  New observing capabilities coming online over the next few years will provide
opportunities for characterization of exoplanet atmospheres. However,
clouds/hazes could be present in the atmospheres of many exoplanets, muting the
amplitude of spectral features. We use laboratory simulations to explore
photochemical haze formation in H2-rich exoplanet atmospheres at 800 K with
metallicity either 100 and 1000 times solar. We find that haze particles are
produced in both simulated atmospheres with small particle size (20 to 140 nm)
and relative low production rate (2.4 x 10-5 to 9.7 x 10-5 mg cm-3 h-1), but
the particle size and production rate is dependent on the initial gas mixtures
and the energy sources used in the simulation experiments. The gas phase mass
spectra show that complex chemical processes happen in these atmospheres and
generate new gas products that can further react to form larger molecules and
solid haze particles. Two H2-rich atmospheres with similar C/O ratios (~0.5)
yield different haze particles size, haze production rate, and gas products,
suggesting both the elemental abundances and their bonding environments in an
atmosphere can significantly affect the photochemistry. There is no methane
(CH4) in our initial gas mixtures, although CH4 is often believed to be
required to generate organic hazes. However, haze production rates from our
experiments with different initial gas mixtures indicate that CH4 is neither
required to generate organic hazes nor necessary to promote the organic haze
formation. The variety and relative yield of the gas products indicate that CO
and N2 enrich chemical reactions in H2-rich atmospheres.
