  We suggest that the carrier of the unidentified infrared emission (UIE) bands
is an amorphous carbonaceous solid with mixed aromatic/aliphatic structures,
rather than free-flying polycyclic aromatic hydrocarbon (PAH) molecules.
Through spectral fittings of the astronomical spectra of the UIE bands, we show
that a significant amount of the energy is emitted by the aliphatic component,
implying that aliphatic groups are an essential part of the chemical structure.
Arguments in favor of an amorphous, solid-state structure rather than a
gas-phase molecule as a carrier of the UIE are also presented.
