  Photochemistry induced by stellar UV flux should produce haze particles in
exoplanet atmospheres. Recent observations indicate that haze and/or cloud
layers exist in the atmospheres of exoplanets. However, photochemical processes
in exoplanetary atmospheres remain largely unknown. We performed laboratory
experiments with the PHAZER chamber to simulate haze formation in a range of
exoplanet atmospheres (hydrogen-rich, water-rich, and carbon dioxide-rich at
300, 400, and 600 K), and observed the gas phase compositional change (the
destruction of the initial gas and the formation of new gas species) during
these experiments with mass spectrometer. The mass spectra reveal that distinct
chemical processes happen in the experiments as a function of different initial
gas mixture and different energy sources (plasma or UV photons). We find that
organic gas products and O2 are photochemically generated in the experiments,
demonstrating that photochemical production is one of the abiotic sources for
these potential biosignatures. Multiple simulated atmospheres produce organics
and O2 simultaneously, which suggests that even the co-presence of organics and
O2 could be false positive biosignature. From the gas phase composition
changes, we identify potential precursors (C2H2, HCN, CH2NH, HCHO, etc.) for
haze formation, among which complex reactions can take place and produce larger
molecules. Our laboratory results indicate that complex atmospheric
photochemistry can happen in diverse exoplanet atmospheres and lead to the
formation of new gas products and haze particles, including compounds (O2 and
organics) that could be falsely identified as biosignatures.
