  We present a Spitzer InfraRed Spectrometer search for 10-36 micron molecular
emission from a large sample of protoplanetary disks, including lines from H2O,
OH, C2H2, HCN and CO2. This paper describes the sample and data processing and
derives the detection rate of mid-infrared molecular emission as a function of
stellar mass. The sample covers a range of spectral type from early M to A, and
is supplemented by archival spectra of disks around A and B stars. It is drawn
from a variety of nearby star forming regions, including Ophiuchus, Lupus and
Chamaeleon. In total, we identify 22 T Tauri stars with strong mid-infrared H2O
emission. Integrated water line luminosities, where water vapor is detected,
range from 5x10^-4 to 9x10^-3 Lsun, likely making water the dominant line
coolant of inner disk surfaces in classical T Tauri stars. None of the 5
transitional disks in the sample show detectable gaseous molecular emission
with 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 spectral
type; no disks around our sample of 25 A and B stars were found to exhibit
water 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 water
emission to be detected by Spitzer. Some Herbig Ae/Be stars show tentative
H2O/OH emission features beyond 20 micron at the 1-2 level, however, and one of
them shows CO2 in emission. We argue that the observed differences between T
Tauri disks and Herbig Ae/Be disks is due to a difference in excitation and/or
chemistry depending on spectral type and suggest that photochemistry may be
playing an important role in the observable characteristics of mid-infrared
molecular line emission from protoplanetary disks.
