  Recently, mid-infrared instruments have become available on several large
ground-based telescopes, resulting in data sets with unprecedented spatial
resolution at these long wavelengths. In this paper we examine
'ground-based-only' diagnostics, which can be used in the study of star-forming
regions in starburst galaxies. By combining output from the stellar population
synthesis code Starburst 99 with the photoionization code Mappings, we model
stellar clusters and their surrounding interstellar medium, focusing on the
evolution of emission lines in the N- and Q-band atmospheric windows (8-13 and
16.5-24.5 micron respectively) and those in the near-infrared. We address the
detailed sensitivity of various emission line diagnostics to stellar population
age, metallicity, nebular density, and ionization parameter. Using our model
results, we analyze observations of two stellar clusters in the overlap region
of the Antennae galaxies obtained with VLT Imager and Spectrometer for mid
Infrared (VISIR). We find evidence for clumpy, high density, ionized gas. The
two clusters are young (younger than 2.5 and 3 Myr respectively), the
surrounding interstellar matter is dense (10^4 cm^-3 or larger) and can be
characterized by a high ionization parameter (logU > -1.53). Detailed analysis
of the mid-infrared spectral features shows that a (near-)homogeneous medium
cannot account for the observations, and that complex structure on scales below
the resolution limit, containing several young stellar clusters embedded in
clumpy gas, is more likely.
