C₆₀ AS A PROBE FOR ASTROPHYSICAL ENVIRONMENTS

Abstract

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.

GTC/CanariCam Mid-IR Imaging of the Fullerene-rich Planetary Nebula IC 418: Searching for the Spatial Distribution of Fullerene-like Molecules

“…We note that our mid-IR images do not provide definitive proof about the dominant excitation mechanism of the fullerene emission (e.g., thermal or fluorescence emission); as it is also found in the literature (e.g., Bernard-Salas et al 2012;Otsuka et al 2014;Brieva et al 2016). Our observations may suggest that fullerenes could be attached to dust grains due to the considerable similarity between the spatial distribution of the IR emission in the Q1 and Q4 filters.…”

Search for Hydrogenated C60 (Fulleranes) in Circumstellar Envelopes

“…Even using the latest data, the observed band ratios are too dispersed to be assigned to either fluorescence or thermal excitation models. Brieva et al (2016) attributed this inconsistency to contamination of the band strengths from UIE or other emissions, which if true would severely limit the utility of C 60 infrared bands as a probe of physical conditions in circumstellar environments.…”

A Small Fullerene (C24) may be the Carrier of the 11.2 μm Unidentified Infrared Band

“…We note that Brieva et al 2016 discussed the use of the fullerene C 60 as a probe of astrophysical environments. They considered the relationship of IR band ratios with respect to local physical conditions.…”