogenic ion trap led to the confirmation that C 60 + is responsible for two of the unidentified absorption bands, providing the first clue as to their origin. In this contribution we focus on the experiments that led to this identification. The consequences of this and for other molecules present in such environments is discussed.

Im Jahr 1985 wurde fur das Buckminsterfulleren, C60, eine Fusball-Struktur vorgeschlagen, die 1990 bestatigt wurde. Schon zu Beginn dieser Forschung beschaftigte man sich mit dem Vorkommen von C60 und C60+ im Weltraum, vor allem im Zusammenhang mit den ratselhaften diffusen interstellaren Banden. Diese stammen von Absorptionen, die man in den Spektren des Lichts roter Sterne findet. Das Absorptionsspektrum von C60+ wurde 1993 in einer Neonmatrix bei 5 K gemessen, und es enthielt zwei prominente Banden nahe 9583 A und 9645 A. Anhand dieser Datengrundlage wurde vorhergesagt, in welchem Wellenlangenbereich die Gasphasenabsorptionen von C60+ liegen sollten. In dieser Spektralregion wurden 1994 zwei diffuse interstellare Banden gefunden und C60+ zugeschrieben. Erst 20 Jahre spater konnte man das Absorptionsspektrum von C60+ unter ahnlichen Bedingungen messen, wie sie in den diffusen Wolken vorherrschen. Komplexe Labormessungen bestatigten 2015, dass diese beiden interstellaren Banden tatsachlich von C60+ herruhren.

Electronic spectra in the near infrared of C 60 + with He, Ne, Ar, Kr, H2, D2 and N2 attached have been recorded below 10 K in a cryogenic radio frequency ion trap. Additional absorption bands are observed compared to the spectrum of C 60 + –He. In the case of C 60 + –N2, the strongest one of these shifts to lower energies by 21.3 cm−1 compared to the origin band of C 60 + –He at 10378.5 cm−1. The pattern in the spectrum is dependent on the attached ligand. The gas-phase observations on C 60 + –Ne allow a rationalization of the relative intensities of the absorptions of C 60 + in a neon matrix.

Laboratory measurements are reported that enable a more accurate determination of the characteristics of the near-infrared absorptions of C-60(+) below 10 K. These data were obtained by photofragmentation of C-60(+)-He complexes in a cryogenic trap. Asymmetry in the profiles of the observed 9577 and 9632 angstrom absorption bands of C-60(+)-He is caused by the attachment of the weakly bound helium atom to hexagonal or pentagonal faces of C-60(+). The implication is that the FWHM of the bands in the electronic spectrum of C-60(+) below 10 K is 1.4 angstrom. The effect of C-13 isotopes on the C-60(+) electronic spectrum is experimentally evaluated by measurement of C-12(60)+-He, (C1C59+)-C-13-C-12-He, and (C2C58+)-C-13-C-12-He. Data on the 9365 angstrom absorption band indicate a wavelength shift of about 0.3 angstrom between the former and latter. This result is consistent with models used to interpret the vibrational isotope effect in the Raman spectrum of neutral C-60. The influence of C-13 isotopes on the 9348, 9365, 9428, 9577, and 9632 angstrom diffuse interstellar bands is expected to be minor considering other broadening factors that affect astronomical observations. The presented data also provide more accurate relative intensities of the five interstellar bands attributed to C-60(+).

The combination of a 3 K cryogenic radiofrequency ion trapping apparatus and a laser vaporization source is described. This instrument was constructed for the synthesis and characterization of gas phase molecules, particularly those that are difficult to make using traditional organic chemistry routes. The flexible time scale for storage and relaxation afforded by the trap enables spectroscopic investigation of ions that are challenging to cool using supersonic expansions. Routine in situ tagging of cations with helium facilitates one-photon experiments. The potential of this instrument is demonstrated by providing data on the 2B2 ← X 2A1 electronic transition of cyclic C6+, a system that had thus far evaded spectroscopic interrogation in the gas phase.

In 1985 the football structure of C60 , buckminsterfullerene was proposed and subsequently confirmed following its macroscopic synthesis in 1990. From the very beginning the role of C60 and C60+ in space was considered, particularly in the context of the enigmatic diffuse interstellar bands. These are absorption features found in the spectra of reddened star light. The first astronomical observations were made around one hundred years ago and despite significant efforts none of the interstellar molecules responsible have been identified. The absorption spectrum of C60+ was measured in a 5 K neon matrix in 1993 and two prominent bands near 9583 A and 9645 A were observed. On the basis of this data the likely wavelength range in which the gas phase C60+ absorptions should lie was predicted. In 1994 two diffuse interstellar bands were found in this spectral region and proposed to be due to C60+ . It took over 20 years to measure the absorption spectrum of C60+ under conditions similar to those prevailing in diffuse clouds. In 2015, sophisticated laboratory experiments led to the confirmation that these two interstellar bands are indeed caused by C60+ , providing the first answer to this century old puzzle. Here, we describe the experiments, concepts and astronomical observations that led to the detection of C60+ in interstellar space.

