Absorption Spectroscopy of Solid-Phase Fullerene C 60 between 1.65 and 2.78 μm

International audienceFullerenes are the largest molecules identified so far in the interstellar medium. The spectroscopic properties of C60 and its charge variants have been extensively studied in the mid-infrared, near-ultraviolet, and visible regions. On the other hand, the near-infrared (NIR) spectral region has remained relatively unexplored, in particular for neutral C60. Here, we report the first measurements of the absorption spectra of solid C60 from 6052 to 3596 cm–1 (from 1.65 to 2.78 μm) recorded over the 11–300 K temperature range. The NIR spectra of C60 microcrystals embedded in solid KBr were measured using a low-temperature helium cryo-cooling setup coupled to a Fourier transform infrared spectrometer. The observed spectra of C60 from 1.65 to 2.78 μm show highly structured absorption features. Compared to isolated molecules, the lower symmetry of C60 microcrystals optically activates many fundamental vibrational modes, which combine (up to 4-quanta) and contribute to the rich observed spectrum. Quantum chemical calculations and group theoretical analysis were performed for a tentative assignment of the observed combination and overtone bands to the calculated transitions of both regions by identifying the IR-active T1u symmetries. A total of 3895 combination and overtone transitions were calculated in the region between 1.65 and 2.78 μm, amongst which 1862 transitions belong to the IR-active T1u symmetry, expected to be observed in the absorption spectrum of gas-phase C60. This work offers some guidance for future low-temperature gas-phase NIR absorption spectroscopic studies of C60. Measurements of vibrational overtone bands of C60 are extremely timely for the identification of the carriers of NIR DIBs (diffuse interstellar bands) in the context of the upcoming launch of the James Webb Space Telescope