  UV radiation can induce photochemical processes in exoplanet atmospheres and
produce haze particles. Recent observations suggest that haze and/or cloud
layers could be present in the upper atmospheres of exoplanets. Haze particles
play an important role in planetary atmospheres and may provide a source of
organic material to the surface which may impact the origin or evolution of
life. However, very little information is known about photochemical processes
in cool, high-metallicity exoplanetary atmospheres. Previously, we investigated
haze formation and particle size distribution in laboratory atmosphere
simulation experiments using AC plasma as the energy source. Here, we use UV
photons to initiate the chemistry rather than the AC plasma, since
photochemistry driven by UV radiation is important for understanding exoplanet
atmospheres. We present photochemical haze formation in current UV experiments,
we investigated a range of atmospheric metallicities (100x, 1000x, and 10000x
solar metallicity) at three temperatures (300 K, 400 K, and 600 K). We find
that photochemical hazes are generated in all simulated atmospheres with
temperature-dependent production rates: the particles produced in each
metallicity group decrease as the temperature increases. The images taken with
atomic force microscopy show the particle size (15-190 nm) varies with
temperature and metallicity. Our laboratory experimental results provide new
insight into the formation and properties of photochemical haze, which could
guide exoplanet atmosphere modeling and help to analyze and interpret current
and future observations of exoplanets.