In the last 30 years, our research has focused on laboratory measurements of the electronic spectra of organic radicals and ions. Many of the species investigated were selected based on their potential astrophysical relevance, particularly in connection with the identification of appealing candidate molecules for the diffuse interstellar absorptions. Notably, carbon chains and derivatives containing hydrogen and nitrogen atoms in their neutral and ionic forms were studied. These data could be obtained after developing appropriate techniques to record spectra at low temperatures relevant to the interstellar medium. The measurement of gas phase laboratory spectra has enabled direct comparisons with astronomical data to be made and though many species were found to have electronic transitions in the visible where the majority of diffuse bands are observed, none of the absorptions matched the prominent interstellar features. In 2015, however, the first carrier molecule was identified: C60+. This was achieved after the measurement of the electronic spectrum of C60+-He at 6K in a radiofrequency ion trap.

The electronic spectrum of the fullerene dication C702+ has been measured in the gas phase at low temperature in a cryogenic radiofrequency ion trap. The spectrum consists of a strong origin band at 7030 Å and two weaker features to higher energy. The bands have FWHMs of 35 Å indicating an excited state lifetime on the order of one-tenth of a picosecond. Absorption cross-section measurements yield (2 ± 1) × 10−15 cm2 at 7030 Å. These results are used to predict the depth of diffuse interstellar bands (DIBs) due to the absorption by C702+. At an assumed column density of 2 × 1012 cm−2 the attenuation of starlight at 7030 Å is around 0.4% and thus the detection of such a shallow and broad interstellar band would be difficult. The electronic spectrum of C602+ shows no absorptions in the visible. Below 4000 Å the spectra of C60, C60+ and C602+ are similar. The large intrinsic FWHM of the features in this region, ~200 Å for the band near 3250 Å, make them unsuitable for DIB detection.

A universal method has been developed for measuring spectra of molecular ions in a 22-pole radio frequency trap at 5 K. It is based on laser induced inhibition of complex growth (LIICG). The first successful measurements have been demonstrated on the A2Πu ← X2Σg+ electronic transition with some thousand N2+ ions, helium densities of 1015 cm–3, and storage times of 1 s. The reduction of the number of N2+–He complexes is the result of an interplay between excitation, radiative and collisional cooling, ternary association, and collision induced dissociation, which is explained by a kinetic model.

The electronic spectra of isotopologues of protonated coronene in the gas phase were measured at a vibrational and rotational temperature of ~10K in a 22-pole ion trap. The (1) 1A′←X 1A′ transition of these polycyclic aromatic hydrocarbon cations with one to three carbon-13 have origin band maxima that blue-shift successively by 0.03nm. All isotopologues show distinct vibrational structure in the (1) 1A′ state. These results are compared with the effect of 13C substitution on the near infrared electronic absorptions of C60+. The (1) 1A←X 1A electronic transition of monodeuterated coronene was also recorded and its origin band is red-shifted to that of protonated coronene by 0.8nm. The implications for astronomical observations are considered.

Campbell et al. recently revised, by a few tenths of an Å, the wavelengths for their low-temperature laboratory measurements of fullerene C60+ bands. This accounts for the perturbation caused by the He atom to the C60+-He spectrum. Here, we demonstrate that the revised laboratory wavelengths improve coincidence with the 9632, 9577, 9428, 9365, and 9348 diffuse interstellar bands detected towards the stars HD 46711, HD 169454, and HD 183143.

Galazutdinov et al. (2017) recently claimed that the relative strengths of the 9577 and 9632 Å diffuse interstellar bands (DIBs) are too poorly correlated to be caused by a single source, the C^+60 ion. Their conclusion is based on theoretical modeling of contaminating stellar Mg ii lines at 9631.9 and 9632.4 Å and UVES spectra. This contradicts their earlier result and those of several others that the two DIBs are closely correlated and, within the errors and effects of stellar blends, exhibit an intensity ratio consistent with that found in the 6 K laboratory spectrum of C^+60. We consider the use of close spectral standards to be superior to model atmosphere calculations in correcting for contamination by the Mg ii lines. We have examined some of the same UVES spectra and demonstrate that a lack of suitably observed telluric standards makes it impossible to adequately correct for telluric water vapor contamination, leading to unreliable continuum levels. The possible effects of higher temperatures, in the 30–100 K range, on the C^+60 electronic absorption band profiles, and their relative intensities, are also considered.

