C60 as a probe for astrophysical environments

TL;DR

This paper investigates the vibrational modes of C60 molecules in space, analyzing their emission strengths to understand excitation mechanisms and proposing C60 as the carrier of an unidentified infrared band.

Contribution

It provides reproducible intrinsic strength ratios for C60 vibrational modes and explores non-thermal excitation processes affecting their emission in astrophysical environments.

Findings

Observed emission ratios cannot be explained by fluorescence or thermal emission alone.

C60 likely contributes to the 6.49 micron emission band in space.

Physical processes beyond thermal or UV excitation influence C60 vibrational populations.

Abstract

The C60 molecule has been recently detected in a wide range of astrophysical environments through its four active intramolecular vibrational modes (T1u) near 18.9, 17.4, 8.5, and 7.0 microns. 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 17.4/18.9 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 microns which has been observed in fullerene-rich environments.