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Mid-infrared (IR) observations of polycyclic aromatic hydrocarbons (PAHs) andmolecular hydrogen emission are a potentially powerful tool to derive physicalproperties of dense environments irradiated by intense UV fields. We presentnew, spatially resolved, \emph{Spitzer} mid-IR spectroscopy of the highUV-field and dense photodissocation region (PDR) around Monoceros R2, theclosest ultracompact \hII region, revealing the spatial structure of ionizedgas, PAHs and H$_2$ emissions. Using a PDR model and PAH emission featurefitting algorithm, we build a comprehensive picture of the physical conditionsprevailing in the region. We show that the combination of the measurement ofPAH ionization fraction and of the ratio between the H$_2$ 0-0 S(3) and S(2)line intensities, respectively at 9.7 and 12.3 $\mu$m, allows to derive thefundamental parameters driving the PDR: temperature, density and UV radiationfield when they fall in the ranges $T = 250-1500 $K, $n_H=10^4-10^6$cm$^{-3}$,$G_0=10^3-10^5$ respectively. These mid-IR spectral tracers thus provide a toolto probe the similar but unresolved UV-illuminated surface of protoplanetarydisks or the nuclei of starburst galaxies.

Mid-infrared PAH and H2 emission as a probe of physical conditions in extreme PDRs