  Photochemistry has the potential to substantially impact the atmospheric
composition of exoplanets with consequences on the radiative transfer, thermal
structure and dynamics of the atmospheres, particularly in UV-rich stellar
environments. Here, we present the results of a first laboratory experimental
simulation of photochemistry in carbon-rich exoplanet atmospheres at elevated
temperatures. Evolution of gas-phase molecular composition was quantitatively
monitored with infrared spectroscopy and mass spectrometry. We found that H2/CO
gas compositions can change significantly from thermal equilibria compositions
when irradiated with Lyman-alpha photons at temperatures ranging from 600 K to
1500 K. Carbon dioxide and water were found to be the main products caused by
photolysis, while formation of methane was also observed to a lesser extent. We
find that photochemistry efficiency is strongly correlated with increasing
temperature. Our finding that water is efficiently produced by photochemistry
in a super Solar C/O=1 environment, representing C enhancement relative to
solar values C/O ratio = 0.54, has significant implications for the
interpretation of many exoplanet transmission spectra. We also find the
formation of an organic solid condensate at 1500 K and under Lyman-alpha
UV-radiation, confirming the possibility of forming photochemical hazes in
hot-Jupiter exoplanet atmospheres with an enhanced C/O ratio compared to Solar.
