We present Spitzer Space Telescope archival mid-infrared ( mid-IR ) spectroscopy of a sample of eleven planetary nebulae ( PNe ) .
The observations , acquired with the Spitzer Infrared Spectrograph ( IRS ) , cover the spectral range 5.2-14.5 \mu m that includes the H _ { 2 } 0-0 S ( 2 ) to S ( 7 ) rotational emission lines .
This wavelength coverage has allowed us to derive the Boltzmann distribution and calculate the H _ { 2 } rotational excitation temperature ( T _ { \mathrm { ex } } ) .
The derived excitation temperatures have consistent values \simeq 900 \pm 70 K for different sources despite their different structural components .
We also report the detection of mid-IR ionic lines of [ Ar iii ] , [ S iv ] , and [ Ne ii ] in most objects , and polycyclic aromatic hydrocarbon ( PAH ) features in a few cases .
The decline of the [ Ar iii ] / [ Ne ii ] line ratio with the stellar effective temperature can be explained either by a true neon enrichment or by high density circumstellar regions of PNe that presumably descend from higher mass progenitor stars .