Fullerenes and proto-fullerenes in interstellar carbon dust
  Laboratory spectra of hydrogenated amorphous carbon (HAC) particles prepared
under a variety of conditions show spectral features at 7.05, 8.5, 17.4 and
18.9 {\mu}m (1418, 1176, 575 & 529 cm-1) that have been associated with
emission from C60 molecules. These lines occur in the spectra even though C60
molecules as such are not present in our samples. It appears that these four
spectral lines in HAC can instead be associated with precursor molecules or
"proto-fullerenes" that subsequently react to yield C60. We develop a model
tracing the evolution and de-hydrogenation of HAC dust and show that the
observation of an emission feature at 16.4 {\mu}m (610 cm-1) in astronomical
spectra signals the presence of the pentagonal carbon rings required for the
formation of fullerenes. We suggest that the set of four IR emission lines
previously identified with C60 in many objects that also show the 16.4 {\mu}m
feature and other polycyclic aromatic hydrocarbon bands arise from
proto-fullerenes rather than C60. Tc1 is an example of a source in which
de-hydrogenation has proceeded to the point where only fullerenes are present.
