  This study explores the molecular origins of plateaus and continuum
underlying IR and FIR bands emitted by compact nebulae, especially
proto-planetary nebulae. Computational organic chemistry codes are used to
deliver the vibrational integrated band intensities of various large, typical
carbonaceous structures. These spectra are composed of a rather continuous
distribution of weak modes from which emerge the fingerprints. The 6 to 18-mu
region is interspersed with a great many weak lines, to which the plateaus are
assigned. Similarly, the far IR spectrum is ascribed to the phonon (skeletal)
spectrum which is readily identified beyond 18 mu. The absorptivities and
absorption cross-sections per interstellar H atom deduced from these spectra
are comparable with those of laboratory dust analogs and astronomical
measurements, respectively. Moreover, the 5-35 mu spectra of two typical
Proto-Planetary Nebula were reasonably well simulated with combinations of
molecules containing functional groups which carry the 21- and 30-mu bands, and
molecules devoid of these but carrying strong phonon spectra. These results may
help understand the emergence of plateaus, the origin of continua underlying
FIR bands, as well as the composition of circumstellar dust.
