C60 as a probe for astrophysical environments
  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.
