  We present a sequence of high resolution (R~20,000 or 15 km/s) infrared
spectra of stars and brown dwarfs spanning spectral types M2.5 to T6.
Observations of 16 objects were obtained using eight echelle orders to cover
part of the J-band from 1.165-1.323 micron with NIRSPEC on the Keck II
telescope. By comparing opacity plots and line lists, over 200 weak features in
the J-band are identified with either FeH or H2O transitions. Absorption by FeH
attains maximum strength in the mid-L dwarfs, while H2O absorption becomes
systematically stronger towards later spectral types. Narrow resolved features
broaden markedly after the M to L transition. Our high resolution spectra also
reveal that the disappearance of neutral Al lines at the boundary between M and
L dwarfs is remarkably abrupt, presumably because of the formation of grains.
Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed by
condensation. The neutral potassium (K I) doublets that dominate the J-band
have pressure broadened wings that continue to broaden from ~50 km/s (FWHM) at
mid-M to ~500 km/s at mid-T. In contrast however, the measured
pseudo-equivalent widths of these same lines reach a maximum in the mid-L
dwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines without
extensive pressure-broadened wings, indicative of a lower gravity atmosphere.
Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH and
H2O features, confirming its status as a rapid rotator. In contrast to other
late T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absence
of potassium even at high resolution, which may be a metallicity effect or a
result of a cooler, higher-gravity atmosphere.
