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The C 60 molecule has been recently detected in a wide range of astrophysical environments through its four active intramolecular vibrational modes (T 1u ) near 18.9 µm, 17.4 µm, 8.5 µm, and 7.0 µm. The strengths of the mid-infrared emission bands have been used to infer astrophysical conditions in the fullerene-rich regions. Widely varying values of the relative intrinsic strengths (RIS) of these four bands are reported in laboratory and theoretical papers, which impedes the derivation of the excitation mechanism of C 60 in the astrophysical sources. The spectroscopic analysis of the C 60 samples produced with our method delivers highly reproducible RIS values of 100, 25 ± 1, 26 ± 1 and 40 ± 4. A comparison of the inferred C 60 emission band strengths with the astrophysical data shows that the observed strengths cannot be explained in terms of fluorescent or thermal emission alone. The large range in the observed 17.4 µm/18.9 µm emission ratios indicates that either the emission bands contain significant contributions from emitters other than C 60 , or that the population distribution among the C 60 vibrational modes is affected by physical processes other than thermal or UV excitation, such as chemo-luminescence from nascent C 60 or possibly, Poincaré fluorescence resulting from an inverse internal energy conversion. We have carefully analyzed the effect of the weakly-active fundamental modes and second order modes in the mid-infrared spectrum of C 60 and propose that neutral C 60 is the carrier of the unidentified emission band at 6.49 µm which has been observed in fullerene-rich environments.

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