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We present a Spitzer InfraRed Spectrometer search for 10-36 micron molecularemission from a large sample of protoplanetary disks, including lines from H2O,OH, C2H2, HCN and CO2. This paper describes the sample and data processing andderives the detection rate of mid-infrared molecular emission as a function ofstellar mass. The sample covers a range of spectral type from early M to A, andis supplemented by archival spectra of disks around A and B stars. It is drawnfrom a variety of nearby star forming regions, including Ophiuchus, Lupus andChamaeleon. In total, we identify 22 T Tauri stars with strong mid-infrared H2Oemission. Integrated water line luminosities, where water vapor is detected,range from 5x10^-4 to 9x10^-3 Lsun, likely making water the dominant linecoolant of inner disk surfaces in classical T Tauri stars. None of the 5transitional disks in the sample show detectable gaseous molecular emissionwith Spitzer upper limits at the 1% level in terms of line-to-continuum ratios(apart from H2). We find a strong dependence on detection rate with spectraltype; no disks around our sample of 25 A and B stars were found to exhibitwater emission, down to 1-2% line-to-continuum ratios, in the mid-infrared,while almost 2/3 of the disks around K stars show sufficiently intense wateremission to be detected by Spitzer. Some Herbig Ae/Be stars show tentativeH2O/OH emission features beyond 20 micron at the 1-2 level, however, and one ofthem shows CO2 in emission. We argue that the observed differences between TTauri disks and Herbig Ae/Be disks is due to a difference in excitation and/orchemistry depending on spectral type and suggest that photochemistry may beplaying an important role in the observable characteristics of mid-infraredmolecular line emission from protoplanetary disks.

A Spitzer survey of mid-infrared molecular emission from protoplanetary disks I: Detection rates