  Detection of molecules using infrared spectroscopy probes the conditions and
compositions of exoplanet atmospheres. Water (H2O), methane (CH4), carbon
dioxide (CO2), and carbon monoxide (CO) have been detected in two hot Jupiters.
These previous results relied on space-based telescopes that do not provide
spectroscopic capability in the 2.4 - 5.2 micron spectral region. Here we
report ground-based observations of the dayside emission spectrum for HD
189733b between 2.0-2.4 micron and 3.1-4.1 micron, where we find a bright
emission feature. Where overlap with space-based instruments exists, our
results are in excellent agreement with previous measurements. A feature at
~3.25 micron is unexpected and difficult to explain with models that assume
local thermodynamic equilibrium (LTE) conditions at the 1 bar to 1 x 10-6 bar
pressures typically sampled by infrared measurements. The most likely
explanation for this feature is that it arises from non-LTE emission from CH4,
similar to what is seen in the atmospheres of planets in our own Solar System.
These results suggest that non-LTE effects may need to be considered when
interpreting measurements of strongly irradiated exoplanets.
