2016ApJ...826..122B

The C₆₀ molecule has been recently detected in a wide range of astrophysical environments through its four active intramolecular vibrational modes (T_1u) near 18.9, 17.4, 8.5, 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 C₆₀ in the astrophysical sources. The spectroscopic analysis of the C₆₀ samples produced with our method delivers highly reproducible RIS values of 100, 25±1, 26±1 and 40±4. A comparison of the inferred C₆₀ 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 C₆₀, or that the population distribution among the C₆₀ vibrational modes is affected by physical processes other than thermal or UV excitation, such as chemo-luminescence from nascent C₆₀ or possibly Poincare 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 C₆₀, and propose that neutral C₆₀ is the carrier of the unidentified emission band at 6.49 µm which has been observed in fullerene-rich environments.