  We present a low-resolution (R = 90), 5.5-38 micron spectral sequence of a
sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS)
onboard the Spitzer Space Telescope. The spectra exhibit prominent absorption
bands of H_2O at 6.27 microns, CH_4 at 7.65 microns, and NH_3 at 10.5 microns
and are relatively featureless at lambda > 15 microns. Three spectral indices
that measure the strengths of these bands are presented; H_2O absorption
features are present throughout the MLT sequence while the CH_4 and NH_3 bands
first appear at roughly the L/T transition. Although the spectra are, in
general, qualitatively well matched by synthetic spectra that include the
formation of spatially homogeneous silicate and iron condensate clouds, the
spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9
to 11 microns. We hypothesize that this may be a result of a population of
small silicate grains that are not predicted in the cloud models. The spectrum
of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5-7.5 microns
relative to typical T6 dwarfs and may be a consequence of its mildly
metal-poor/high surface gravity atmosphere. Finally, we compute bolometric
luminosities of a subsample of the M, L, and T dwarfs by combining the IRS
spectra with previously published 0.6-4.1 micron spectra and find good
agreement with the values of Golimowski et al. who use L'- and M'-band
photometry and to account for the flux emitted at lambda > 2.5 microns.
