Cool Stars 14 - Submitted Abstract # 216 This version created on 05 October 2006 Spitzer Spectroscopy of Ultracool Dwarfs Michael Cushing, Steward Observatory, University of Arizona Tom Roellig, NASA Ames Mark Marley, NASA Ames Didier Saumon, LANL We review the mid-infrared spectroscopic observations of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The spectra cover from 5.5 to 38 microns and exhibit prominent absorption bands of water at 6.27 microns, methane at 7.65 microns, and ammonia at 10.5 microns, and are relatively featureless at wavelength longer than 15 microns. Both the 7.65 micron methane band and 10.5 micron ammonia band first appear in the spectral sequence at roughly the L/T transition. The IRS spectra are in general well matched by atmospheric models that include the formation of spatially homogeneous silicate and iron condensate clouds. However, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 microns that we hypothesize may be due to a population of small silicate grains that are not predicted in the cloud models. Finally we describe the effects of non-equilibrium chemistry on the abundance of ammonia in the atmosphere of the T7.5 dwarf GL 570D. We find that the IRS spectrum of Gl 570D, and in particular the 10.5 micron band of ammonia, can only be fitted by reducing the abundance of ammonia by ~1 order of magnitude from the value obtained with chemical-equilibrium models. ----------------------------------