  We have undertaken a program to observe emission lines of SIV 10.5, NeII
12.8, NeIII 15.6, & SIII 18.7 um in a number of extragalactic HII regions with
the Spitzer Space Telescope. We report our results for the nearly face-on
spiral galaxy M83. The nebulae selected cover a wide range of galactocentric
radii (R_G). The observations were made with the Infrared Spectrograph in the
short wavelength, high dispersion configuration. The above set of 4 lines is
observed cospatially, thus permitting a reliable comparison of the fluxes. From
the measured fluxes, we determine the ionic abundance ratios including
Ne++/Ne+, S3+/S++, and S++/Ne+ and find that there is a correlation of
increasingly higher ionization with larger R_G. By sampling the dominant
ionization states of Ne and S for HII regions, Ne/S ~ (Ne+ + Ne++)/(S++ + S3+).
Our findings of ratios that exceed the benchmark Orion value are more likely
due to other effects than a true gradient in Ne/S. Both Ne and S are primary
elements produced in alpha- chain reactions. It is expected that Ne/S remains
relatively constant throughout a galaxy. This type of observation and method of
analysis has the potential for accurate measurements of Ne/S, particularly for
HII regions with lower metallicity & higher ionization than those here, such as
those in M33. Our observations may also be used to test the predicted ionizing
spectral energy distribution of various stellar atmosphere models. We compare
the fractional ionization ratios <Ne++>/<S++> & <Ne++>/<S3+> vs. <S3+>/<S++>
with predictions made from our photoionization models using several
state-of-the-art stellar atmosphere model grids. A second paper using Spitzer
observations of HII regions in the galaxy M33 will follow.
