  Photochemical hazes are suspected to obscure molecular features, such as
water, from detection in the transmission spectra of exoplanets with
atmospheric temperatures < 800 K. The opacities of laboratory produced organic
compounds (tholins) from Khare et al. (1984) have become a standard for
modeling haze in exoplanet atmospheres. However, these tholins were grown in an
oxygen-free, Titan-like environment that is very different from typical
assumptions for exoplanets, where C/O~0.5. This work presents the 0.13-10
micron complex refractive indices derived from laboratory transmission
measurements of tholins grown in environments with different oxygen abundances.
With the increasing uptake of oxygen, absorption increases across the entire
wavelength range, and a scattering feature around 6 micron shifts towards
shorter wavelengths and becomes more peaked around 5.8 micron, due to a C=O
stretch resonance. Using GJ 1214b as a test-case, we examine the transmission
spectra of a sub-Neptune planet with C/O ratios of solar, 1, and 1000 to
evaluate the effective differences between our opacities and those of Khare.
For an atmosphere with solar hydrogen and helium abundances, we find a
difference of 200-1500 ppm, but for high-metallicity (Z=1000) environments, the
difference may only be 20 ppm. The 1-2 micron transmission data for GJ 1214b
rule out the Titan-like haze model, and are more consistent with C/O=1 and
C/O=solar haze models. This work demonstrates that using haze opacities that
are more consistent with underlying assumptions about bulk atmospheric
composition are important for building self-consistent models that
appropriately constrain the atmospheric C/O ratio, even when molecular features
are obscured.
