  We analyze low resolution Spitzer infrared (IR) 5-14 micron spectra of the
diffuse emission toward a carefully selected sample of stars. The sample is
composed of sight lines toward stars that have well determined ultraviolet (UV)
extinction curves and which are shown to lie beyond effectively all of the
extinguishing and emitting dust along their lines of sight. Our sample includes
sight lines whose UV curve extinction curves exhibit a wide range of curve
morphology and which sample a variety of interstellar environments. As a
result, this unique sample enabled us to study the connection between the
extinction and emission properties of the same grains, and to examine their
response to different physical environments. We quantify the emission features
in terms of the PAH model given by Draine & Li (2007) and a set of additional
features, not known to be related to PAH emission. We compare the intensities
of the different features in the Spitzer mid-IR spectra with the Fitzpatrick &
Massa (2007) parameters which describe the shapes of UV to near-IR extinction
curves. Our primary result is that there is a strong correlation between the
area of the 2175 A UV bump in the extinction curves of the program stars and
the strengths of the major PAH emission features in the mid-IR spectra for the
same lines of sight.