Laboratory measurements of the gas-phase spectrum of C60+ confirm that the diffuse interstellar bands observed at 9,632 angstroms and 9,577 angstroms arise as a result of C60+ in the interstellar medium. Lick Observatory astronomer Mary Lea Heger first observed what were to be called 'diffuse interstellar bands' in 1919. These are absorption lines seen towards reddened stars, and although hundreds are now known, until now none of the molecules giving rise to them have been conclusively identified. In 1994, Bernard Foing and Pascale Ehrenfreund reported two diffuse interstellar bands with wavelengths close to those of the absorption bands of fullerene C60+ measured in a neon matrix. A more certain identification awaited the gas-phase spectrum of C60+. John P. Maier and colleagues now present laboratory measurements of the gas-phase spectrum of C60+ and confirm that the diffuse interstellar bands that Foing and Ehrenfreund observed do arise from C60+. As C60 has already been detected in various nebulae by detection of its infrared spectrum, this new observation in the Milky Way can only add to current interest in the role of astronomical fullerenes. The diffuse interstellar bands are absorption lines seen towards reddened stars1. None of the molecules responsible for these bands have been conclusively identified2. Two bands at 9,632 angstroms and 9,577 angstroms were reported in 1994, and were suggested to arise from C60+ molecules (ref. 3), on the basis of the proximity of these wavelengths to the absorption bands of C60+ measured in a neon matrix4. Confirmation of this assignment requires the gas-phase spectrum of C60+. Here we report laboratory spectroscopy of C60+ in the gas phase, cooled to 5.8 kelvin. The absorption spectrum has maxima at 9,632.7 ± 0.1 angstroms and 9,577.5 ± 0.1 angstroms, and the full widths at half-maximum of these bands are 2.2 ± 0.2 angstroms and 2.5 ± 0.2 angstroms, respectively. We conclude that we have positively identified the diffuse interstellar bands at 9,632 angstroms and 9,577 angstroms as arising from C60+ in the interstellar medium.

The origin of the attenuation of starlight in diffuse clouds in interstellar space at specific wavelengths ranging from the visible to the near-infrared has been unknown since the first astronomical observations around a century ago. The absorption features, termed the diffuse interstellar bands, have subsequently been the subject of much research. Earlier this year four of these interstellar bands were shown to be due to the absorption by cold, gas phase molecules. This discovery provides the first answer to the problem of the diffuse interstellar bands and leads naturally to fascinating questions regarding the role of fullerenes and derivatives in interstellar chemistry. Here, we review the identification process placing special emphasis on the laboratory studies which have enabled spectroscopic measurement of large cations cooled to temperatures prevailing in the interstellar medium. This article is part of the themed issue ‘Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene’.

The wavelengths of the strongest absorptions in the electronic spectrum of C+60 have been determined by experimental investigation into the perturbation caused by the helium in the laboratory spectra of C+60 - Hen(n = 1–3). The extrapolation of these gives absorption bands of bare C+60 at 9348.4, 9365.2, 9427.8, 9577.0, and 9632.1 A, with ±0.2 A as the 2σ uncertainty. The laboratory data are compared with the complete set of astronomical observations reported in the literature. The spectral characteristics are found to be in agreement with five diffuse interstellar bands, for which the systematic uncertainties are larger than for the laboratory data.

Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation C60+ is responsible for four diffuse interstellar bands (DIBs). These absorptions correspond to the strongest bands of the lowest electronic transition. The gas phase spectrum below 10 K is reported here for the full wavelength range encompassed by the electronic transition. The absorption spectrum of C70+, with its origin band at 7959.2 Å, has been obtained under similar laboratory conditions. Observations made toward the reddened star HD 183143 were used in a specific search for the absorption of these fullerene cations in diffuse clouds. In the case of C60+, one further band in the astronomical spectrum at 9348.5 Å is identified, increasing the total number of assigned DIBs to five. Numerous other C60+ absorptions in the laboratory spectrum are found to lie below the astronomical detection limit. Special emphasis is placed on the laboratory determination of absolute absorption cross-sections. For C60+ this directly yields a column density, N(C60+) of- 2x10^13 cm-2 in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the C70+ electronic transition in the range 7000–8000 A is spread over many features of similar strength. Absorption cross-section measurements indicate that even for a similar column density, the individual absorption bands of C70+ will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of 2 mA to the equivalent width.

Based on gas-phase laboratory spectra at 6 K, Campbell et al. (2015) confirmed that the diffuse interstellar bands (DIBs) at 9632.7 and 9577.5A are due to absorption by the fullerene ion C60+. They also reported the detection of two other, weaker bands at 9428.5 and 9365.9A. These lie in spectral regions heavily contaminated by telluric water vapour lines. We acquired CFHT ESPaDOnS spectra of HD183143 close to the zenith and chopped with a nearby standard to correct for the telluric line absorption which enabled us to detect a DIB at 9365.9A of relative width and strength comparable to the laboratory absorption. There is a DIB of similar strength and FWHM at 9362.5A. A stellar emission feature at 9429A prevented detection of the 9428.5A band. However, a CFHT archival spectrum of HD169454, where emission is absent at 9429A, clearly shows the 9428.5A DIB with the expected strength and width. These results further confirm C60+ as a DIB carrier., Comment: 16 pages, 3 figures, accepted for publication in ApJ Letters