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Infrared spectra, optical constants and temperature dependences of amorphous and crystalline benzene ices relevant to Titan

Benzene ice contributes to an emission feature detected by the Cassini Composite InfraRed Spectrometer (CIRS) near 682 cm^{-1} in Titan's late southern fall polar stratosphere. It is as well one of the dominant components of the CIRS-observed High Altitude South Polar (HASP) ice cloud observed in Titan's mid stratosphere during late southern fall. Titan's stratosphere exhibits significant seasonal changes with temperatures that spatially vary with seasons. A quantitative analysis of the chemical composition of infrared emission spectra of Titan's stratospheric ice clouds relies on consistent and detailed laboratory transmittance spectra obtained at numerous temperatures. However, there is a substantial lack of experimental data on the spectroscopic and optical properties of benzene ice and its temperature dependence, especially at Titan-relevant stratospheric conditions. We have therefore analyzed in laboratory the spectral characteristics and evolution of benzene ice's vibrational modes at deposition temperatures ranging from 15 K to 130 K, from the far- to mid-IR spectral region (50 - 8000 cm^{-1}). We have determined the amorphous to crystalline phase transition of benzene ice and identified that a complete crystallization is achieved for deposition temperatures between 120 K and 130 K. We have also measured the real and imaginary parts of the ice complex refractive index of benzene ice from 15 K to 130 K. Our experimental results significantly extend the current state of knowledge on the deposition temperature dependence of benzene ice over a broad infrared spectral range, and provide useful new data for the analysis and interpretation of Titan-observed spectra.