We present a sequence of high resolution ( R \sim 20,000 or 15 km s ^ { -1 } ) 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 \mu m 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 H _ { 2 } O transitions .
Absorption by FeH attains maximum strength in the mid-L dwarfs , while H _ { 2 } O 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 \sim 50 km s ^ { -1 } ( FWHM ) at mid-M to \sim 500 km s ^ { -1 } 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 H _ { 2 } O 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 .