  We report observations of the transiting extrasolar planet, HD 209458 b,
designed to detect the secondary eclipse. We employ the method of `occultation
spectroscopy', which searches in combined light (star and planet) for the
disappearance and reappearance of weak infrared spectral features due to the
planet as it passes behind the star and reappears. Our observations cover two
predicted secondary eclipse events, and we obtained 1036 individual spectra of
the HD 209458 system using the SpeX instrument at the NASA IRTF in September
2001. Our spectra extend from 1.9 to 4.2 microns with a spectral resolution of
1500. We have searched for a continuum peak near 2.2 microns (caused by CO and
water absorption bands), as predicted by some models of the planetary
atmosphere to be approximately 6E-4 of the stellar flux, but no such peak is
detected at a level of about 3E-4 of the stellar flux. Our results represent
the strongest limits on the infrared spectrum of the planet to date and carry
significant implications for understanding the planetary atmosphere. In
particular, some models that assume the stellar irradiation is re-radiated
entirely on the sub-stellar hemisphere predict a flux peak inconsistent with
our observations. Several physical mechanisms can improve agreement with our
observations, including the re-distribution of heat by global circulation, a
nearly isothermal atmosphere, and/or the presence of a high cloud.
