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We present a survey of mid-infrared gas-phase lines toward a sample of 76circumstellar disks around low mass pre-main sequence stars from the Spitzer"Cores to Disks" legacy program. We report the first detections of [Ne II] and[Fe I] toward classical T Tauri stars in ~20% respectively ~9% of our sources.The observed [Ne II] line fluxes and upper limits are consistent with [Ne II]excitation in an X-ray irradiated disk around stars with X-ray luminositiesL_X=10^{29}-10^{31} erg s^{-1}. [Fe I] is detected at ~10^-5-10^-4 L_Sun, butno [S I] or [Fe II] is detected down to ~10^{-6} L_Sun. The [Fe I] detectionsindicate the presence of gas-rich disks with masses of >~0.1 M_J. No H_2 0-0S(0) and S(1) disk emission is detected, except for S(1) toward one source.These data give upper limits on the warm (T~100-200K) gas mass of a few Jovianmasses, consistent with recent T Tauri disk models which include gas heating bystellar radiation. Compact disk emission of hot (T>~500K) gas is observedthrough the H_2 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The linefluxes are, however, higher by more than an order of magnitude than thosepredicted by recent disk models, even when X-ray and excess UV radiation areincluded. Similarly the [Ne II]/H_2 0-0 S(2) ratios for these sources are lowerthan predicted, consistent with the presence of an additional hot molecular gascomponent not included in current disk models. Oblique shocks of stellar windsinteracting with the disk can explain many aspects of the hot gas emission, butare inconsistent with the non-detection of [S I] and [Fe II] lines.

c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H_2 gas-phase lines