The study of hot Jupiter planet atmospheres is giving rise to a surprising and intriguing diversity. Large differences in night/day contrast, chemical abundances, and physical properties are just a few examples of the variety found. A further recent addition to this complex picture is the discovery of a very strong emission feature (a factor of 4 increase in flux) at 3.2 (micron) in the transiting exoplanet HD 189733 b. Such emission has been associated to the fluorescence of the methane molecule in the heavily irradiated atmosphere of this hot Jupiter. The only viable way to move forward and understand how hot Jupiter atmospheres work is to gather data for additional planets with different irradiations. We propose Gemini/NIRI observations of the transiting planet XO-2 b in the L' band to reconstruct its emission spectrum. The target has been selected to obtain high-precision measurements and thus provide insight into the mechanisms that govern the atmospheres of hot Jupiter planets, including the newly proposed non- equilibrium chemistry.