We have measured the near-normal-incidence infrared spectral reflectance of aqueous solutions of sulfuric acid having H2SO4 concentrations of 75% and 95.6% by weight and have employed Kramers–Kronig analyses to obtain values of the optical constants n(ν) and k(ν) in the spectral range 400–6000 cm−1. The optical constants of these solutions at 250 and 300 K are compared. In spectral regions remote from strong absorption bands the values of the refractive indices n(ν) obtained at 250 K agree with the values given by Lorentz–Lorenz correction of the indices n(ν) obtained at 300 K. All absorption bands observed at 300 K are also present at 250 K with slight shifts in frequency and significant differences in k(ν) at the band maxima. The present results give further support to the view that the clouds of Venus are composed of droplets of aqueous solutions of sulfuric acid with concentrations of approximately 75% H2SO4 by weight.
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