  We investigate the low-mass population of the young cluster IC348 down to the
deuterium-burning limit, a fiducial boundary between brown dwarf and planetary
mass objects, using a new and innovative method for the spectral classification
of late-type objects. Using photometric indices, constructed from HST/NICMOS
narrow-band imaging, that measure the strength of the 1.9 micron water band, we
determine the spectral type and reddening for every M-type star in the field,
thereby separating cluster members from the interloper population. Due to the
efficiency of our spectral classification technique, our study is complete from
approx 0.7 Msun to 0.015 Msun. The mass function derived for the cluster in
this interval, dN/dlogM \propto M^{0.5}, is similar to that obtained for the
Pleiades, but appears significantly more abundant in brown dwarfs than the mass
function for companions to nearby sun-like stars. This provides compelling
observational evidence for different formation and evolutionary histories for
substellar objects formed in isolation vs. as companions. Because our
determination of the IMF is complete to very low masses, we can place
interesting constraints on the role of physical processes such as fragmentation
in the star and planet formation process and the fraction of dark matter in the
Galactic halo that resides in substellar objects.
