The diffuse interstellar bands are absorption lines seen towards reddened stars. None of the molecules responsible for these bands have been conclusively identified. Two bands at 9,632 ångströms and 9,577 ångströms 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 matrix. 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 ångströms and 9,577.5 ± 0.1 ångströms, and the full widths at half-maximum of these bands are 2.2 ± 0.2 ångströms and 2.5 ± 0.2 ångströms, respectively. We conclude that we have positively identified the diffuse interstellar bands at 9,632 ångströms and 9,577 ångströms as arising from C60(+) in the interstellar medium.

Recent low-temperature laboratory measurements and astronomical observations have proved that the fullerene cation + C 60 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 + C 70 , with its origin band at Å 7959.2 , 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 + C 60 , one further band in the astronomical spectrum at Å 9348.5 is identified, increasing the total number of assigned DIBs to five. Numerous other + C 60 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 + C 60 this directly yields a column density, ( ) + N C 60 , of´-2 10 cm 13 2 in diffuse clouds, without the need to rely on theoretical oscillator strengths. The intensity of the + C 70 electronic transition in the range 7000-8000 Å 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 + C 70 will be too weak to be detected in the astronomical spectra, which is confirmed giving an upper limit of 2 mÅ 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 9577.5Å are due to absorption by the fullerene ion C + 60 . They also reported the detection of two other, weaker bands at 9428.5 and 9365.9Å. These lie in spectral regions heavily contaminated by telluric water vapour lines. We acquired CFHT ESPaDOnS spectra of HD 183143 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 9365.9Å of relative width and strength comparable to the laboratory absorption.There is a DIB of similar strength and FWHM at 9362.5Å. A stellar emission feature at 9429Å prevented detection of the 9428.5Å band. However, a CFHT archival spectrum of HD 169454, where emission is absent at 9429Å, clearly shows the 9428.5Å DIB with the expected strength and width. These results further confirm C + 60 as a DIB carrier.

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 A 2Π u ← X 2Σ g + 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 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 -= + n C He 1 3 n 60 ( -). The extrapolation of these gives absorption bands of bare + C 60 at 9348.4, 9365.2, 9427.8, 9577.0, and 9632.1 Å, with ±0.2 Å 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.