  We present a new method to decompose the emission features of polycyclic
aromatic hydrocarbons (PAHs) from mid-infrared spectra using theoretical PAH
templates in conjunction with modified blackbody components for the dust
continuum and an extinction term. The primary goal is to obtain robust
measurements of the PAH features, which are sensitive to the star formation
rate, in a variety of extragalactic environments. We demonstrate the
effectiveness of our technique, starting with the simplest Galactic
high-latitude clouds to extragalactic systems of ever-increasing complexity,
from normal star-forming galaxies to low-luminosity active galaxies, quasars,
and heavily obscured infrared-luminous galaxies. In addition to providing
accurate measurements of the PAH emission, including the upper limits thereof,
our fits can reproduce reasonably well the overall continuum shape and
constrain the line-of-sight extinction. Our new PAH line flux measurements
differ systematically and significantly from those of previous methods by ~15%
to as much as a factor of ~6. The decomposed PAH spectra show remarkable
similarity among different systems, suggesting a uniform set of conditions
responsible for their excitation.
